Molluscs

Peter Shaw

Phylum Mollusca

  One of the most important phyla in terms of species richness, and nearly ubiquitous in most ecosystems They invariably have a eponymously soft, mucus-covered body. Many forms have a hard, calcareous shell protecting most of the body.

The mollusc body

  

Is unsegmented Coelomate, but coelom is reduced to pericardium + gonadial cavity.

The main body cavity is a haemocoel.

Primitively: shelled, with 2 broad body regions: 1: a non-muscular visceral hump staying protected inside the shell, wherein most action is ciliary.

2: a muscular foot, with mouth + sense organs at anterior end which protrudes from the shell

The ancestral mollusc

  A favourite parlour game for invert. Zoologists is to create a hypothetical ancestral mollusc.

In 1957 a monoplacophoran

Neopilina galatheae

was dredged up from 5000m off Mexico, and was found to fit the bill - except that, rather embarrassingly, it showed signs of metamerism.

Ancestral mollusc (hypothetical)

Phyletic affinities

 Are very much disputed, but current best guess is close to some obscure worm-like phyla: sipunculids, echiurids, and more distantly annelids. These are now classed with lophophorate phyla as a subgroup within the lophotrochozoa.

 This scenario assumes a ciliary/gliding animal which in one radiation acquires a shell, the other evolves lophophorate filter feeding

Respiration

A single ctenidium (typically triangular, leaf-like)   

Most molluscs are aquatic and bear paired gills or ctenidia, lying in the mantle cavity. These bear many triangular leaflets, and water is carried over these by ciliary action.

Water is “tasted” before flowing over the gill by a sense organ the osphradium.

The anus and renal pores lie in the exhalant stream.

Respiration contd.

Pneumatostome, leading To mantle-cavity ‘lung’ in banana slug    Neopilina has no ctenidia, but 5 pairs of muscular gills along its body. Primitive or derived?

Terrestrial gastropods often lose ctenidia, and have “lungs” in their place.

(And a few of the air-breathing pulmonates then returned to the sea!!)

 

Mouthparts

Usually consist of a unique molluscan organ: the radula. This is a muscular cord which bears sharp chitinous teeth, which rasps slowly away at the surface of food grating fragments into the mouth.

The radula is born on a supporting structure the odontophore.

More on mouthparts

Radula teeth vary immensely with diet, an extreme being Conus, whose teeth are hollow venom-injecting needles.

In Chitons, the radulae are tipped with magnetite for extra hardness.

In cephalopods the radula is still present but supplimented by a parrot-like beak.

Feeding

   Primitively this involved food particles being scraped off a surface by the radula/odontophore and carried in a mucus rope into the stomach, where it is size-sorted by cilia.

Filter-feeders retain the mucus rope, but collect particles off the ctenidia.

Cephalopods bite lumps of flesh with a chitinous beak prior to radula.

The shell

   This is invariably calcium carbonate (aragonite, not calcite - the crystal structure differs slightly) reinforced with chitin.

The superficial appearance differs hugely between groups, from a tight spiral through loose tubes to nearly flat.

In fact one growth equation with 3 biologically realistic parameters can fit the whole spectrum One weird footnote from a weird system: a snail, unnamed in 2003, inhabiting the deep-sea black smokers, has been found to supplement its normal carbonate shell with scales of iron sulphide.

Classification

  This seems to have changed over the years, due to increased ‘splitting‘ of obscure minor forms in recent years To a good first approximation there are 5 main classes, but you need to know of another three oddball classes.

Principal mollusc classes

Mollusca

Cephalopods Octopi, squids, cuttlefish etc Gastropods Slugs, snails winkles etc Polyplacophora Chitons, coat-of-mail shells Scaphopods elephant tusk shells Lammellibranchia bivalves: mussels, oysters etc

Minor classes

Molluscs Monoplacophora Neopilina Aplacophora neomeniorphs Chaetodermomorpha

Polyplacophora

     Chitons, or coat of mail shells Primitive marine molluscs with 8 small dorsal shells, partly or wholly buried in flesh.

This is not segmentation but an adaptation to curling in tight crevices Feeding is by grazing on algae.

Mantle cavity runs along side, almost up to head, with paired gills along its length

Scaphopods

   “Tusk shells”: 350 spp mantle secretes a tubular shell, open at both ends Burrow in soft marine sediments, collecting food particles with sticky clubbed captactula

    

Bivalvia =

Lammellibranchia = Pelecypoda

C. 20,000 spp Sedentary/ sessile molluscs encased in paired shells Lacking eyes, radula and tentacles, though these may re-evolve around mantle edge Foot retained, often used to burrow into sediment Live by filter feeding, mainly using ctenidia as filtering surfaces

Gastropods

    The most numerous group of molluscs, with c. 76,000 species (75%) Mainly shelled, except for the nudibranchs The shell is always single, usually spiral Locomotion is by muscular waves passing along the foot, except for a few tiny spp that use cilia

Gastropods, 2

    A typical gastropod is a squat mollusc crawling with its muscular foot.

The cephalic sensory organs and radula are well developed but can retract fully into the shell Eg common limpet Patella Many gastropods have a calcareous/chitinous plug to the shell - the operculum

Torsion

   All have undergone a curious modification of body form known as torsion The visceral hump is rotated 180 o so that the mantle cavity, gills and anus all point anteriorly Perhaps to allow the head to retract into the generous space of the mantle cavity + allow osphradium to sense oncoming water

Gastropods: torsion

   But having your anus discharging over your head is poor design, even for a mollusc The solution is to modify the exhalent stream:  evolve a special slit in the shell  lose one ctenidium (R side) giving a unidirectional flow  Loss of R gill => loss of R kidney, osphradium + R side heart  L (inhalent) mantle protrudes as a siphon Many spp have undergone 90 o detorsion!

Gastropod orders

Prosobranchs ancestral gastropods Gastropods Opisthobranchs Mainly marine pulmonates mainly terrestrial

Prosobranchs

    Mainly marine “snails”, with traditional body/shell design Most graze algae, sessile animals, or sediment Many have notable shells, ie cowries One order Stenoglossa have effective predators: Conus spp hunt+kill fish!

Opisthobranchs

    Marine, shell-less: sea slugs, sea hares, pteropods etc.

Some are pelagic, hunting cnidaria one group are pelagic filter feeders using mucus nets Ctenidia are reduced or absent: the body surface is often adequate, or 2ndry gills have evolved

Pulmonates

    Mainly terrestrial radiations Mantle cavity becomes an air breathing lung with a contractile opening the pneumatostome Most have a shell, but stylommatophora have lost it: land slugs A few pulmonates have returned to water!

Class: Cephalopoda

Nautilida Nautilus Ammonites extinct Octopoda Teuthida squids Vampyrorpha 'Vampire squids' Sepiida cuttlefish

Cephalopods

     The most intelligent, fastest moving and highly modified molluscs all are marine predators with good/ excellent vision include the largest invertebrate (giant squids) foot is modified into multiple tentacles with suckers swim by jet propulsion

Cephalopods

  We are lucky to have alive an ancestral form, Nautilus (6 spp). This has a spiral shell used for protection and buoyancy.

The eye is a pinhole camera with no lens but a contractile iris It lives as a deep water scavenger, probably explaining its survival of the K/T boundary event Its fossil ancestors had straight shells (<= 4m long)

Ammonites

    

Were large open-water Nautiloids that went extinct at the end of the Cretaceous Up to 2m diameter shells which differed from Nautilus by heavy sculpturing of sutures We can only guess at their ecology ?pelagic hunters Some big bulky forms ?plankton feeders Evolutionary trends of their shells are complex if not haphazard!

Coleoidea

 Is the sub-class containing the remaining cephalopods  octopoda  teuthida  sepiida  Vampyromorpha  These have a closed blood circulation, non-ciliated gills, a good compound eye and well developed CNS. Are oddly semelparous

Cephalopod sex These animals have some remarkable reproductive behaviour.

Sexes are separate and fertilisation is internal, but there is no penis. Instead one of the male’s arms is differentiated (usually with smaller suckers at the tip), is used to pull the torpedo shaped chitinous sperm packet out of his genital opening, and insert it into the female’s body cavity.

In some deep sea squids the sperm packet may be jammed into non-standard regions - the mantle, or a tentacle base. How this effects sperm transfer is still unclear - these animals are effectively impossible to study alive.

Hectoctylus arm on South African diamond squid

Note the reduced suckers on the hectocotylus arm