Phylum: Mollusca
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PHYLUM: MOLLUSCA Authors Dai Herbert1, Georgina Jones2 and Lara Atkinson3 Citation Herbert DG, Jones GJ and Atkinson LJ. 2018. Phylum Mollusca In: Atkinson LJ and Sink KJ (eds) Field Guide to the Ofshore Marine Invertebrates of South Africa, Malachite Marketing and Media, Pretoria, pp. 249-320. 1 University of KwaZulu-Natal, School of Life Sciences, Pietermaritzburg, South Africa 2 Southern Underwater Research Group, Kommetjie, Cape Town 3 South African Environmental Observation Network, Egagasini Node, Cape Town 249 Phylum: MOLLUSCA (excluding Cephalopoda) Sea snails, sea slugs, bivalves, tusk shells and chitons Molluscs are one of the most diverse invertebrate Subclass Caenogastropoda groups with more than 100 000 described species A very diverse group including the periwinkles, and approximately 3 154 marine species recorded in cowries, wentletraps, moon snails, murex shells, South Africa. Organisms belonging to this phylum whelks, volutes and cone shells. are highly diverse but can be identiied by several commonly shared traits, including a mantle, the Subclass Heterobranchia presence of a radula, the configuration of the These are more advanced gastropods including sea nervous system and usually the presence of a shell slugs as well as freshwater and terrestrial snails and that encases the mollusc’s soft body for protection. slugs. The mantle plays an important role in respiration and excretion, while also creating the shell by secreting Class Bivalvia calcium and conchiolin. The radula or rasping tongue acts as the primary feeding organ, and is used Subclass Protobranchia by both herbivorous and carnivorous species for This group includes nut clams with taxodont hinge ingesting food. Along with the main characteristics dentition, as well as the awning clams with their of molluscs, the presence of a foot should also be over-grown periostracum. Most are deposit feeders, noted. This is adapted for numerous locomotive but the awning clams feed via sulphide-oxidising purposes such as burrowing into sediment, gliding bacteria in their gills. or swimming (nudibranchs), attachment to hard surfaces (limpets) and directing jet propulsion Subclass Pteriomorphia (cephalopods). Reproduction varies among classes This group includes ark shells, almond arks, dog and fertilisation may be external or internal. In cockles, wing oysters, mussels, pen shells, file marine species the sexes are usually separate, but shells oysters, thorny oysters and scallops. Most some, such as the nudibranchs, are hermaphrodite, of these organisms are sedentary and attach to with both male and female sex organs. All molluscs the substratum by means of byssus threads or are produce eggs and these can hatch as free-swimming cemented in place. Others like the larger scallops planktonic larvae or there may be no pelagic phase and some ile shells can actively swim. Interior and the young hatch as miniature crawling adults. frequently nacreous. Nearly all are suspension- Molluscs act as an important source of food for many feeders. marine ish and mammals as well as for humans, and play a critical economic role in many countries. They Subclass Heterodonta also act as bio-indicators that can be used to monitor Includes the lucinas, jewel boxes, cockles, mactras, the health of the aquatic environment. wedge shells, tellins, venus clams and piddocks. Heterodont bivalves have a complex hinge made Molluscs can be divided into ive principal classes, up of low numbers of diferent types of teeth and namely Gastropoda, Bivalvia, Scaphopoda, the shell lacks nacre. These organisms often burrow Polyplacophora and Cephalopoda. Species into the sediment and are suspension-feeders, but representing each of these classes are included in the lucinids feed via sulphide-oxidising bacteria in this guide. Cephalopoda are addressed in a separate their gills. section due to the large number of species and their importance as a ishery. Subclass Anomalodesmata This group includes some of the most specialised Class Gastropoda of all bivalves, some of which are carnivores. Many are associated with soft sediments in deep water. Subclass Vetigastropoda Examples include the Pandora clams, cuspidariids This group includes the abalones, key-hole and slit and watering pot shells. limpets, top-shells and turban shells. In many of these, the shell interior is nacreous (made of mother- of-pearl). 250 Phylum: Mollusca Class Scaphopoda Collection and preservation The appropriately named tusk shells are a distinctive For morphological study most shelled gastropods, group of molluscs found in association with soft and bivalves, tusk shells and chitons are best frozen as unconsolidated substrata into which they burrow. quickly as possible. After thorough freezing they They are selective predators of micro-invertebrates can be allowed to thaw and quickly thereafter they living within the sediment. should be preserved in 80% ethanol. If the animals are large, the ethanol will need to be replaced after Class Polyplacophora 24 to 48 hours. For DNA studies the entire living animal (with shell cracked) should be preserved in Better known as chitons or coat-of-mail shells, these 96+% ethanol. If the animal is large, smaller pieces of molluscs are easily identiied on account of the eight the foot can be excised and placed in 96+% ethanol articulating dorsal plates and the surrounding girdle. and the remainder treated as for morphology above. They range from the intertidal to great depths Care must be taken to label the excised tissue and are nearly always attached to rocks or hard samples so that they do not become dissociated surfaces. Most are grazing herbivores, but some, from the rest of the animal. Ideally chitons should be with anteriorly enlarged girdles, are predators of pressed lat when placed in preservative to prevent small invertebrates. them from curling up. Class Cephalopoda Shell-less sea slugs (nudibranchs) can be preserved in 70% ethanol, 4% formalin, or bufered and isotonic See separate section. 3.7% glutaraldehyde solution, and in 96% ethanol for molecular studies. Sea slug specimens can be relaxed in isotonic MgCl₂ solution (7%) (or menthol crystals) until unresponsive to touch. References Barnes RSK, Calow P, Olive PJW, Golding DW and Spicer JI. 2001. The Invertebrates, A Synthesis (3 ed.). UK: Blackwell Science. Beesley PL, Ross GJB and Wells A. (eds) 1998. Mollusca: The Southern Synthesis. Fauna of Australia. Vol. 5. CSIRO Publishing: Melbourne, Part A. xvi 563 pp. Part B viii 565–1234 pp. Jones G. 2008. A ield guide to the marine animals of the Cape peninsula. Southern Underwater Research Group Press, Hout Bay. Kilburn RN and Rippey E. 1982. Sea shells of southern Africa. Johannesburg. Macmillan. p. 59, pl. 13. Ruppert EE, Fox RS and Barnes RD. 2004. Invertebrate Zoology (7 ed.). Brooks/Cole. Sturm, CF. Pearce, TA & Valdés, A. 2006. The Molluscs: A Guide to their Study, Collection and Preservation. American Malacological Society, Pittsburgh, PA, USA. pp. xii + 445. Trueman ER and Clarke MR. 1988. The Mollusca (Vol. 11) Form and function. Academic Press Inc., California. 251 Phylum: Mollusca Gastropod shell terminology Adapted by LS Davis from Kilburn RN and Rippey E. 1982. Sea shells of southern Africa. Johannesburg. Macmillan. p. 73 Bivalve shell terminology Adapted by LS Davis from Kilburn RN and Rippey E. 1982. Sea shells of southern Africa. Johannesburg. Macmillan. p. 73 252 Phylum: Mollusca Calliotropis granolirata (Topshl) Phylum: Mollusca Class: Gastropoda Subclass: Vetigastropoda Order: Seguenziida Family: Eucyclidae Genus: Calliotropis Species: granolirata Common name: Cape cog shell 10 mm 10 mm Distinguishing features Similar species Shell small, with conical spire and rounded base; None on Agulhas Bank. sculptured by strong spiral cords bearing well- developed granules; spire whorls with three cords References above and including periphery; base with four cords; Barnard KH. 1963. Contributions to the knowledge of umbilicus closed; aperture nacreous (mother-of- South African marine Mollusca. Part IV. Gastropoda: pearl) when fresh. Prosobranchiata: Rhipidoglossa, Docoglossa. Tectibranchiata. Polyplacophora. Solenogastres. Colour Scaphopoda. Annals of the South African Museum Uniformly milky-white to pale buf, lustreless. 47(2): 201–360. p. 260. Herbert DG. 2015. An annotated catalogue and Size bibliography of the taxonomy, synonymy and Length (height) up to 13 mm. distribution of the Recent Vetigastropoda of South Africa (Mollusca). Zootaxa 4049(1): 1–98. p. 29. doi. Distribution org/10.11646/zootaxa.4049.1.1. South African endemic. To date known reliably only from deep water of the Cape Agulhas–Cape Point region, to depths of 2 750 m. More accurate locality data is urgently needed. 253 Phylum: Mollusca Calliostoma perfragile (CaScot) Phylum: Mollusca Class: Gastropoda Subclass: Vetigastropoda Order: Trochida Family: Calliostomatidae Genus: Calliostoma Species: perfragile Common name: Agulhas calliostoma 22 mm 16 mm Distinguishing features Distribution Shell top-shaped, with conical spire and somewhat South African endemic. Agulhas Bank (Cape canyon lattened base; spire whorls slightly convex, suture to southern Transkei), perhaps also KwaZulu-Natal, shallowly indented; periphery roundly angular, but 100-350 m. not keeled; sculptured by spiral cords of which the irst two to three below suture are inely granular, the Similar species others smooth; cord intervals often with a ine spiral Calliostoma ornatum, a shallower water species from thread; base smoother with several