Tuesday, 22 October 2024
Yunfuconcha bimenta: A Heteroconch Bivalve from the Middle Ordovician of Guangdong Province, China.
Monday, 10 April 2023
Trying to identify an invasive colonial Sea Squirt from the Gulf of California.
Invasive species are one of the greatest challenges faced by conservationists today, often rapidly taking over ecosystems where they have no natural enemies and displacing endangered or commercially important local species in the process. The first step in controlling an invasive species is identifying it, which enables appropriate control systems to be introduced. However, this is often difficult and complicated, as invasive species are not necessarily well understood, or even known, in their own ecosystems. This is particularly true in marine environments, where our taxonomic understanding of many groups is poor, and increasingly plagued by a lack of skills as few young scientists are attracted into the field. This is despite the problem of invasive species being particularly acute in marine environments, where international shipping traffic has made it easy for many benthic 'fouling' organisms to rapidly establish global distributions.
Ascidians, or Sea Squirts, are the most abundant class of the Subphylum Tunicata and are distributed along shorelines worldwide. They are sessile marine invertebrates and are widely used as a model organism for developmental and evolutionary studies. Ascidians exhibit multiple morphological characteristics, from small colonial to colorful and large solitary forms. They are divided into three major well-accepted orders, namely, Phlebobranchia, Aplousobranchia, and Stolidobranchia, based on the branchial sac morphology of the adults. However, the class Ascidiacea is paraphyletic (i.e. not everything thought to be descended from the last common ancestor of the group is considered to be an Ascidian) with the Phlebobranchia and Aplousobranchia showing a close relationship with Thaliaceae (Pyrosomes, Salps, and Doliolids), a non-Ascidian Tunicate class, whereas the Stolidobranchia remains a distinct and monophyletic group. Over the course of several decades, the Ascidiacea have been shown to be an important class of ecological species because of their invasive potential along with their ability to adapt to new environments. Transportation of Ascidians attached to ship hulls as fouling material and within the ballast water of ships has enabled them to invade many new territories. This phenomenon has major impacts on local marine biodiversity as well as aquaculture industries. Therefore, the Ascidiacea are now considered as important model species for the study of non-indigenous species worldwide.
Sea Squirts are particularly problematic from a taxonomic point of view, with a simple body plan which provides few diagnostic features, and very few scientists specializing in their taxonomy. To make matters worse, the group appears to be rife with cryptic species (i.e. species which are physically identical or nearly identical, but biologically distinct), so that many identified species are likely to be clusters of similar-looking species, often with different ecological constraints. Many species of Sea Squirt have proven to be adept at colonizing new environments, where they are often identified as new species, so that particularly successful invasive species often have several different names, further adding to the taxonomic confusion within the group. A combination of careful physical examination and genetic analysis can hopefully unravel some of these problems, particularly in the identifying of pseudo-indigenous species, i.e. invasive species thought to be native to their new habitat because their origin is unknown.
Eleven invasive species of Sea Squirt had been identified from Mexican waters by 2014, five of them from the Gulf of California. In 2015, a colonial Sea Squirt previously unknown to the region was identified in Ensenada de La Paz, which spread rapidly, causing a mass mortality event in Pen Shells, Atrina maura, a Bivalve species commercially important locally, whose large shells provided an ideal substrate for the invaders. The Pen Shell fishery was already suffering from over-harvesting, with a moratorium on their collection introduced in 2013 to allow the population to recover, something which appears to have been significantly hampered by the arrival of the Ascidians (no significant variation in other environmental variables was detected which coincided with the die-back). A subsequent biofouling experiment carried out in Bahía de La Paz found that the Sea Squirts, tentatively identified as Distaplia cf. stylifera, were the most abundant macro-organisms settling on a series of silicone resin coated metal panels placed in the water for two months. Furthermore, the Sea Squirt colonies provided a substrate upon which 28 species of epibiont Polychaete Worms were able to settle, further adding to the biofouling. Subsequent studies of the Sea Squirts, which have continued to suppress Shellfish populations, and become a serious biofouling problem in the area, have used the name Distaplia stylifera, although this is only an assumption, based upon a rough similarity to descriptions and the fact that it is a common invasive species. No formal taxonomic identification of the invader has been attempted to date, something which may be hampering efforts to find an effective control.
In a paper published in the journal ZooKeys on 5 April 2023, Betzabé Moreno-Dávila and Leonardo Huato-Soberanis of the Programa de Ecología Pesquera at the Centro de Investigaciones Biológicas del Noroeste, Jaime Gómez-Gutiérrez of the Departamento de Plancton y Ecología Marina at the Instituto Politécnico Nacional, Carolina Galván-Tirado of the Consejo Nacional de Ciencia y Tecnología, Carlos Sánchez of the Departamento de Ciencias Marinas y Costeras at the Universidad Autónoma de Baja California, Teresa Alcoverro of the Department of Marine Ecology at the Centre for Advanced Studies of Blanes, Eduardo Balart, also of the Programa de Ecología Pesquera at the Centro de Investigaciones Biológicas del Noroeste, and Xavier Turon, also of the Department of Marine Ecology at the Centre for Advanced Studies of Blanes, present the results of a study of the invasive Sea Squirt from Ensenada de La Paz, which examines the Mexican species' morphology and compares it to previous descriptions from other parts of the world, as well as attempting a genetic analysis.
Bahía de La Paz is located on the southeastern tip of the Baja California Peninsula. The Ensenada de La Paz is separated from Bahía de La Paz by a 12 km long sandbar known as El Mogote. The Ensenada de La Paz has an area of about 45 km² and a maximum depth of 10 km and a bottom covered by patches of sand and mud-silt. The Ensenada de La Paz and the Bahía de La Paz are connected by a shallow channel about 1 km wide and 4 km long, upon which are seven yacht docks associated with the city of La Paz, while at the entrance lies the Pichilingue commercial harbour.
Colonies representing three different colour-morphs of the invasive Sea Squirt (white, orange, and purple) were collected from four sites within the Ensenada de La Paz, for a total of twelve colonies, which were preserved for morphological analysis as well as having their DNA extracted for genetic analysis. The colonies were found growing at depths of 0.5-3 m, growing on the shells of Sea Pens, a PVC pipe, buoys, a rope, and wooden docks.
The colonies were predominantly orange, and approximately mushroom-shaped, with mottled white markings around the common cloacal-aperture, with purple and white colonies also present within the population. They reach a maximum of about 2 cm, with the head having a diameter of up to 2.5 cm. The colonies are more heavily pigmented around their tips than at the base, with pigment only remaining in this area after preservation. The tunics of the colonies are firm, and the stalks sometimes branch, so that two or more colonies share a common base; some colonies lack stalks altogether, forming cushion-shaped masses that spread over the substrate. The head of each colony is shared by one or more zooid systems, each sharing a common cloacal-aperture, which is surrounded by a single or double ring of zooids.
The zooids are up to 5 mm in length, excluding the gonadal sacs. Each zooid is divided into a thorax and abdomen. Two sacs are attached to this structure, a smaller one, containing the gonads, is attached to the right posterior side of the abdomen. The second sac contains embryos and developing larvae, and is often longer than the zooid. This second sac is attached to the posterior part of the pharynx by a thin peduncle.
The oral siphon on the thorax is smooth-rimmed or has six slight lobulations, and a large atrial opening which exposes most of the branchial sac. A wide flap-like lid with smooth or lobed margins called the atrial languet lies on top of the atrial opening. This languet is crossed by several transverse muscular bands. Each side of the thorax has about 30 bands of longitudinal muscle. About 14 simple oral tentacles are also present on the thorax. The pharynx has four stigmata rows clearly divided by parastigmatic vessels, with the first two rows typically having 18-19 stigmata, and the posterior two rows having 15-16. Three simple dorsal languets are found between these rows, slightly displaced to the left.
Within the abdomen lies an elongated and curved stomach, the wall of which has more than 20 fine longitudinal plications (folds), which can be seen on the inner and outer surfaces when the specimens are sectioned, although they are sometimes interrupted or divided. A short post-stomach connects to an enlarged mid-intestine at the bottom of the gut-loop. The distal intestine runs to the anterior, and ends in a bilobed anus at the base of the atrial aperture. A pyloric (mucus) gland between the stomach and the intestine and continues anteriorly forming sinuous tubules over the intestine in front of the stomach.
The gonads are held within a pedunculated sac, with one or two oocytes at the bottom, above which lie a cluster of five or six elongated or wedge-shaped testes. A common sperm duct arises posteriorly from the cluster of testes, but turns anteriorly at its very beginning, without overlapping the oocytes.
All of the colonies examined had larvae incubating in long sacs that reach posteriorly deeper than the zooids themselves in the colonies. These sacs typically contained one or two well developed larvae, about 1.3 mm in length, plus three embryos. When fully developed the larvae reach about 1.5 mm, and possess three adhesive papillae, two dorsal and one ventral, with a globular ampulla each in the stalks. The four rows of stigmata have developed by this stage, with an incipient abdomen folded under the branchial sac. A sensory vesicle contains two pigmented spots, with a larger one above and a smaller one below, though these can be hard to differentiate, as they are close together and the larvae are not transparent.
Distaplia stylifera was originally described in the Red Sea, and subsequently has been recorded from several areas of the Indo-Pacific, Australia, the Philippines, and the South China Sea, as well as. under the possible synonym Distaplia mikropnoa, from Palau. There have been reports from the Mediterranean, although these are considered dubious. It has also been reported in the Western Atlantic, from the coast of North Carolina south as far as Jamaica, and at several locations in the Caribbean, and possible further south around Sao Paulo in Brazil. It has not, however, previously been reported from the Eastern Pacific.
Despite successful amplification of DNA apparently collected from the Ascidian colonies, all of this proved, upon analysis, to be closer to that of Algae, Bacteria, or Fungi, than to any Animal, despite great care being taken to avoid any contamination. For this reason the taxonomic comparison had to be made entirely upon physical examination of the specimens, and comparison to other members of the genus Distaplia.
The taxonomy of the genus Distaplia is mainly based on characters such as colony shape, arrangement of zooids in systems, presence or not of gonadal sac, stigmata per row, stomach shape and external surface, and muscle arrangement. Based upon these criteria, the Ensenada de La Paz specimens would appear to be entirely consistent with Distaplia stylifera. However, Moreno-Dávila et al. note that the original description of the species was made in 1874, and like a lot of descriptions from this period, was a lot less detailed than descriptions made by modern taxonomists. Furthermore, that description was made upon a specimen that was probably a juvenile, lacking larvae and having under-developed gonads.
Descriptions of populations of modern populations of 'Distaplia stylifera' show some variation, making it possible that the global population is in fact a cluster of closely related species. Notably, the specimens collected at Ensenada de La Paz all had parastigmatic vessels, something also reported in almost all specimens collected from Australia, as well as specimens collected from the Caribbean. An absence of such vessels has been noted in specimens from Madagascar, and it has been suggested that the presence or absence of these vessels might be a feature which can be used to differentiate between Distaplia stylifera and Distaplia mikropnoa. However, a number of other features can be used to differentiate Distaplia mikropnoa, which include a long double rows of zooids converging to the terminal common cloacal apertures, a long post-pyloric part of the gut loop, and a lack of a gastric reservoir. Furthermore, in Distaplia mikropnoa the course of the gastro-intestinal ducts that does not cross from the stomach to the ascending limb of the gut loop but extends down the descending loop. Based upon this, Patricia Kott concluded that the two species are both valid, but that they have widely confused within the literature. Notably, she concluded that the Palau population belonged to Distaplia stylifera rather than Distaplia mikropnoa.
Oval follicles have been suggested as another feature which could be used to differentiate the two species, with specimens which have up to 15 oval follicles assigned to Distaplia mikropnoa, while those having only five or six, which would include the Ensenada de La Paz population, belong to Distaplia stylifera.
The sperm duct is another feature apparently variable in populations assigned to Distaplia stylifera, with some populations having a sperm duct running posteriorly and making one or several loops over the oocytes before turning anteriorly, while in other populations, including the Ensenada de La Paz population, it is straight. Furthermore, in some populations, including Ensenada de La Paz, the gonadal sac is attached via a peduncle, while in others it is almost flush with the abdomen, separated by a wide neck.
Moreno-Dávila et al. suggest that this wide range of variable features indicate that 'Distaplia stylifera' is probably a cluster of closely related species, often mistaken with the closely related Distaplia mikropnoa. This probably cannot be resolved without a more detailed taxonomic study, using both morphological and genetic methods to analyse populations from different regions of the world. Nevertheless, the reporting of a member of the species cluster from the Eastern Pacific represents a significant range-expansion for a group previously known from tropical regions of the Western Atlantic, Indian Ocean, and Western Pacific.
Moreno-Dávila et al. were unable to extract DNA from their specimens of 'Distaplia stylifera', and note that no records exist within the GenBank and BOLD public databases. The co-generic Distaplia bermudensis is present in these databases, and shows a 14–20% genetic variance between different populations and morphotypes, indicating either a remarkable level of genetic variation within a single species, or that Distaplia bermudensis is also a species cluster.
This taxonomic uncertainty makes it difficult to assess to what extent the different populations of 'Distaplia stylifera' represent introductions or local species, although Moreno-Dávila et al. note that the populations from North Carolina and the Caribbean appear to strongly favour lagoons and artificial structures, which is behaviour typical of invasive fouling organisms, and that this population appears to be expanding southwards to the coast of Brazil, where again it has been found only on artificial substrates.
Despite this uncertainty, Moreno-Dávila et al. believe that the Ensenada de La Paz population does represent a high-impact invasive species, and given the absence of any other known populations of 'Distaplia stylifera' in the Eastern Pacific, and the behavioural similarities of this species to the Western Atlantic populations, one which has probably arrived via the Panama Canal. probably crossing the canal on a large ship, then secondarily transferring to a smaller vessel capable of entering the harbours around La Paz.
See also...
Follow Sciency Thoughts on Facebook.
Follow Sciency Thoughts on Twitter.
Tuesday, 10 August 2021
Inversidens rentianensis: A new species of Freshwater Mussel from Jiangxi Province, China.
The genus Inversidens was established in 1911 by the German zoologist Fritz Haas, to describe two species of Freshwater Mussels (Uniomoids) from Japan, Inversidens brandtii and Inversidens parcedentata. Haas revisited the genus in 1969 adding a further four species, Inversidens reinianus, Inversidens haconensis, and Inversidens japanensis, from Japan, and Inversidens pantoensis, from China. Howeverr, other mallacologists subsequently removed several of these, so that only two species Inversidens brandtii from Japam and the Chinese Inversidens pantoensis now remain.
In a paper published in the journal ZooKeys on 3 August 2021, Ruiwen Wu of the School of Life Science at Shanxi Normal University, Xiongjun Liu, of the School of Life Science at Jiaying University, Takaki Kondo of the Division of Natural Science at Osaka Kyoiku University, and Shan Ouyang and Xiaoping Wu of the School of Life Sciences at Nanchang University, describe a new species of Inversidens from Jiangxi Province in China.
The new species is named Inversidens rentianensis, where 'rentianensis' means 'from Rentian' , in reference to the town of Rentian, which is near where the species was discovered. The shell of the new species is subtriangular and inflated, with a rounded anterior margin, an almost straight ventral margin, and an obliquely arc-shaped posterior margin. The umbos (prominences above the hinge) are well developed, but slightly eroded. Only one tooth is present on each valve. The shell is covered by a shiny black or brownish yellow skin, the shell itself is a reddish-brown colour. The shells are 43–52 mm in length and 29–36 mm in height. The species is known only from the Mianshui River near Renrian.
Species of Freshwater Mussels can be difficult to tell apart, as they tend to be morphologically variable, responding to their local environment, and show a high degree of physical convergence with other species. To counter these problems, Wu et al. carried out a genetic analysis using the mitochondrial cytochrome oxidase subunit I gene sequence, which has been widely used to delimitate Freshwater Mussel species. This analysis recovered Inversidens rentianensis as the sister species to Inversidens brandtii, enabling them to be confident of its assignment to the genus Inversidens.
See also...
Follow Sciency Thoughts on Facebook.
Follow Sciency Thoughts on Twitter.
Saturday, 1 May 2021
Authorities in California concerned by rising use of hydrolic pumps to harvest Clams.
The California Department of Fish and Game has raised concerns about the increasingly widespread use of hydraulic pumps to harvest Clams on the northern coasts of the state. The pumps, which first appeared about four years ago, use a mechanism similar to a bicycle pump to inject water into the sediment around the Clams, liquifying the sediment and making them easier to extract. This enables the clammers to extract many more Clams in a given period of time than was possible with more traditional methods, such as digging with a garden fork.
The use of hydraulic pumps has expanded rapidly in the past year, as thousands of people laid off from their regular employment due to the Covid-19 pandemic have turned to hunting and gathering activities as pastimes which can be carried out in socially distanced ways, and which provide some food for the table and possibly a bit of extra income. This has put particular pressure on protected Gaper and Washington Clams, both of which are subjected to harvesting restrictions. Harvesting of these large Clams is restricted to ten per person per day, which is not unreasonable with traditional techniques, but with the pumps it is often possible to harvest this many Clams in an hour, with daily yields consequently being much higher. To make matters worse, the pumps have made it viable to target areas which are more difficult to access with traditional techniques, usually because they are not exposed by the tide for very long, thereby endangering Clam populations which have, until now, acted as reserves from which more accessible populations can be re-seeded. This overharvesting led to a ban on the pumps being introduced in February 2021, but this has proved difficult to enforce, with many harvesters apparently either ignorant of or indifferent to the ban.
As well as the large numbers of amateur Clam harvesters, professional Clam poachers have become a serious problem on parts of the coast. These poachers harvest particularly large numbers of Clams, not just for personal use, but to be sold on into the commercial seafood industry. The advent of hydraulic pumps has made it much easier for these poachers, not just to extract many more Clams, but to extract them without any chips to the shells, which is common with traditional digging techniques. This often enables the poachers to pass off Californian Gaper Clams as more valuable Geoduck Clams from the coast of Washington, which can sell for up to US$80 per kg.
See also...
Follow Sciency Thoughts on Facebook.
Follow Sciency Thoughts on Twitter.
Friday, 12 March 2021
Dreissena polymorpha: United States Geological Survey finds invasive Zebra Mussels in pet stores in 21 states.
A citizen’s report of an invasive Zebra Mussel, Dreissena polymorpha, found in an aquarium Moss package found in a pet store prompted a United States Geological Survey expert on invasive aquatic species to trigger nationwide alerts that have led to the discovery of the destructive shellfish in pet stores in at least 21 states from Alaska to Florida, according to a press release issued on 8 March 2021.
Amid concerns that the ornamental aquarium Moss balls containing Zebra Mussels may have accidentally spread the pest to areas where it has not been seen before, federal agencies, states, and the pet store industry are working together to remove the Moss balls from pet store shelves nationwide. They have also drawn up instructions for people who bought the moss balls or have them in aquariums to carefully decontaminate them, destroying any Zebra Mussels and larvae they contain using one of these methods: freezing them for at least 24 hours, placing them in boiling water for at least one minute, placing them in diluted chlorine bleach, or submerging them in undiluted white vinegar for at least 20 minutes. The decontamination instructions were developed by the U.S. Fish and Wildlife Service, the USGS and representatives of the pet industry.
Zebra Mussels are an invasive, fingernail-sized Mollusc native to freshwaters in Eurasia. They clog water intakes for power and water plants, block water control structures, and damage fishing and boating equipment, at great cost. The federal government, state agencies, fishing and boating groups and others have worked extensively to control their spread.
In 1990, in response to the first wave of Zebra Mussel invasions, the USGS set up its Nonindigenous Aquatic Species Database, which tracks sightings of about 1270 non-native aquatic Plants and Animals nationwide, including Zebra Mussels. State and local wildlife managers use the database to find and eliminate or control potentially harmful species.
The coordinator of the Nonindigenous Aquatic Species Database, USGS fisheries biologist Wesley Daniel, learned about the presence of Zebra Mussels in Moss balls on 2 March and alerted others nationwide about the issue. Moss balls are ornamental plants imported from Ukraine that are often added to aquariums.
'The issue is that somebody who purchased the Moss ball and then disposed of them could end up introducing Zebra Mussels into an environment where they weren’t present before,' Daniel said. 'We’ve been working with many agencies on boat inspections and gear inspections, but this was not a pathway we’d been aware of until now.'
On 25 February, an employee of a pet store in Seattle, Washington, filed a report to the database that the employee had recently recognised a Zebra Mussel in a Moss ball. Daniel requested confirming information and a photograph and received it a few days later.
Daniel immediately notified the aquatic invasive species coordinator for Washington State and contacted invasive species managers at the USGS and USFWS. He visited a pet store in Gainesville, Florida, and found a Zebra Mussel in a Moss ball there. At that point federal non-indigenous species experts realized the issue was extensive.
The USFWS is coordinating the response along with the USGS. The U.S. Department of Agriculture, several state wildlife agencies and an industry group, the Pet Industry Joint Advisory Council, are also taking steps to mitigate the problem. National alerts have gone out from the USFWS, the federal Aquatic Nuisance Species Task Force and regional aquatic invasive species management groups. Reports of Zebra Mussels in Moss balls have come from Alaska, California, Colorado, Florida, Georgia, Iowa, Massachusetts, Michigan, Montana, Nebraska, Nevada, New Mexico, North Dakota, Oklahoma, Oregon, Tennessee, Vermont, Virginia, Wisconsin, Washington and Wyoming.
'I think this was a great test of the rapid-response network that we have been building,' Daniel said. 'In two days, we had a coordinated state, federal and industry response.'
The USGS is also studying potential methods to help control zebra mussels that are already established in the environment, such as low-dose copper applications, carbon dioxide and microparticle delivery of toxicants.
Sightings of Zebra Mussel or other non-indigenous aquatic Plants or Animals in the US can be reported here.
See also...
Follow Sciency Thoughts on Facebook.
Follow Sciency Thoughts on Twitter.
Thursday, 14 January 2021
European Flat Oysters return to Belfast Lough.
The demise of the European Native Oyster, Ostrea edulis, from that of a keystone species to an obscure Bivalve, throughout the majority of its natural range has been well documented. A number of factors have been associated with the demise of global estuarine Oyster populations between the mid-1700s and late-1800s; industrial pollutants, coastal development, increases in sewage outflow and growing urbanisation. However, the most prominent drivers related to historical losses in Native Oyster populations have been identified as overfishing and disease. The combination of these stressors devastated stocks to such an extent that the Oyster still remains extinct from many of its historically prolific sites more then 100 years after its disappearance. Instances whereby the Native Oyster has returned unaided are rare and those which have been documented were often the result of an aquaculture spawning event. Pro-active interventions using stock augmentation are therefore considered vital if Ostrea edulis is to make a return to its historic locations. As a result, numerous Ostrea edulis restoration programmes are currently underway throughout the UK and Europe in an attempt to address the Oysters decline.
Belfast Lough in Northern Ireland once accommodated a substantial population of Native Oysters. The first report of a recognised commercial oyster fishery in the Lough was in 1780 when it was stated that 'the Oyster is dredged from September to May by 27 boats and 123 fishers all of whom can read with the exception of two'. The report also implied that Oyster stocks were in a state of decline as a boat could dredge 800 to 1200 large Oysters a day at a price of 4–7 shillings per 100 in the year of 1800. However, by 1819, a boat was dredging between 100 and 300 Oysters a day at a price of 8–18 shillings per 100. Since the official closure of the Belfast Lough fishery in 1903 there have been no reports of wild Ostrea edulis, no aquaculture stocks and no restoration attempts.
An investigation into the 1897, 1898 and 1901 enteric fever epidemics of Belfast City by the physician Dara Mair stated that, ‘the working class of Belfast were heavy consumers of shellfish Including Periwinkles, Cockles, and Mussels but not Oysters as these have been practically extinct for many years’’. The Irish Fisheries Commission considered the Belfast Lough Oyster fishery officially closed in their report of 1903. The renowned malacologist Nora Fisher McMillan did not record any living Ostrea edulis specimens during meticulous surveys of the Lough between 1928 and 1929. John Gee and Kieth Wilson some fifty years later still failed to document the Oyster on a comprehensive Molluscan species list and more recently, in 1999, malacologists from the Ulster Museum Belfast could not detect any living native Oysters during an extensive subtidal and intertidal investigation. The 2002 Joint Nature Conservation Committee survey of the Lough was also unsuccessful in locating any live specimens and in 2017 the Sanitary Review of Belfast Lough only documented the presence of sub-fossil shell.
These historical accounts and surveys confirm the European Native Oyster as being absent from Belfast Lough for over 100 years. However, there have recently beeb unconfirmed reports of solitary Ostrea edulis along the intertidal zone of the Lough.
In a paper published in the journal Regional Studies in Marine Science in December 2020, David Smyth, Maria Hayden-Hughes, Jenna Alexander, and Philippa Bayford of the School of Ocean Science at Bangor University Wales, and Louise Kregting of the School of Natural and Built Environment at Queen’s University Belfast, present the results of a study which investigated these unsubstantiated sightings of individual Ostrea edulis to ascertain if an unassisted recovery had indeed occurred after more than a century.
Belfast Lough is a fully marine inshore body of water located on the east coast of Northern Ireland. The Lough is a relatively shallow marine bay roughly 21 km long and 11 km wide, with a max depth of 23 m covering an area of 130 km² with annual seawater temperatures of between 2 to 21°C. The intertidal zones of both shores are characterised by a sandy mud substrate with high volumes of overlaying shell material. The three most abundant contributing species to substrate mixes are; Mytilus edulis, Cerastoderma edule and Artica islandica. The main freshwater input into Belfast Lough is via the River Lagan, at a mean flow of 8.521 cubic metres per second.
The River Lagan is impounded by a floodgate, the Lagan Weir, which typically only allows water exchange for 2 hours either side of high tide. According to the water balance index there is a clear dominance of the tidal dynamics against river dynamics in the Lough. Belfast Lough is also subject to intensive anthropogenic stressors. The major industrial shipping port at the head of the Lough manages over 80% of Northern Ireland’s petroleum and oil imports. Furthermore, Belfast Port handles more than 7000 vessels per year with an average freight through flow of 24.6 million tonnes. The inner lough accommodates 21 licenced Blue Mussel, Mytilus edulis, mariculture beds which are fished by dredge. Fishing activity in the outer Lough focuses on pot fishing, Scallop dredging and bottom-trawling for Dublin Bay Prawns (Langoustine), Nephrops norvegicus.
In order to ascertain if Ostrea edulis had settled along the intertidal zone of the Lough, both east and west shores were surveyed between May and June 2020 using belt transects of 5 m carried out 1 m from the low water mark on tides lower than 0.8 m below datum chart. Ten × 1 km transects were completed along the west shore starting at Shellbank and finishing at Kilroot. Dangerous sheer rock terrain and fine sand substrates limited the east shore survey to six × 1 km transects starting at, Fujitsu Shore and finishing at Rock Beach.
If an Oyster was recorded along a transect, a 20-minute timed search was carried out at the site of settlement within a 5 × 20 m survey plot. Substrate type was recorded at all sites and all oyster height measurements recorded in-situ using Vernier callipers. Specimens were photographed using a Canon Powershot G16 on auto setting:19 mm focal length, ISO 100, 1/50 s at f/5.6. 16:9 aspect ratio with a reference in frame for scale. Images were calibrated using Coral Point Count to the suggested known overhead distance of (1 m). The image measuring application in Coral Point Count was applied to each image to obtain morphometric data which could be applied to the shell age associations for Ostrea edulis.
As this was a baseline investigation, statistical analysis was limited. However, a t-test was carried out to examine the total number of Oysters recorded on the east and west shores to determine if settlement was governed by location. In order to ascertain if a difference existed in the size of Oysters recorded at each site a one-way permutational multivariate analysis of variance was carried out between site and size of individual oysters using PAST vr3.4.
Within the intertidal zone of Belfast Lough, live Ostrea edulis were recorded at six sites. The west shore had five sites and the greatest number of Oysters with 32 individuals, the smallest being 27 mm in length and the largest 112 mm. The size and age variations between sites indicated that recruitment has been on-going within the Lough over the past 8–10 years. The average age of Oysters on the west shore were between 3–4 years with an average shell height of 64 mm. The east shore had one settlement site with nine individuals, the smallest being 33 mm and the largest 72 mm. The average age within the assemblage was 1–2 years with an average shell height of 50 mm. However, it must be emphasised that there may be variations within these age determinations these estimates were based on Ostrea edulis specimens from Essex which sits three lines of latitude below the sites in Belfast Lough.
Intertidal Ostrea edulis assemblages: (4) Gideon’s Green, (5) Hazelbank, (6) Whiteabbey, (7) Jordanstown, (8) Greenisland and (13) Kinnegar Barracks. Map generated using Arc Map 10.7.1 spatial analysis WGS84 coordinate geometry used throughout. Smyth et al. (2020).
A t-test was carried out using PAST vr3.4 between the total Oyster abundance of west and east shore assemblages, no significant difference was detected. Furthermore, permutational multivariate analysis of variance analysis did not detect any significant differences between Oyster size and site. A basic substrate description was assigned to each survey transects and live Oyster sites.
Unsubstantiated reports of solitary European Flat Oysters on the shores of Belfast Lough suggested that Ostrea edulis had returned to the waterway after more than a century of absence and the findings presented by Smyth et al. have categorically shown that this is indeed the case. A total of 42 Oysters were recorded with sizes ranging from 28–112 mm indicating a population consisting of both 0–1-year juveniles and adults of over 8 years. These variations in age are significant and indicate that recruitment had been taking place unnoticed within the Lough for at least 8-10 years.
While the number of Oysters does not signify a sustainable population, it does raise questions as to; where did they come from and what allowed the settlement of larvae to occur? There have been no attempts at Ostrea edulis aquaculture within the Lough’s catchment and therefore the recent settlements must have been incidental. An understanding of recent abiotic and biotic changes within the Lough may offer some explanations as to how Ostrea edulis has managed to make a return. An, important recent development within the Lough to be noted before assumptions are made occurred in 2016 with the dredging of the central channel. This was carried out to accommodate an increase in shipping traffic which had been steadily growing since 2000 when 5336 vessels visited Belfast Port to more than 6800 in 2018. As a result, over 400,000 m³ of dredge spoil were removed to widen the main shipping routes and accommodate deeper drafts.
The hypothesis that shipping ballast water spreads Molluscan species has been postulated since the late 1800s and is now a well-documented route for invasive and native species into new areas. The transportation of the Pacific Oyster, Crassostrea (or Magallana) gigas, larvae has been periodically detected in ballast water sources. It has been hypothesised that ballast water transfer in the 1990s was the cause of Crassostrea gigas introductions into the east coast of Scotland as there had been no aquaculture ventures for the species in the region.
The increases in shipping to Belfast Port over the last decade offer the possibility for a ballast water induced spread via the transfer of pelagic larvae through ballast deposition. However, the reality of this is doubtful as the global sources of planktonic Ostrea edulis larvae are extremely rare and mortalities within the pelagic stage of the life cycle are considerable.
A more probable explanation is that adult Oysters were introduced through the commercial Blue Mussel fishery. The Lough has 21 licenced subtidal Mytilus edulis mariculture sites spread over a number of plots located either side of the shipping channel. The fishery plots are seeded with juvenile Mussels dredged from various locations in the Irish Sea. It may be that within some of the dredged seed that a number of fecund Ostrea edulis were collected and relayed. However, the translocation of seed Mussel has been carried out for over 30 years with no Ostrea edulis settlements recorded. Therefore, if the mariculture plots have been the source of Oyster larvae, it must have been from seed deployed within the last ten years. This is significant as records of the seed Mussel site locations for the 2008, 2009 and 2010 relays may reveal a possible undiscovered Irish Sea population of Native Oysters.
In addition, abiotic or biotic changes within Lough over the last 10 years could have created conditions conducive to promoting the pooling and settlement of Oyster larvae. Recent bathometric changes through the deepening of the central shipping channel and the resulting subsequent alterations to fine-scale hydrodynamics, may offer an explanation. However, fine-scale particle tracking and hydrodynamic modelling of the watercourse would be required to confirm this theory and unfortunately this was economically beyond the resources of Smyth et al.'s short survey.
Belfast Port sits at the head of the Lough and is the largest commercial harbour in Northern Ireland. Anthropogenically introduced disturbances such as dredging are often associated with detrimental changes in marine habitats. However, in the case of Belfast Lough the increases in shipping and the subsequent dredging induced bathymetrical changes may have created a situation whereby the hydrodynamic regimen is now in a position to once again permit Ostrea edulis larval settlements.
Increases in vessel activity and their subsequent wakes in particular have been shown to be instigators of environmental change within soft sediment benthic communities. Wake generated water velocity has led to instances of increased siltation in shallow estuarine systems and the subsequent smothering of bivalve beds and the burying of hardshell substrates. However, this is not always the situation and a number of factors need to be considered when assessing the effects of vessel wakes. Sediment composition, channel geometry, distance to shore, the number of successive vessels and their speed of passage can all influence wake effects.
In 2012 Kyle Demes, Rebecca Kordas, and Jennifer Jorve, showed in British Columbia that primary production actually increased on rocky shore sites which lay favourable distances from intermittent ferry wake pulses. The distance from the wake’s origin can have a significant influence on the degree of environmental stress. In some instances, Crassostrea virginica Oyster reef systems in shallower estuarine regions were displaced by wakes with heights as shallow as 2 cm. However, wakes were shown to have had no impact on recruitment. Indeed, the dislodgement of Oyster assemblages may have actually aided the geographical expansion of Crassostrea virginica within estuaries.
The combined effect of regular wake travel to and from the shore can result in cleaning substrate surfaces of sediment through a washing effect. This process can lead to increases in species abundance and richness along intertidal zones which are situated sufficiently far enough from the initial high velocity water pulse.
Smyth et al.'s studty has revealed no Oyster settlement close to the Port entrance even though shell substrate was substantial. Therefore, the speculation that the mariculture plots were a source of larvae becomes ever more likely as settlements were localised to sites within the boundaries of the Mussel fishery lays and between 3–7 km from the head of the Lough. A distance which appears to have been suitably far enough from initial wake amplitude to allow shell substrates to remain clear of siltation, while tidal water retention remained sufficient to enable Ostrea edulis pediveligers the opportunity to undergo cementation. The provision of clean settlement material and sufficient water retention have been recognised as optimal for larvae to settle and metamorphose.
The substrate mixes along Lough’s intertidal zones provide some excellent settlement areas, however the findings of the survey identified assemblages in close proximities to each other. Suggesting, that the assumptions as to how the Native Oysters returned is not confined to one factor but to a combination of the proposed hypothesises. The unassisted reoccurrence of Native Oysters in Belfast Lough is undoubtably a unique event with many questions needing addressed before a pathway of re-establishment can be confirmed.
The 2020 documented settlements of Ostrea eduli in Belfast Lough have raised a number of interesting hypotheses. Further research as to how the return of the Oyster was induced is required. In depth investigations into the effects of pollution and dredging impacts were practically and financially beyond the scope of Smyth et al.'s study. The main aim of this manuscript was to document that Oysters are once again present in Belfast Lough after a century of absence. The initiation of future studies should be considered a matter of urgency, as a better understanding of abiotic and biotic parameters within the Lough could greatly benefit numerous European Native Oyster restoration projects which are currently underway.
See also...