molluscs 02

7/28/2019 Molluscs 02

1/83

Mollusca

mollusks

bltdjur

molluscs

7/28/2019 Molluscs 02

2/83

What is a mollusc?

Fundamental organization (hypothetical archimollusc):

- shell secreted by a layer of tissue called the mantle- mouth and anus at opposite end (but in gastropods both anterior)- mantle cavity bears gills (but pulmonate gastropods have no gills)- above mantle cavity is the visceral mass

with gut, nervous, circulatory and muscular system

- shell is of calcium carbonite (calcite or aragonite) (but may be secondaryly lost)- shell typicallyexternal (but in some groups it became internal)- grow by accretion (calcium carbonate is added to the edge of the shell by the mantle)- generally marine (but also few freshwater terrestrial groups)

Mollusca

7/28/2019 Molluscs 02

3/83

Mollusca

systematics main groups

7/28/2019 Molluscs 02

4/83

Mollusca systematics Bivalvia

Bivalvia

7/28/2019 Molluscs 02

5/83

Mollusca systematics Bivalvia

Bivalvia (=Pelecypoda, Lamellibranchia)

pair of valves (right and left valve) bilobed mantle valves articulate along a dorsal hinge line no head typically bilaterally symmetric

(plane of symmetry passing between the valves, = commissural plane) prominent ventral foot known since the Early Cambrian, but diversify not prior to Ordovician but still not a very common faunal element during the Paleozoic

7/28/2019 Molluscs 02

6/83

Mollusca systematics - Bivalvia

Main features of the shell

muscle scars ligament dentition lunule pallial line beak

homomyar internal cardinalia (escutcheon) sinupalliate orthogyrate

heteromyar external lateralia integripalliate prosogyrate !

monomyar amphidetic opisthogyrate

prosodetic

opisthodetic !

7/28/2019 Molluscs 02

7/83

7/28/2019 Molluscs 02

8/83

Gills: Protobranchs (deposit feeders, most primitive)

Filibranchs (suspension feeders)

Eulammelibranchs (suspension feeders)

Septibranchs (carnivores, most derived)

Basic for systematics are the gill type and the hinge dentition

Mollusca systematics Bivalvia gill types

7/28/2019 Molluscs 02

9/83

Mollusca systematics Bivalvia dentition

Dentition: Various types and subtypestaxodont dysodont isodont schizodont desmodont pachydont heterodont

7/28/2019 Molluscs 02

10/83

Types of dentition

Taxodont many small similar teeth & sockets all along hinge plate (e.g., Glycimeris andArca)

Schizodont two or three thick teeth with prominent grooves (e.g., Trigonia)

Dysodont small simple teeth near the edge of the valve (eg Mytilus)

Heterodont few teeth varying in size and shape, distinquished as cardinal teeth, beneath the umbo, and

lateral teeth which lie obliquely along the hinge plate (e.g., most recent bivalves)

Isodont teeth very large and located on either side of a central ligament pit (e.g., Spondylus)

Desmodont teeth very reduced or absent (e.g., Mya) with a large internal process (the chondrophore)

carrying the ligament

Mollusca systematics Bivalvia dentition

7/28/2019 Molluscs 02

11/83

taxodont

Taxodont many

small similar teeth &sockets all along hinge

plate (e.g., Glycimeris

andArca)

7/28/2019 Molluscs 02

12/83

dysodont

no teeth just crenulation

Dysodont

small simple

teeth near the

edge of the

valve (eg

Mytilus)

7/28/2019 Molluscs 02

13/83

heterodont

cardinalia and lateralia

Heterodont few teeth varying in size and shape, distinquished as cardinal teeth,

beneath the umbo, and lateral teeth which lie obliquely along the hinge plate (e.g.,

most recent bivalves)

7/28/2019 Molluscs 02

14/83

isodont

two teeth correspond to two grooves

Isodont teethvery large and

located on either

side of a central

ligament pit (e.g.,

Spondylus)

7/28/2019 Molluscs 02

15/83

schizodont

teeth have crenulations (teeth with teeth)

Schizodont two or three thick teeth with prominent grooves (e.g., Trigonia)

7/28/2019 Molluscs 02

16/83

desmodont

internal process (the chondrophore) carries the ligament

Desmodont teeth very reduced or absent (e.g., Mya) with a large internal

process (the chondrophore) carrying the ligament

7/28/2019 Molluscs 02

17/83

pachydont

Pachydont large, heavy and massive teeth (e.g., rudists)

7/28/2019 Molluscs 02

18/83

Mollusca systematics Bivalvia orientation

Orientation of a bivalve shellwhat is posterior anterior right left ?

ligament typically posterior posterior adductor muscle scar stronger developed pallial sinus posterior / shell gaps posterior posterior part of shell typically better developed

umbo (beak) typically points anterior (prosogyre) byssal notch anterior

Oysters: left valve bigger/cemented

7/28/2019 Molluscs 02

19/83


right

7/28/2019 Molluscs 02

20/83


right

7/28/2019 Molluscs 02

21/83


right

7/28/2019 Molluscs 02

22/83


right

7/28/2019 Molluscs 02

23/83


left

Ecology

7/28/2019 Molluscs 02

24/83

Ecology

marine and fresh water

typically benthic, infaunal or epifaunal include burrowing, browsing, cemented, free lying, swimming, boring forms

filter feeders, deposit feeders, carnivores

Mollusca systematics Bivalvia ecology

Infaunal bivalves

7/28/2019 Molluscs 02

25/83

Infaunal bivalves

Both detrivorous and filtering strategies

Most Palaeozoic groups are infaunal detrivores

Probably the oldest of all bivalve life-modes

Burry thorugh sediment with the muscular foot

Extensions of the mantle (siphons) allow water

transportShell modified to specific substrate requirements

7/28/2019 Molluscs 02

26/83

Sessile Epibenthic bivalves

7/28/2019 Molluscs 02

27/83

Sessile Epibenthic bivalves

Attaches to hard subsrates and becomes immobile

Many groups have evolved this lifemode independently

Allows effective filterfeeding

Mytilus (common blue mussel) and many others attach

by chitinous threads (byssus) secreted by the footOysters attach by cementing one valve (left) to the

substrate and adapt to the shape of the substrate

Motile epibenthic bivalves

7/28/2019 Molluscs 02

28/83

Motile epibenthic bivalves

Lie exposed on the seabed

Mostly filterfeeders

Acute sensory system including photophores (eyes) and

sensory tentacles along the mantle edge

Escape strategy: Rapid closure of the valves createsjetstream and the mussel can thus swim short distances

Some J urassic bivalves may have been permanent

swimmers

Soft sediment recliners and mudstickers

7/28/2019 Molluscs 02

29/83

Some byssally and cementing forms have

evolved secondary soft sediment life-modes

Larvae attaches to small objects and develops

shapes that allows the bivalve to survive on the

sediment surface

Pinnate bivalvesGryphaea (devils toenail)

Reef-forming bivalves

7/28/2019 Molluscs 02

30/83

g

Modern Tridacna clam

Rudists (J urassic-Cretaceous) reef builders

Differential valves

Cone-shaped right valve

Left valve acts as a lid

Probably had symbiotic algae like modernTridacna

Evolved from oysters?

Reef-forming bivalves

7/28/2019 Molluscs 02

31/83

g

7/28/2019 Molluscs 02

32/83

Rock boring bivalves

7/28/2019 Molluscs 02

33/83

g

Several groups of bivalves can produce

livingchambers by boring through rock and wood

Lithophaga

calcareous substrates (corals, limestone

etc.)

Valves without gape

Exclusively chemical excavation

Pholadids

All types of substrates

Wood, corals, granite, lead cables, plastic,

amber etc.

Valves with wide anterior gape

Excavation by movement (abrasion)Shell ornament of teeth and rockfragments

wedged between them act as sandpaper

7/28/2019 Molluscs 02

34/83

Mollusca systematics Cephalopoda

Cephalopoda

Cephalopods

7/28/2019 Molluscs 02

35/83


p p

most highly evolved molluscs (especially eyes and brain) a high level of cephalization (concentration of sensory and neural centers in the head) group includes the modern Nautilus, argonauts, squids, octopuses, cuttlefishes

as well as the fossil ammonites and belemnites 2 main groups: Palcephalopoda (nautilids and endoceratids)

Neocephalopoda (orthoceratids, ammonites, belemnites)

typically bilaterally symmetrical shell, if developed, subdivided in chambers by septae chambers are connected by a tube (siphuncle) hyponome and tentacles are homologue to foot of bivalves and gastropods

mouth with powerful horny beaklike jaws and a radula radula less developed than in gastropods since Late Cambrian

Neocephalopods

7/28/2019 Molluscs 02

36/83

p p

Spirula

Loligo (Squid)

Sepia (Cuttlefish)

Octupus

7/28/2019 Molluscs 02

37/83

Palcephalopoda (Nautilus + fossils)

7/28/2019 Molluscs 02

38/83

Shell terminology

7/28/2019 Molluscs 02

39/83

Mollusca systematics Cephalopoda shell morphology

protoconch

septal neck

peristome

shell wall

growth line

camera /

chamber

septum

aperture

phra

gmoc

one

living

chamb

er

7/28/2019 Molluscs 02

40/83

Mollusca systematics Cephalopoda morphology shell

The suture = junction between septa and shell wall

7/28/2019 Molluscs 02

41/83


saddles: point in apertural directionlobes: point backward

- most important for taxonomy and phylogeny ofAmmonitoidea

- particular types characterize distinct families and orders

prosuture primary suture

Shape of shell

7/28/2019 Molluscs 02

42/83


The cephalopod jaw

7/28/2019 Molluscs 02

43/83

Modern Cephalopods have a horny beak, either two simple plates or more

complex structuresThere is also a radula with rel. simple, undifferentiated teeth

7/28/2019 Molluscs 02

44/83


Classification

7/28/2019 Molluscs 02

45/83

Old: Nautiloidea Ammonoidea Coleoidea

Palcephalopoda (~Nautiloidea) Neocephalopoda (Orthoceratoidea, Ammonoidea, Coleoidea)

Palcephalopodashell well developed and large, originally slightly curvedsiphuncle was situated between the center and the ventral surface.siphuncle generally large with internal deposits (important tax. feature)

Neocephalopodasiphuncle thin and emptyphragmocone originally straight with the siphuncle situated at or near the center

later the position of the siphuncle shifted to the ventral surface (Bactritida),the shell became coiled (Ammonoidea)the shell became internal, reduced or absent (Coleoidea)


Palcephalopoda (= Nautiloidea, + several Paleozoic groups, excl. orthoceratids)

7/28/2019 Molluscs 02

46/83


7/28/2019 Molluscs 02

47/83

Neocephalopoda (= Orthoceratoidea, Ammonoidea, Coleoidea)

Ammonoidea Ammonitida

7/28/2019 Molluscs 02

48/83


Ammonoidea - Ammonitida

Neocephalopoda (= Orthoceratoidea, Ammonoidea, Coleoidea)Ammonoidea heteromorphic ammonites

7/28/2019 Molluscs 02

49/83


Ammonoidea heteromorphic ammonites

SpirocerataceaeMiddle J urassic

Ancyloceratinalatest J urassic to end Cretaceous

ChoristocerataceaeLate J urassic

7/28/2019 Molluscs 02

50/83

Neocephalopoda (= Orthoceratoidea, Ammonoidea, Coleoidea)

7/28/2019 Molluscs 02

51/83

Coleoidea

Coleoids have little skeletal materialConsequently are rare as fossils

Fossisl date back to the CarboniferousProbably derived from orthoconeNeocephaolopds in the Devonian

Neocephalopoda (= Orthoceratoidea, Ammonoidea, Coleoidea)Coleoidea Belemnitida

7/28/2019 Molluscs 02

52/83


Belemnites were squid-like with internal shell (Phragmocone)The posterior of the phragmocone had mineralised deposits

(rostrum or guard)The rostrum is a massive, calcareous structure and hencefossilise extremely well (contrary the phragmocone)Probably worked as counterbalance (compare darts)Belemnites were common in the J urassic and CretaceousNo modern cephalopods produce a rostrum

Evolution

7/28/2019 Molluscs 02

53/83

Mollusca systematics Cephalopoda evolution

Plectronoceras

evolutionary explosionhigh diversity

increase in size

Ecology

entirely marine

7/28/2019 Molluscs 02

54/83

Mollusca systematics Cephalopoda ecology

entirely marine active predators (all are carnivorous)

active swimmersswimming is by rapidly expelling water from the mantle cavity

the water is forced out through the hyponome (jet propulsion)

7/28/2019 Molluscs 02

55/83

Cephalopod eyes

Camera eye fully comparable to ours

7/28/2019 Molluscs 02

56/83

Homo sapiens Octopus

Camera eye fully comparable to ours Famous case of convergent evolution

Forms from skin in the embryo, ours from extension of the brain Nautilus has very primitive, pin-hole camera type eye

Biostratigraphy

especiallyAmmonoidea

7/28/2019 Molluscs 02

57/83

Mollusca systematics Cephalopoda biostratigraphy

especiallyAmmonoidea

and in the Mesozoic

7/28/2019 Molluscs 02

58/83

Mollusca systematics Polyplacophora

Polyplacophora

Polyplacophora (chitons)

primitive molluscs with eight, articulating (overlapping) aragonitic plates

7/28/2019 Molluscs 02

59/83


(except one Palaeozoic lineage had seven)

generally oval in outline with a flattened body creeping foot, a primitive feature in molluscs radula, mineralized with magnetite head is poorly developed the girdle (perinotum), a band of muscular tissue, runs along the dorsal periphery

embedded in the girdle are small calcareous spines, scales or spicules known since the Late Cambrian (isolated plates)

Multiplacophorans

7/28/2019 Molluscs 02

60/83

Polysacos

Stem grouppolyplacophorans?Different numbers ofscleritesBest know is Polysacos

from the Carboniferous17 plates

i l i k d d i th i t tid l

Polyplacophoran Ecology

7/28/2019 Molluscs 02

61/83

marine, commonly occurring on rocks and seaweed in the intertidal zone

few species have also been found at depths down to 5000 meters

photoreceptor cells in the mantle and girdle.

the animal is thus able to detect light, which it responds negatively to

active at night, when they creep over rocks scraping algae

and other microscopic organisms off the surface with their radula


7/28/2019 Molluscs 02

62/83

Mollusca systematics Gastropoda

Gastropoda

7/28/2019 Molluscs 02

63/83

Torsion

twisting of the body [it is entirely different from the spiraling of the shell

7/28/2019 Molluscs 02

64/83

Mollusca systematics Gastropoda torsion

fossil evidence suggests that early, non-twisted molluscs already had coiled shells

some modern gastropods have uncoiled shells, or even no shell at all]

all gastropods undergo torsion during some stage of their development- displacement of many interior organs- digestive tract became U-shaped (anus and nephridia moved anterior)- nervous system acquires a twisted appearance (streptoneury)

Torsion

Advantages:ll d th ill b tt t t fl

7/28/2019 Molluscs 02

65/83

Mollusca systematics Gastropoda torsion

allowed the gills better access to water flow

allowing the animal to withdraw more deeply into the shellthe head was able to retract first (foot last, still able to swim)

Disadvantages:

anus and nephridia anterior the animal would be dumping its waste on its head

7/28/2019 Molluscs 02

66/83

The radula important taxonomic feature in modern gastropods no fossil radula confirmed, although there are descriptions

7/28/2019 Molluscs 02

67/83

Mollusca systematics Gastropoda radula

composed of chitinous material and arranged as a long, coiled band

consists of central, lateral, and marginal teeth

Shell terminology

coiling:

7/28/2019 Molluscs 02

68/83


- dextral- sinistral

Gastropod opercula

7/28/2019 Molluscs 02

69/83

Traditional classification

7/28/2019 Molluscs 02

70/83

Mollusca systematics Gastropoda systematics

Prosobranchia (shelled gastropods in which torsion is complete)

classification based on gill and radula types -- unfortunately!

7/28/2019 Molluscs 02

71/83

Mollusca systematics Gastropoda classification

Archaeogastropoda: holostome aperture = no siphonal canal (since Cambrian)

Mesogastropoda: aperture typically with siphonal canal (since Ordovician)

Neogastropoda: aperture siphonostome, often very long siphonal canal (since Cretaceous)

Incertae Sedis (primitive forms - Archaeogastropoda in part)Order "Tropidodiscida" ("Bellerophontina" in part)

Modern classification

7/28/2019 Molluscs 02

72/83

Order Tropidodiscida ( Bellerophontina in part)

Order Bellerophontida ("Bellerophontina" in part)

Subclass Eogastropoda (primitive forms - Prosobranchia / Archaeogastropoda in part)Order "Platycerida" Order Patellogastropoda (Docoglossa)Order Cocculinida (polyphyletic?)Order Vetigastropoda

Subclass Orthogastropoda (all other gastropods)

Infraclass Neritimorpha (Archaeogastropoda in part)Infraclass ApogastropodaSuperorder Heterobranchia

Order OpisthobranchiaOrder Pulmonata

Superorder Caenogastropoda(Prosobranchia in part)Order ArchitaenoglossaOrder NeotaenioglossaOrder Neogastropoda

Mollusca systematics Gastropoda systematics

Patellogastropoda Neritimorpha

7/28/2019 Molluscs 02

73/83

Cellana radians

Haliotis (Haliotis) midae

Vetigastropoda

Turbo (Dinassovica) imperialis

Amblychilepas scutella

Caenogastropoda

7/28/2019 Molluscs 02

74/83

Oliva (Oliva) sericea textilina

Conus (Asprella) alabaster

Pusionella vulpina Morum (Oniscidia) exquisitum

Murex (Murex) aduncospinosus Malea ringens

Turritella ungulina

Heterobranchia

7/28/2019 Molluscs 02

75/83

Pulmonata

Philine angasi

Ophistobranchia

Glaucilla marginata

Ecology

most are aquatic, marine, brackish and fresh water

several groups lives on land (most are Pulmonates)

7/28/2019 Molluscs 02

76/83

Mollusca systematics Gastropoda ecology

marine forms typically live in shallow waters

highest diversity in tropical waters

but also known from arctic waters and hydrothermal vents in the deep sea

one of the most adaptable forms with respect to:

salinity preassure (water and air) temperature (water and air) humidity

most are herbivores, but also carnivore (Muricidae, Naticidae, Conidae) and omnivore

marine forms typically benthic, but also free swimming and floating forms

freshwater and terrestrial forms at least since Carboniferous

7/28/2019 Molluscs 02

77/83

Palaeozoic gastropods

7/28/2019 Molluscs 02

78/83

Bellerophontids

Planispiral coiling

Selenizone and deep sinus

Selenizone often raised

Extinct

Name derived from ancient Greek

hero Bellerophon in recognition of the

similarity to a greek helmet

Bucanella nana

Sinuites

Bellerophon

7/28/2019 Molluscs 02

79/83


Modern gastropods

Cone shells

7/28/2019 Molluscs 02

80/83

Hunt with poisonous harpoons

Poison sometimes extremely

potent (deadly to humans)

Prey is ingested whole or

scraped with radula

Patellids

7/28/2019 Molluscs 02

81/83

Cap-shaped shell

Sticking to rocks and other hard things

Foot modified to function as a sucker

Why?

ProtectionConserve moisture

Feeding by scraping algae

Secondarily untorted

Obs! Convergent evolution

Predation by Gastropods

Several groups of gastropods feed by drilling holes

in mollusc shells

7/28/2019 Molluscs 02

82/83

Naticid

Muricid

Muricids are epibenthic with often highly ornateshells. Drill holes with straight sides

Naticids are infaunal with very smooth, rounded

shells. Drill countersunk holes by combining acid with

radular activity

Mesozoic marine revolution

7/28/2019 Molluscs 02

83/83

Predator-prey arms race

J urassic to present

Evolution of new predators (e.g. tools)

- Crab and lobster claws

Today: More shells are damaged than in

Palaeozoic

Led to new mollusc adaptions

- glossy shells

- varices on aperture

- narrow aperture

New inventions forced the opponent to

develop new counter methods

Affected all benthic marine animals

molluscs 02

Documents