Leucosolenia

Leucosolenia • Gr. Leukos= white & solen= pipe • Pipe-like body • Asconoid sponge • Represents simplest & most primitive type of structural pattern olynthus type as body organization resembles olynthus stage in ontogeny of Scypha Systematic Position Phylum: Porifera Class: Calcarea Order: Homocoela Genus: Leucosolenia Occurrence • Small, delicate, sessile, branching, colonial & marine sponge • Found growing in shallow water, below low tide mark on seashore rocks, boulders & jetties • Does not live in calm sea water but is found abundantly where wave action is intense • Very sensitive to external conditions • Will die if removed from its habitat • 100 species • Cosmopolitan • L. botryoides, L. complicata & L. variabilis External Morphology • Colony yellowish or whitish • Consists of radially symmetrical, vaseshaped, vertical, hollow & tube-like individuals, united at their bases by irregular horizontal tubes • Attached to substratum by adhesive discs • Upright individuals may reach up to 25 mm in height • Surface of each tubelike individual bears minute pores called ostia or incurrent pores • Pores lead into a central spacious cavity (paragastric cavity or spongocoel) opening to outside via osculum Body Wall • • • • • • • Thin Encloses spongocoel Relatively simple 2-cellular layers Outer pinacoderm Inner choanoderm Non-cellular mesenchyme in between 1. Pinacoderm • Body covered externally by epidermis (pinacoderm) • Single layer of thin & flat polygonal cells (pinacocytes) • Ensures protection to internal organization of body • Pinacocytes hexagonal in surface view • Have a central bulging part with a nucleus • Margins show contractility • So sponge can decrease or increase slightly in size • Within body wall are special, large & tubular cells (pore cells or porocytes) • Supposed to be modified pinacocytes • Each porocyte has a central canal-like space communicating with outside as well as spongocoel • Pores are ostia or dermal pores or incurrent pores • Permit flow of water from outside into spongocoel 2. Choanoderm • Spongocoel is lined internally by gastrodermis (choanoderm) • Consists of a single-layer of flagellated collar cells (choanocytes) • Choanocyte is an ovoid cell with its free end bearing a transparent contractile collar • It surrounds a single, long flagellum arising from basal granule or kinetosome • Nucleus lies at the base or apex of its cell body • Protoplasmic processes of cell body are embedded in mesoglea • Choanocytes are used in feeding & for ensuring flow of water within animal’s body by flagellar beating 3. Mesenchyme • In between pinacoderm & choanoderm is present noncellular mesenchyme • Secreted by both body layers • Consists of a gelatinous matrix of protein (mesoglea) • Contains several types of amoebocytes & minute spicules (CaCO3) a. Amoebocytes • Are of many types depending upon the shape of their pseudopodia & function • Large sized primary amoebocytes with blunt pseudopodium & large nucleus (archaeocytes) • Contain much RNA & carry on all functions essential for life of sponge • Self replicating • Also capable of giving rise to all other types of amoebocytes (totipotent) • Scleroblasts are skeleton forming amoebocytes • Calcoblasts as form calcareous spicules • Thesocytes with rounded pseudopodia have reserve food material • Chromocytes carry color pigments, responsible for color of sponge • Gland cells secrete slime • Cells with thin branching pseudopodia (collencytes) forming a sort of connective tissue & probably nerve cells • Myocytes contractile, thin cellsaround osculum (sphincter) b. Spicules • • • • Skeletal elemants Small or large Formed of crystalline CaCO3 Needle-like spicules are monoaxons • Some are tetraxons with 4rays • Some become tetraxons become triaxons by loss of one ray • Spicules remain embedded within mesenchyme • Many of them protrude through pinacoderm • Impart roughness to sponge’s body • All orient in same direction Canal System • Ostia, spongocoel & osculum together form a canal system characteristic of all sponges • Ascon type • Simplest type of canal system • Waters enters through ostia into spongocoel lined with choanocytes & leaves through osculum Physiology 1. Locomotion & Behavior Incapable of locomotion It is supposed to possess local contractile powers that appear to be mostly restricted to region of osculum Reactions to stimuli are very slow & responses are seen several minutes after application of stimulus 2. Water Current Vital life processes of a sponge are dependent upon a continuous uninterrupted flow of water through its porous body They is essential because they are sessile & have no other means of getting food & O2 or getting rid of wastes Water current is caused by constant beating of flagella of millions of choanocytes Water enters spongocoel through ostia & exits through osculum Flow of water is controlled by closing & opening of ostia Flow of water through a sponge can be demonstrated by adding carmine particles to water containing a living sponge These particles are seen to enter body of sponge through ostia & pass out through osculum Water current brings food & O2 & removes excretory as well as reproductive elements 3. Digestion Food chiefly consists of plankton, i.e., microscopic animals & plants & organic particles Choanocytes capture & digest the food Amoebocytes receive the partly digested food particles from choanocytes, complete digestion & distribute the digested food from cell to cell by diffusion Amoebocytes digest food intracellularly within their food vacuoles The reaction of food vacuoles is first acidic & later alkaline Undigested residue is eliminated by the amoebocytes into spongocoel 4. Respiration Respiration, involving exchange of O2 from sea water & CO2 produced within the living cells is accomplished by diffusion 5. Excretion Elimination of metabolic wastes (chiefly NH3) also takes place by diffusion through the general surface of body Reproduction •Reproduces asexually as well as sexually 1. Asexual Reproduction •Occurs by branching & budding a. Branching • New horizontal branches arise from stolon, grow over rocks & give rise to new, erect, vase-shaped cylinders • When an upright branch attains sufficient size its top breaks through as an osculum • Regeneration is also common & a complete individual or colony will grow from almost any broken piece of sponge b. Budding • This is accomplished by evagination of body wall near the base of a vertical tube in the form of bud • Bud grows in size & breaks off an osculum at its free end thus becoming additional individual of the colony • May again bud off new individuals 2. Sexual Reproduction • Monoecious • No special gonads are formed • Sperm & ova are derived from archaeocyte by gametogenesis • Sperms are released into sea water & they make their way into another sponge to a region of mature ovum • Some workers believe that sperms are transported to mature ova by amoebocytes 3. Development • Ovum is fertilized by a sperm within maternal body wall forming a diploid zygote • Zygote undergoes equal & holoblastic cleavage developing into a hollow blastula (coeloblastula) • Its wall consists of a single layer of narrow, elongated, columnar & flagellated cells • At the posterior pole of larva occurs a group of large, rounded, granular & non-flagellated cells • These are believed to be archaeocytes which form all future arcaheocytes & reproductive cells of sponge • These granular cells later wander into cavity of embryo • Some of the adjacent flagellated cells also lose their flagella, become amoeboid & pass into internal cavity • The larva now reaches a stage corresponding to the planula larva of coelenterates • Sterogastrula or parenchymula consisting of an external layer of flagellated cells & inner mass of amoeboid cells • It has no mouth opening • Parenchymula swims freely for some hours • Then it becomes fixed by its anterior pole & develops into a flat plate with an irregular outline • Most of the amoeboid cells migrate to external surface passing between flagellated cells & form the pinacoderm & mesenchyme • Flagellated cells thus, enclosed become the choanocytes • A central cavity or spongocoel appears which increases in size becomes lined by choanocytes & opens to outside by osculum • Certain non-flagellated cells in the wall of sponge or porocytes become perforated to form incurrent pores or ostia • Monoaxon & triradiate spicules are secreted by the scleroblasts or modified amoeboid cells • Within a few days of its attachment the larva is converted into the adult asconoid sponge Thank You Your Feedback on: [email protected]