Phuket Marine Biological Center Special Publication 21(3): 591-594 (2000) 591

MOLLUSCS: AMICROCOSM OF IIV\MRTEBRA TE ZOOLOGY

AN INTRODUCTION TO THE GREAT PHYLUM

Alan J. Kohn

Uniuersity of Washington, USA Programme (TMMP) 592 Tropical Marine Mollusc

GENERAL OVERVIEW 2, Diversity .No single "hallmark;" array of (Telegraphic style) traits. 3, plan . Head, foot, visceral mass cov- Success ! More than 50,000 extant species, about Body a shell. (But 25.000 extinct. ered by and usually regions are 2nd largest phylum (1/20 species inArthro- demarcations between body poda; 2 x species in Vertebrata). not always clear). Ubiquitous: most marine, to 10km; + fresh- water, land; Tnn ceNnnar,IZED MoLLUSC: Density: to 40,000/m2 External morphology Size: 1 mm to about 16 m length! Fossil record: Lower Cambrian to Recent. Body regions: head, foot, visceral mass, (Lower Cambrian 545 my before present) mantle, and shell. Lower Cambrian classes: Gastropoda, Mantle cavity: Contains gills ("ctenidia") , and Monoplacophora. divide mantle cavity into inhalent and Upper Cambrian classes: Polyplacophora, exhalent chambers. Importance of cilia' Cephalopoda. Pathway of water flow in mantle cavity' Ordovician: Scaphopoda + extinct classes. Shell: CaCO3 + proteins;crystal architec- No fossil record: Aplacophora ture. Intern q,l organ systems Mot t uscs AND MAN of generalized mollusc Food, money, jewellery, pests, vectors of jaws, radula; alimentary parasites, etc. Digestive: Mouth, tract, glands. Nervous: Cerebral, pedal, pleural, visceral Molluscs IN SCIENCE ganglia, or more, or none. Basic nerve function; nerve cell networks Circulatory: Heart (3 chambers) i dorsal; iinked to behaviour. vessels; hemocoel; open system; relation to ("nephridia"). THREEANATOMICAL excretory system Coelom: Reduced. Occurs in pericardium, GENERALISATIONS excretory, and reproductive systems. ABOUT MOLLUSCA Reproduction and Development: Most dioecious. Spiral, unequal, determinate 1, Complexity . Organ-system grade; all cleavage. Gastrula to larva; functional systems are well-developed; bi- expands along secondary axis forming vis- Iateral symmetry, 3 primary ceII layers, ceral mass, mantle-shell, foot. Pre-oral cili- coelom (small: main bodY cavitY is ated band of trochophore, becoming the hemocoel).

Body axes: Axis Organ systems Power source Major functions (Somatic) Prirnary foot muscle Iocomotion, feeding (Anterior-Posterior) head,

respiration, digestion, Secondary (Visceral) visceral mass' cilia (mainlyDorsal-Ventral) mantle'shell reproduction Phuket Marine Biological Center Special publication 21(J): 591_594 (2000) 593

velum of larva (in some classes). Metamorphosis. Class POLYPLACOPHORA "many plates" The chitons MOLLUSCAN COMPLEXITY AND DIVERSITY About 600 species; Morphologically most Characters of 7 Classes in brief uniform class. Adapted to hard substratum, shallow wa- Class APLACOPHORA ter, but to 7,000 m! Head: definite "no plates" but reduced; visceral mass: depressed; foot: large, flat; Mantle: girdle, 8-piece shell, with thin outer 2nd smallest (about 150 species); no shells; cuticle; shape ofshell plates. no fossils. Shallow subtidal to 4000m. Shell: Outer shell layer CaCO3 but more or Wormlike, with mantle of thickened cuticle, Iess soft, pigmented, perforated: many CaCO3 spicules. with sense organs. Inner shell layer extends under Head poorly developed; Foot vestigial or in front yielding flexibility, strength. absent; no visceral mass or 2nd axis. Feeding and digestion: Most chitons herbivo- Mantle cavity small, posterior; 1 pair cte- rous, with typicaily molluscan system. nidia or several respiratory papillae; radula Habits and diversity: Class GASTROPODA Order/SubcIass/Class "stomach foot" CAUDOFOVEATA The snails, nudibranchs, etc. About 25 species, head-down infaunal de- posit feeders in mud, posterior end up. The largest molluscan class: About 8b,000 Recent species, most varied habitats. Order/Subclass/Class "Definition": Molluscs that have undergone SOLENOGASTRES torsion. About 125 species, ectoparasitic on Cnidaria; Primitive members retain broad flat foot; more distinct head than CAUDOFOVEATA. head well developed. Visceral mass: much enlarged along 2nd Class MONOPLACOPHORA axis, coiled. "one plate" Mantle-shell: enlarged, coiled, specifically conispiral; mantle cavity typically anterior. Torsion: 180'twist of visceral mass on head- Lower Cambrian to Devonian (until 1957 foot. when Neopllina was discovered!) Smallest class: about 20 extant species. All Class marine, on hard substrate in 180-4000 m. BIVALVIA Head anterior, ventral, like chitons; Foot "two-valved shells" flat, circular, weak; Visceral mass conical; Clams, mussels, , , etc. Mantle, shell limpet-like; Mantle cavity a lateral groove. About 10,000 species; marine + freshwater; Neopilina:5-8 pairs muscle scars; bilateral Lower Cambrian-present. Mainly Mesozoic- symmetry; limpet-like (cap-shaped shell). present. Nervous system ladder-Iike: 2 pairs longi- Secondary axis important; Head absent; tudinal nerves, 10 pairs connectives, no gan- compressed, usually large, digging, may at- glia. Cf. Polyplacophora. tach by byssal threads, sucker. Visceral mass extensive D-V. (TMMP) 594 Tlopical Marine Mollusc Programme

Mantle-Shell usually encloses entire body; posterior opening. decalcified dorsally, of 2 valves with hinge, No ctenidia. teeth. : C-spring or resilium: resilin (protein similar to rubber). Antagonises ad- ductor muscle(s). Mantle cavity large, down CLASS CEPHALOPODA and forward. "They Bear Their Feet on Their Ctenidia: long (D-V), enlarged (A-P),lamel- Heads" laelfiIaments, main food collecting organs; Squids, Octopus ciliary pumping and ciliary-mucus feeding. Mantle assumes head, gill man- functions; Muscle molluscs: Biggest (16 m), smartest, tle fusion, siphons. U-shaped water flow fastest marine invertebrates; inventors of pattern, chambers inhalent, exhalent in submarine, smoke screen, jet propulsion. mantle cavity. All marine, ca. 600 species, 7,000 fossil spe- types related suspension and de- to cies; Upper Cambrian origin; most members posit feeding. of class Mesozoic, most extinct at end-Cre- taceous mass extinction. Secondary body axis predominates, primary Class SCAPHOPODA axis reduced. "spade-footed" Head highly developed. Tusk shells Foot much modified, into arms + siphon (=funnel). About 300 species; morphologically con- Visceral mass elongate, coiled in 1 subclass. servative; all marine in soft sediment, from Shell usually reduced or absent; Mantle intertidal to muscular (as is 2nd axis!). more than 3,000 m. Size up to 15 cm length. Orientation of body: functional vs. morpho- Active subsurface predators, mainly on logical. foraminiferans. The cephalopod personality Secondary axis emphasised; Head a buccal (In contrast to other molluscs): Adaptations tube with two groups of many long and thin for active lifestyle: rapid muscular move- tentacles ("captacula"). ment, large size complex body plan; impor- Buccal mass with very large radula. tance of head: sense organs: vision impor- Foot very large, piston-like, digging. tant; centralised nervous system; behaviour: Visceral mass elongate; increased dependence on prior experience. Mantle in tubular shell, with mantle cavity Skeletal and circulatory systems. Muscular open at both ends; water circulation in + out hydrostats.