A new species of Giant Clam.

Giant Clams of the genus Tridacna are large Bivalve Molluscs in the Cockle Family (Cardiidae). They are extremely distinctive, both for their large size and their bright colouration, which is caused by symbiotic algae that live within the flesh of their mantles. Giant Clams are found throughout the Indian and Pacific Oceans, as well as in the Red Sea. There are currently eight described species (though there is some dispute among taxonomists, as species can be hard to tell apart), seven of which are listed on the International Union for the Conservation of Nature's Red List of Endangered Species. The Clams are threatened by habitat loss, as well as overharvesting; they are taken from the wild for their edible flesh, their shells and (increasingly) for sale in the aquarium trade.

A Giant Clam in the Red Sea. Woods Hole Oceanographic Institution.

In a paper published in the journal PLoS One on 20 November 2013, a team of scientists led by Thomas Huelsken and Jude Keyse of the School of Biological Sciences at the University of Queensland publish the results of a genetic study into the phylogeny of the genus Tridacna, which unexpectedly revealed the presence of a previously unknown species of Giant Clam.

The new species was found from the Ningaloo Reef of Western Australia east to the Solomon Islands and north as far as Taiwan, and is quite possibly also to be found further afield, since the species was discovered accidentally while sampling Clams thought to be of a different species, rather than by active collecting. The new species is morphologically indistinguishable from Tridacna maxima (the Small Giant Clam), but was more closely related to Tridacna crocea and Tridacna squamosa ( The Boring and Fluted Giant Clams). 

The species is not formally named in the paper, possibly because the study was based upon genetic analysis of tissue collected from living, wild Clams, whereas the designation of a species would require a type specimen in a museum collection, to which other putative members of the same species could be compared.

See also Predation of Freshwater Mussels in the Early Cretaceous of Spain, Opportunistic Bivalves during the Early Jurassic Toarcian Oceanic Anoxic Event, A new species of Pea Clam from the Miocene of Bosnia and Herzegovina, Iceberg damage in an Antarctic Clam and The evolution of Galeommatoid Bivalves.

Follow Sciency Thoughts on Facebook.

Opportunistic Bivalves during the Early Jurassic Toarcian Oceanic Anoxic Event.

The Toarcian Oceanic Anoxic Event is an extinction event that took place in the Early Jurassic, about 183 million years ago. It took place in four phases, thought to have been related to Milankovitch Cycles. During each phase the temperature of the global ocean is thought to have risen abruptly by as much as 13℃, leading to a depletion in oxygen levels in the oceans, followed by an extinction event. Warmer waters are less able to retain oxygen, and this is thought to have been made worse by an increase in runoff from the continents due to higher rainfall, and a breakdown in ocean currents caused by the warming of deep oceanic waters. Each of these phases is marked by a distinct shift in Carbon, Oxygen and Strontium isotope ratios, an extinction event in the fossil record and the deposition of vast amounts of organic matter which has led to extensive hydrocarbons deposits at these levels in many places around the world. The Early Jurassic was considerably warmer than today, and it is thought that the warmest points on the Milankovitch Cycles (which are driven by cyclic variations in the Earth's orbit) raised global temperatures above a tipping point which led to runaway warming.

In a paper published in the journal Geology on 6 September 2013, Bryony Caswell of the School of Environmental Sciences at the University of Liverpool and Angela Coe of the Department of Environment, Earth and Ecosystems at the Centre for Earth, Planetary, Space and Astronomical Research at the The Open University, examine the behavior of populations of two species of opportunistic Bivalve, Bositra radiata and Pseudomytiloides dubius during the Toarcian Oceanic Anoxic Event in deposits at Whitby near North Yorkshire, England.

During the run up to the initial event the deposits were dominated by the Bivalve Bositra radiata, a Posidoniid Clam related to modern Scallops, quickly came to dominate to fauna, forming monospecific pavements and growing to sizes not achieved prior to the onset of the event. 

Bositra radiata shell pavement from Hawsker Bottoms, North Yorkshire. Scale bar is in milimeters. Caswell et al. (2009).

During the remaining three events the Inoceramid Clam Pseudomytiloides dubius (also related to modern Scallops) quickly came to dominate the faunas, again reaching larger sizes during the event. The two species appear to compete during the onset of the third event, but Pseudomytiloides dubius quickly came to dominate, suggesting that the two species competed for some resource (probably food) and that Pseudomytiloides dubius was able to outcompete Bositra radiata.

Pseudomytiloides dubius from Port Mulgrave in North Yorkshire. Scale bar is in milimeters. Caswell et al. (2009).

Bositra radiata and Pseudomytiloides dubius are both thought to have been opportunistic species. Both were small (even when reaching exceptional sizes), and are thought to have had short generation times and high larval production rates, probably comparable to the modern Mulinia lateralis, a small Surf Clam which can breed at two months old and seldom lives more than two years, which does well in anoxic conditions. Such organisms can thrive in environments even where they are exterminated each year, since they will have time to reproduce and can recolonize from adjacent areas.

The approximate location of the study area. Google Maps.

The Toarcian Oceanic Anoxic Event is of particular interest today as we are living in time when rising global temperatures, combined with local anoxic zones in parts of the oceans caused by excess runoff due to human activities, for example in the Black Sea and the Gulf of Mexico. This makes it important to understand how organisms cope (or don't) with such events, and what the long-term impact on the global biosphere is likely to be, particularly if these events become larger and more frequent.

See also A new species of Pea Clam from the Miocene of Bosnia and Herzegovina, Iceberg damage in an Antarctic Clam, The evolution of Galeommatoid Bivalves, Symbiosis and the success of Galeommatoid Bivalves and A new species of Scallop from Western Australia.

Follow Sciency Thoughts on Facebook.

The evolution of Galeommatoid Bivalves.

Galeommatoid Bivalves are a large group of Molluscs that inhabit a broad range of environments and often form commensal relationships with a broad range of other invertebrates. Their classification has been somewhat uncertain, with the group divided into either two (Galeommatidae and Lasaeidae) or four (Galeommatidae, Lasaeidae, Kelliidae and Montacutidae) families.

In a paper published in the journal BMC Evolutionary Biology on 6 September 2012, a group of scientists led by Ryutaro Goto of the Graduate School of Human and Environmental Studies at Kyoto University and the Department of Marine Ecosystem Dynamics at the Atmosphere and Ocean Research Institute at The University of Tokyo, publish the results of a genetic study into relationships within the Galeommatoid Bivalves.

Goto et al. found that the Galeommatoid Bivalves are split into six clades (distinct evolutionary lineages), but that these bore no relationship to either previous classification of the group. 

Galeommatoid Bivalves are known to colonise a number of very different invertebrate hosts, notably Crustaceans, Sea Cucumbers, Spoon Worms, Sipunculan Worms, Brachiopods, Bryozoans, Annelids, and other Bivalves. Typically when epibiotic animals (animals that live on the surface of other organisms) switch hosts they do so between closely related species, with jumps between distantly related species being rare. However each clade of Galeommatoid Bivalves contained species which lived on very different hosts, with host species apparently being no guide to relationships within the Galeommatoid Bivalves. When parasites colonise new hosts they have to learn to get past the defences of the new organism, however Galeommatoid Bivalves are not true parasites; they live on the bodies of other invertebrates but gain nutrition by filter feeding from the water column, and few (if any) animals seem to have defences against this sort of colonisation, apparently making it easy for Galeommatoid Bivalves to switch between unrelated hosts.

The Galeommatoid Bivalve Arthritica japonica that attaches directly onto the body surface of intertidal Crabs. Goto et al. (2012).

Neaeromya rugifera that attaches onto the abdomen of Upogebid Shrimps. Goto et al. (2012).

Devonia semperi (top) and Anisodevonia ohshimai (bottom), which attach to the body surfaces of the burrowing Sea Cucumbers. Goto et al. (2012).

Byssobornia yamakawai on an Echiuran (Spoon) Worm. Goto et al. (2012).

Litigiella pacifica on the body of the Sipunculan worm, Siphonosoma cumanense. Goto et al. (2012).

Finally Goto et al. identified four ways in which Galeommatoid Bivalves colonised their hosts. Two of these, living inside shells used by Hermit Crabs and living inside the esophaguses of Sea Cucumbers, were utilised by single species, suggesting that these were unique evolutionary innovations, with little taxonomic significance. The remaining two methods, colonising the surface of the bodies of the host animals, and colonising the burrows of the hosts, were found in a variety of unrelated forms, suggesting that Galeommatoid Bivalves are also able to switch easily between these lifestyles.

The Galeommatoid Bivalve Ephippodonta gigas that lives in the burrows of Thalassinidean Shrimps. Goto et al. (2012).

Curvemysella paula, lives inside shells carried by Hermit Crabs. Goto et al. (2012).

This suggests that Galeommatoid Bivalves are extremely elastic in their ability to colonise new hosts and therefore new environments, which helps to explain the success of the group, even if it does make its taxonomy difficult to understand.

See also Symbiosis and the success of Galeommatoid Bivalves, The biology of pumice rafts, Deep-sea Gastropods from Miocene Cold Seeps and Whale-falls in Japan, Thirteen new species of interstitial Gastropods from New Zealand, and A new species of Scallop from Western Australia.

Follow Sciency Thoughts on Facebook.

Symbiosis and the success of Galeommatoid Bivalves.

Galeommatoid Bivalves are small (under 20 mm, mostly under 10 mm) nondescript Clams found in both soft and hard bottomed marine environments (which is unusual, as most Bivalves specialize in one or the other). They are thought to be one of the most successful groups of Bivalves, with over 500 species named, which is thought to be a fraction of the total number. Despite this the group have a very limited fossil record, suggesting their success and diversity is a recent accomplishment. Many species of  Galeommatoids are known to be commensals, living on the bodies of a very wide range of invertebrate hosts, but not gaining any nutrition from them.

In a paper published in the journal PLoS One on 8 August 2012, Jingchun Li and Diarmaid Ó Foighil of the Museum of Zoology and Department of Ecology and Evolutionary Biology at The University of Michigan, Peter Middelfart of the Australian Museum in Sydney cary out a review of the success of the Galeommatoids, in which they conclude that the habit of forming commensal relationships is the key to the groups success.

The non-commensal Galeommatoid Bivalve, Scintilla strangei, in a rock crevice. Li et al. (2012).

Bivalves have two basic defenses against predation, namely armor, in the form of a shell, and hiding, generally by burying themselves. Being very small Galeommatoids cannot grow particularly heavy shells to fight off predation, as many larger Bivalves do. Many non-commensal Galeommatoids live in crevices in rocks, suggesting this may be their ancestral habitat, but only two species of are known to live in soft sediments, both within the top few millimeters of sediment. One of these, Mysella charcoti, has been shown to be able to survive passage through the intestine of carnivorous Fish, suggesting that this species has been able to use armor as a primary defense. However Mysella charcoti is restricted to Antarctic waters, where it encounters a very limited range of predators, and it is unlikely that this strategy would be of much use in a more diverse community where the Clams might encounter other feeding techniques, such as the drilling of some Gastropods and Octopus.

Bivalves that live in soft sediments have to be able to keep in contact with the surface in order to be able to be feed and breath. This is usually done by means of a siphon that reaches from the buried shell to the surface, and which can be withdrawn when danger threatens. Thus for most Bivalves the length of the siphon dictates how deep into the sediment the animal can hide. The record for the deepest known burrowing Bivalve currently goes to the Geoduck (Panopea generosa), a very large burrowing Clam, from the west coast of North America, which can reach siphon lengths of up to a meter. However the tiny (under 5 mm) Galeommatoid Clam Mysella vitrea can live deeper than this; Mysella vitrea lives on the bodies of Ghost Shrimps (Trypaea australiensis) which dig deep burrows then pump water through them with their tales, a resource that the Clam is also able to share.

(Top) The Geoduck(Panopea generosa), the largest known burrowing Bivalve, can grow up to one meter in length and burrow as deep as its siphon is long. Biggest Menu. (Bottom) The commensal Galeommatoid Clam Neaeromya rugifera, lives on the body of a Mud Shrimp, Upogebia pugettensis, and can live as deep as the Shrimp does. Li et al. (2012).

Free living Galeommatoids are most abundant on hard substrates, such as rocks or corals, where they gain protection by growing wedged inside cracks and crevices, but a number of species also live commensally on rock-or-coral boring organisms, such as boring Bivalves, Sipunculan Worms and Slow Shrimps. While these forms can often also live free of their hosts, the commensal relationship does open up a large number of additional colonization sites, giving these species an advantage over similar clams that do not do this. It is likely, therefore, that these clams developed the habit of living commensally on burrowing organisms in hard surfaced environment, but that once they had acquired the ability it enabled them to spread rapidly into soft bottomed environments and to undergo an evolutionary radiation which has led to the success of the modern group. 

The commensal Galeommatoid Clam Scintillona bellerophon attached to its host, the burrowing Sea Cucumber Leptosynapta clarki.

See also The biology of pumice rafts, Deep-sea Gastropods from Miocene Cold Seeps and Whale-falls in Japan, Thirteen new species of interstitial Gastropods from New Zealand, A new species of Scallop from Western Australia and The Snail that eats Crabs.

Follow Sciency Thoughts on Facebook.

A new species of Scallop from Western Australia.

Scallops of the genus Pecten are distributed more-or-less globally in temperate waters, and are very well studied as they are important commercial species. Tropical members of the genus are less common, and less well understood, with species having low abundances and deeper habitats, making them less attractive commercially and harder to study.

In a paper published in the journal Molluscan Research on 30 March 2012, Peter Duncan of the Faculty of Science, Health and Education at the University of the Sunshine Coast and the School of Ocean Sciences at Bangor University, and independent researcher Gary Wilson, describe a new species of Scallop from the north coast of Western Australia.

The new species is named as Pecten dijkstrai, in honor of Henk Dijkstra, an expert on Australian Scallops. It is named from specimens collected from two sites off the north coast of Western Australia, roughly 200 km apart. The first of these sites being off the coast of Gnaraloo, the other in the Exmouth Gulf. Both sites were situated at depths of about 40 m.

Map showing the known and potential ranges of P. dijkstrai. Duncan & Wilson (2012).

P. djikstrai is a medium sized Scallop with a shell roughly 43 × 48 mm; it is deeply convex, and shells can reach 55 mm thick (unusual in a Scallop). It has 14-16 prominent radial ribs, and is an off-white colour, with extensive pink-to-dark-pink markings and (sometimes) black or white chevrons.

Pecten djikstrai. (A) Exterior left valve. (B) Interior left valve. (C) Interior right valve. (D) exterior right valve. Duncan & Wilson (2012).

Duncan and Wilson suspect that P. djikstrai may be present along much of the north coast of Australia, and may also be present in Indonesia, where Scallops are known to exist, that have previously been attributed to an East Asian species, P. excavatus, which is not found close to, or in a similar climate to, the Indonesian locations.

See also News species of Velvetfish from the Kimberly Coast of Western Australia, The Snail that eats Crabs and Drilling for oil on the Ningaloo Reef.

Follow Sciency Thoughts on Facebook.