Mollusca* 🐌

ZO 350 - Animal Phylogeny & Diversity Week 6 - Mollusca

Phylum Mollusca Basic Info

~100,000 species, largest marine phylum, some terrestrial or freshwater

loss of diversity, true coleom, bilateral cephalization, no segmentation (as a group), all 3 kinds of support structures

Malacology

the study of molluscs

Biological Community

large biomass, calcium distribution

→ host for flatworm parasites

Human Interactions

food, money (art), housing material, medical research (venoms and antibiotics), damages ships (shipworms), agricultural pests, ecological indicators, invasive species

Head-foot Region

contains feeding, cephalic sensory, and locomotor organs

Visceral Mass

contains digestive, circulatory, respiratory, and reproductive organs

Head

well-developed w/mouth and sensory organs

Radula

unique to molluscs, found in most (except bivalves)

rasping, tongue-like organ w/up to 250,000 teeth

Foot Functions

attachment - disc of limpets

digging - foot of bivalves

gliding - gastropods

siphon - jet/arms of cephalopods

Mantle

dorsal body wall, secretes shell and covers visceral mass

Mantle Cavity

houses gills/lungs (gas exchange), digestive and excretory waste, gametes empty into here

Periostracum

outer protein layer, thick in freshwater mollusks, and protects against acid from leaf decay

Prismatic Layer

main part of the shell w/closely packed calcium carbonate (CaCO3)

Nacre

innermost layer of the shell, secreted continuously throughout life, "mother of pearl

Digestion

radula; anus empties out into mantle cavity

Circulatory

open system, pumping heart, few vessels

Cephalopods - closed system

Respiratory

gill or lung (vascularized chamber)

Excretory

kidneys or nephridia

kidney ducts also discharge sperm and eggs

Nervous/Sensory

cerebral ganglia (brain) near pharynx

2 nerve cord pairs in foot and lower viscera

Reproduction & Development

most have separate sexes; some gastropods are hermaphroditic

some external and some internal fertilization

Trochophore Larvae

veliger = more developed larvae

glochidia = specialized parasitic larvae of many bivalves

Defense

shell, venom/poison, ink, mucus, camo, mutualism, etc.

Diversity

C. Gastropoda — snail

C. Cephalopoda — octopus

C. Bivalvia — clam

C. Polyplacophora — chiton

C. Scaphopoda — tusk shell

Class Gastropoda

most diverse class; 70,000 species

all environments - marine, freshwater, terrestrial

Gastropoda Shells

main defense, one-piece

ancestral shells flat spiral → helix

operculum = covers aperture to prevent water loss

Gastropoda Torsion

mantle cavity and anus rotate

results: viscera shift left to right, nerve cords form figure 8

fouling = anus empties over mouth, sense organs → gill move

Gastropoda Feeding

radula adaptations = feeding variations

herbivore, carnivore, scavenger

Gastropoda Respiration

gill (mantle cavity) or “lung” (vascular area in mantle)

pneumostome = lung opening to outside

Gastropoda Reproduction

dioecious and hermaphroditic species

NO self-fertilization

land snails lay eggs in gelatinous masses

love darts 🙂

Gastropoda Anatomy

Class Cephalopoda

nautiluses, cuttlefish, squids, octopuses

foot modified into tentacles + funnel

ALL predators; strong beak-like jaws grasp prey

Cephalopoda Appendages

nautilus = 60-90 tentacles, no suckers

squid/cuttlefish = 8 arms w/suckers + 2 tentacles

octopuses = 8 arms w/suckers

Cephalopoda Shells

nautilus = gas chambers in shell allow swimming, animal inhabits last chamber, siphuncle extends through chambers and adjusts gases

cuttlefish = “cuttlebone” in mantle

squid = “pen” in mantle

octopus = completely lost

Cephalopoda Circulation/Respiration

increase in oxygen demands increase in size

1) ventricle contracts → oxygenated blood to body

2) deoxygenated blood returns to branchial hearts

3) branchial hearts contract to gills (exchange gases)

4) blood returns to ventricle

Cephalopoda Nervous/Sensory

large diameter nerve (speed w/o myelination)

brain w/cartilaginous cranium

sense organs: well-developed; complex eyes + photosensitive chromatophores

tool use!

Cephalopoda Movement

expel water through siphon; controls direction and force/speed

squid/cuttlefish: lateral fins act as stabilizers

octopus: crawls on sea floor or swims around with webbing

Cephalopoda Reproduction

sexes separate, internal fertilization, eggs are laid

some take care of the eggs until they die ☹

Cephalopoda Defense

color change/camouflage: skin texture changes, can imitate other animals

chromatophores = cells in skin expand and contract

ink: sac empties into rectum, secretes melanin, pseudomorph

octopuses & cuttlefish produce venom in salivary glands

Cephalopoda Anatomy

Class Bivalvia

clams, scallops, oysters, mussels, shipworms

2 shells, sedentary, suspension feeders

reduced head and no radula

Bivalvia Shells

closed by adductor muscles

umbo = oldest part of shell, growth occurs outward in rings

Bivalvia Feeding

1) H2O moves over gills

2) gills secrete mucus and traps particles

3) food in mucus moves via grooves at gills’ edge

4) cilia move mucus to mouth

Class Polyplacophora

chitons

flattened with 8 dorsal plates

prefer rocky intertidal zones, scrape algae from the rocks

Class Scaphopoda

tusk shells

sedentary, marine

tubular shell, open both ends

mantle-fused → tube around viscera; foot/tentacles from larger end burrow

eats detritus caught on tentacles