MolluscsPeter 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

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.A single ctenidium

(typically triangular, leaf-like)

Respiration contd.

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!!)

Pneumatostome, leadingTo mantle-cavity ‘lung’ in banana slug

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

C e ph a lo po dsO c top i, squ id s, cu tt le f ish e tc

G a stro po dsS lu gs , sn a i ls w ink le s e tc

P o lyp lacop ho raC h ito ns , co a t-o f-m a i l she lls

S cap ho po dse le ph an t tu sk she lls

L am m ellib ran ch iab iv a lv e s: m u sse ls , o yste rs e tc

Mollusca

Minor classes

M on op lacop ho raN eop i lina

n e om e n io rp hs C h ae to d erm o m o rp ha

A p la cop ho ra

M o llu scs

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 180o 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 90o detorsion!

Gastropod orders

P roso bra nchsa n ce s tra l ga stro po ds

O p isth ob ran chsM ain ly m a rine

p u lm on atesm ain ly te rres tria l

G a stro po ds

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 NautilidaNautilus

Am m onitesextinct

Octopoda

Teuthidasquids

Vam pyrorpha'Vam pire squids'

Sepiidacuttlefish

                                                                            

   

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 eventIts 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.

Note the reduced suckers on the hectocotylus arm

Hectoctylus arm onSouth African diamond squid