Molluscs (phylum Mollusca) represent a diverse group of between 100,000 and 200,000 species!

Here are some representative examples (see below for details)

Marine molluscs occur from the intertidal to the abysal depths and range in size from tiny snails to giant clams that weigh 180 kg and squid that stretch almost 20 m and weigh several tons!

These include: clams, snails, and cephalopods (octopus and squid)

In addition to filling a variety of niches in many different habitats, molluscs are important to humans for food (though sometimes toxic) and jewelry.

Their fossil shells provide important clues to evolutionary patterns

The simplified organs and neuronal systems are useful for biochemical research.

All molluscs have a basic head-foot body plan but, unlike the annelids, they lack body segments

They have paired gills (ctenidia) that are usually protected by a fold of skin called the mantle

The mantle secretes a calcium carbonate shell that provides protection to most molluscs.

Some molluscs primarily eat algae, while others are pure carnivores. Deep sea species are generally detritivores.

Six Classes of mollusc are important members of the marine community

The "living mollusc fossils" (Class Monoplacophora)
Chitons (Class Polyplacophora)
Clams (Class Bivalvia)
Snails (Class Gastropoda) Classic whorled shell	Basic gastropod anatomy Larval gastropod Torsion during development
Octopus and Squid (Class Cephalapoda)	Chambered Nautilis
Tusk shells (Class Scaphopoda)

All classes of mollusc except the bivalve possess a chitinous ribbon in the foregut called the radula... this can be modified for many specialized purposes

Many molluscs are hermaphrodites, although eggs and sperm often mature at different times.within the same animal.

Bivalves, such as the European oyster show strong patterns of lunar reproductive synchrony corresponding to spring tides.

The life cycle of bivalves includes a trochophore larval stage

The jointed invertebrates (phylum Arthropoda)... the planets most successful multicellular organisms

Arthropods are characterized by:

external chitinous skeleton

segmented body

jointed appendages

Marine arthropods belong to one of two sub-phyla: Chelicerata and Crustacea (terrestrial representatives include insects and spiders).

Total living arthropod diversity is estimated at around a billion billion (10¹⁸) species!!...

only about 1 million have been described (mostly insects)

50,000 are crustaceans.

All arthropods have separate sexes... often transfer sperm via specialized appendages.

Chelicerata

Most marine chelicerates are extinct...

The horseshoe crab (Limulus polyphemus) abounds along the Atlantic and Gulf coasts of the United States.

Limulus are large (to .5 m across) and heavily armored (thick shelled), with to main body sections.

First section: six pairs of appendages, the first are movable claws called chelicerae.

Second section: several pairs of appendages, including highly modified "book gills" that also aid in swimming.

Limulus prey on benthic invertebrates in shallow water.

They form large mating aggregations along the east coast.

Smaller males cling to larger females as the move onshore to lay eggs.

Eggs and larvae represent important food resources for migrating shorebirds.

Crustacea

Crabs, Lobsters and Shrimp:

Decapods (they have five pairs of legs)

Two antennae

Compound eyes (usually on stalks)

A dorsal cover of the cephalothorax called a carapace.

Important member of many marine communities

Most are scavengers (e.g. Cancer, Panularus)

Some herbivores (Loxorhynchus and Mithrax)

Some carnivores (Callinectes)

Females generally hold eggs externally beneath the telson (tail)

Males have evolved elaborate behavioral strategies for attracting and guarding females

Eggs and larvae are generally released on outgoing tides

Barnacles are sessile, shallow water (intertidal) crustaceans that filter feed during periods of immersion.

 

Echinoderms (Sea stars, brittle stars, sea cucumbers, and sea urchins)

Echinoderms are deuterostomes. They, along with chordates (vertebrates) differ from other coelomates (Protostomes) by virtue of their zygote development

	Protostomes	Deuterostomes
Cleavage	Spiral and determinate	Radial and indeterminate
Fate of Blastopore	Forms the mouth	Usually forms the anus
Coelom formation	Schizocoelous: Solid masses of mesoderm split to form the coelom	Enterocoelous: Outpockets of the gut cavity form the coelom

Echinoderms are a diverse (~7000 spp) group of sessile or slow moving marine organisms with secondarily-derived radial symmetry on a pentagonal body plan

They have a water vascular system that permits the motion of extensions called tube feet.

Each tube foot is connected to a bulb like structure called an ampulla (looks and acts like a rubber bulb)

Water is exchanged across a dorsal sieve plate called a madreporite.

The outer skin encloses a skeleton of interlocking calcified plates called ossicles.

In urchins and sea stars, Pedicillariae (small specialized ossicles) can pinch to protect against predators and prevent fouling

Sexes are separate and reproduction sexual, however some echinoderms can regenerate entire bodies from a body fragment.

Six classes within the phylum:

Asteroidea (sea stars) are common carnivores of nearshore habitats. They consume prey by everting their stomachs

Ophiuroidea (brittle stars and basket stars) have distinct central disks and long flexible arms. Most can move quickly and are voracious carnivores. Most are nocturnal and many use bioluminescence to deter predators. Basket stars eat zooplankton.

Echinoidea (sea urchins and sand dollars) possess a remarkable five-toothed, jaw-like structure called an Aristotle's lantern that is formed from specialized ossicles and ligaments. They are primarily herbivores and their feeding can destroy kelp beds.

Crinoidea (sea lilies) are relatively primitive, cup shaped echinoderms that trap zooplankton with sticky tube feet.

Holothuroidea (sea cucumbers) are relatively soft-bodied when relaxed and can evert their stomach to deter predators.

Concentricycloidea (sea daisies) are recently discovered small animals that live on water logged wood in the deep sea

 

Urochordates (the sea squirts)

Sea squirts are invertebrate members of the phylum Chordata (to which all vertebrates belong)

All chordates possess an embryonic notochord that develops into a dorsal, hollow nerve cord.

They also have, for at least part of their development, a post-anal tail and pharyngeal slits that allow water that enters the mouth to pass back out of the body without entering the digestive tract. This allows for much greater levels of gas exchange and respiration.

Sessile sea squirt or tunicates are usually simple-looking barrel shaped organisms filter water from an incurrent siphon across pharyngeal slits to a body cavity with mucous sheets that trap food particles. All traces of the notochord, nerve cord and tail are lost in the adult form.

Some tunicates can concentrate toxic metals in their skin to deter predation... others can secrete low concentration sulfuric acid.

Planktonic forms (called salps) occur in the open ocean.

 

A review of some important "moments" in the evolution of multicellular animals

1) The Parazoa/Metazoa Split: No true tissues (sponges) vs. true tissues (all other multicellular animals)

2) The Radiata/Bilaterala Split: Cniderians and Ctenophores vs the rest of the higher inverts

Two distinguishing features

a) Radial vs Bilateral symmetry (allows cephalization)... careful here...radial symmetry is also a secondarily evolved characteristic (e.g. some echinoderms)

b) Dipoblastic (two germ layers) vs. triploblastic (three germ layers)

	Radiata (dipoplastic)	Bilaterala (triploblastic)
Top and bottom	Yes	Yes
Head and rear ends	No	Yes
Left and right	No	Yes

3) The Acoelomate/Coelomate Split: No body cavity (flatworms) vs. a body cavity

Body cavities allow for a much greater potential for motion as muscles can exert force off of the cavity

All animals with a body cavity have some form of blood vascular system: blood that circulates through a network of spaces or vessels.

Animals with a body cavity not entirely lined by mesoderm (roundworms and ribbon worms) are called pseducoelomates.

4) The Protostome/Deuterostome Split: Mollusks, annelids, and arthropods vs echinoderms and chordates (see above for characteristic differences in zygote development