Molluscs Have Soft Bodies That Are Usually Covered by a Shell

Chapter 9

Key Concepts

•  Molluscs have soft bodies that are usually covered by a shell.

•  Molluscs are important herbivores and carnivores in the marine environment.

•  Polychaete diversity stems from the evolution of a segmented body that allows increased motility.

•  In addition to being important consumer organisms, polychaetes are the primary prey of many marine animals and play an important role in recycling nutrients.

•  Arthropods have external skeletons, jointed appendages, and sophisticated sense organs.

•  Crustaceans make up a majority of the zooplankton that are a major link between phytoplankton and higher-order consumers in oceanic food webs.

•  Nematodes are abundant and important members of the meiofauna.

•  Arrowworms are carnivorous zooplankton.

•  Echinoderms exhibit radial symmetry as adults.

•  Echinoderms have internal skeletons and a unique water vascular system that functions in locomotion, food gathering, and circulation.

•  Acorn worms are benthic suspension feeders and deposit feeders.

•  Invertebrate chordates include tunicates, salps, larvaceans and lancelets.

Molluscs

•  Phylum Mollusca

•  One of the most successful groups

•  Have soft bodies, usually covered by a calcium carbonate shell

•  Wide range of sizes, lifestyles and relationships to humans

Molluscan Body

•  2 major parts:

–  head-foot

–  visceral mass

•  circulatory, digestive, respiratory, excretory, reproductive systems

•  Mantle

–  protective tissue covering soft parts

•  mantle cavity

•  Radula

–  a ribbon of tissue containing teeth

Molluscan Shell

•  Secreted by the mantle

•  Normally comprises 3 layers:

–  periostracum

–  prismatic layer

–  nacreous layer

Chitons

•  8 shell plates

•  Attach tightly to rocks, in intertidal zone

•  Most scrape algae off the rocks

Gastropods

•  Class Gastropoda

•  unshelled or a univalve shell

–  operculum

•  Feeding and nutrition

–  exhibit wide variety of feeding styles

•  Naked gastropods – Nudibranchs

–  cerata

–  stinging cells as defensive weapons

–  bright colors indicate toxicity to predators

•  Reproduction and development

–  separate sexes

–  internal fertilization

–  some shed eggs directly into the sea

–  2 types of free-swimming larva

•  trochophore

•  veliger

–  some are hermaphroditic

–  some without planktonic larval stage

Bivalves

•  shells divided into 2 valves

•  includes:

–  clams

–  oysters

–  mussels

–  scallops

–  shipworms

•  Bivalve anatomy

–  no head or radula

–  laterally compressed bodies

–  shell halves attached dorsally

•  umbo

•  adductor muscles

–  inhalant and exhalant openings

–  Siphons – facilitate filter feeding while buried

•  Reproduction in bivalves

–  majority have separate sexes

–  fertilization usually occurs in the water column

–  trochophore and veliger stages

–  some hermaphroditic

Cephalopods

•  Named after the foot

•  tentacles projects used in prey capture, defense, reproduction and sometimes locomotion

•  most lack shells

•  Types of cephalopods

–  nautiloids

•  large, coiled shells

–  siphuncle

•  head has 60-90 tentacles

•  jet propulsion

•  diurnal migration

•  eat hermit crabs and scavenge on the bottom

•  food is stored in crop

–  coleoids

•  cuttlefish

•  squids

•  octopods

•  coleoids

•  swim by jet propulsion or by fin undulation

•  have the most advanced, complex nervous system among invertebrates

•  Color and shape in cephalopods

–  body movements and color changes are used in communication

–  chromatophores

•  Feeding and nutrition

–  carnivores

–  beak-like jaws

–  diet varies with habitat

•  Reproduction in cephalopods

–  sexes are separate

–  often includes courtship display

–  modified arm used to transfer spermatophore

–  some females incubate eggs

–  usually reproduce once and then die

Sipunculids

•  solitary benthic worms

•  live in burrows

•  peanut worms

•  suspension or deposit feeders

•  separate sexes, external fertilization

Annelids: The Segmented Worms

•  bodies are divided into segments

–  hydrostatic skeleton

–  setae

•  Types of marine annelids

–  polychaetes

–  echiurans

–  pogonophorans

Polychaetes

•  Most common marine annelids

•  Traditionally divided into 2 groups:

–  errant polychaetes

–  sedentary polychaetes

•  Feeding and digestion

–  predators, filter or suspension feeders

–  digestive tract is usually a straight tube

–  deposit feeders

•  selective and nonselective deposit feeders

•  Reproduction in polychaetes

–  asexual reproduction via budding or fragmentation

–  most reproduce only sexually

–  gametes are released into the water column

–  epitoky

–  swarming

Echiurans

•  Mostly deposit feeders

–  typically have a flat proboscis to collect particles

•  separate sexes, shed gametes into the water column, and have a planktonic larval stage

Pogonophorans

•  Live in buried tubes

•  cylindrical body with a ring of tentacles

•  Lack mouth or digestive tract

•  May absorb nutrients dissolved in seawater or obtain nourishment from chemosynthetic bacteria

Nematodes

•  Most numerous animals on earth

•  Critical role as scavengers

•  Some are parasitic

•  Most are hermaphroditic

Ecological Role of Marine Worms

•  Nutrient cycling

•  Predator-prey relationships

•  Symbiotic relationships

Arthropods

•  75% of identified species

•  molting exoskeleton

•  divided into segments

•  jointed appendages

•  developed nervous systems

•  2 major groups of marine arthropods:

–  chelicerates

–  mandibulates

Chelicerates

•  Primitive group

•  6 pairs of appendages

•  Horseshoe crabs

–  3 body regions

•  cephalothorax

•  abdomen

•  telson

–  carapace

•  Sea spiders

–  4+ pairs of legs

–  male carries eggs

–  palps

–  feed on juices from cnidarians and other soft-bodied invertebrates, using a long sucking proboscis

Mandibulates

•  Crustaceans—marine mandibulates

–  3 main body regions:

•  head

•  thorax

•  abdomen

–  appendages:

•  2 pairs of sensory antennae

•  mandibles and maxillae

Decapods

•  5 pairs of walking legs

•  chelipeds

•  wide range in size

•  Specialized behaviors

–  hermit crabs

–  decorator crabs

–  common blue crabs

•  Nutrition and digestion

–  chelipeds

–  appendages are used for scavenging

–  some are deposit or filter feeders

•  Reproduction

–  sexes usually separate

–  males have modified appendages modified

•  spermatophores

•  copulatory pleopods

–  most brood their eggs

–  larval stages:

•  zoea larval stage

•  nauplius larva

Mantis Shrimp

•  specialized predators

•  2nd pair of thoracic appendages

•  reproduction

–  some pair for life

–  larvae hatch from egg mass

–  retain planktonic form for up to 3 months

Krill

•  Pelagic, 3-6 cm long

•  Filter feeders

•  Most are bioluminescent

–  photophore

–  swarms

•  Food source

Amphipods

•  Laterally compressed, with posterior 3 pairs of appendages directed backward

•  Many are burrowers

•  Most are detritus feeders or scavengers

•  Young resemble adults upon hatching

Copepods

•  Largest group of small crustaceans

•  Abundant member of the zooplankton

•  Mostly suspension feeders

•  Males use spermatophores

•  Eggs are shed and hatched into water

Barnacles

•  Only sessile crustaceans

•  Most have calcium carbonate shell

•  Attach to a hard surface

•  Filter feed using cirripeds

•  Reproduction

–  hermaphroditic

–  long, extensible penis

–  nauplius larvae develop into cyprid larvae

Ecological Roles of Arthropods

•  Arthropods import are source of food

–  copepods form link in marine food chain

–  krill are consumed in large quantities

•  Arthropods as symbionts

–  cleaning shrimps

–  ecto and endo parasites for fish

–  barnacles are commensal

•  Role of arthropods in recycling and fouling

Arrowworms

•  Phylum Chaetognatha

–  common and planktonic

–  carnivorous, voracious feeders

–  4 – 14 grasping spines

Echinoderms: Animals with Spiny Skins

•  Sea stars, sea urchins and sea cucumbers

•  Bilateral symmetry in larvae, radial symmetry in adults

Echinoderm Structure

•  Endoskeleton

–  ossicles

–  pedicellariae

•  Water vascular system

–  madreporite

–  tube feet

Sea Stars

•  Central disk with 5+ rays

•  ambulacral grooves

•  Aboral surface

•  Feeding in sea stars

–  most are carnivores or scavengers

–  sea stars envelope and open bivalves, evert a portion of the stomach, and insert it into the bivalves to digest them

•  Reproduction and regeneration

–  sea stars can regenerate rays

–  most have separate sexes, eggs and sperm released into the water

Ophiuroids

•  Benthic with 5 spiny arms

•  Lack pedicellariae

•  Tube feet lack suckers

•  Feeding in ophiuroids

–  brittle stars usually filter feed

–  deposit feeders use their podia

–  basket stars suspension feed

Ophiuroids

•  Reproduction and regeneration in ophiuroids

–  autotomize

–  asexual reproduction by division

–  planktonic larvae metamorphose into adults before settling

Sea Urchins and their Relatives

•  Body enclosed by test

•  Benthic

•  Moveable spines

•  Feeding in echinoids

–  regular echinoids – grazers, Aristotle’s lantern

–  feeding in irregular urchins - deposit feeders

Sea Cucumbers

•  Class Holothuroidea

•  Respiratory trees

•  Sexes are generally separate

•  Eggs may be brooded or incubated

•  Larvae are planktonic

•  Feeding in sea cucumbers

–  deposit or suspension feeders

–  oral tentacles

•  Defensive behavior

–  Cuvierian tubules

–  can eviscerate

Crinoids

•  Feather stars, sessile with grasping cirri

•  Suspension feeders

•  Can regenerate lost arms

•  Separate sexes shed

•  larvae have free-swimming stage

Ecological Roles of Echinoderms

•  Spiny skins deter most predators

•  Predators of molluscs, cnidarians, crustaceans

•  Urchins eat kelp, coral

•  Sea cucumber poison, holothurin, has potential as a medicine

Hemichordates

•  Sessile, burrow in sediments

•  Large proboscis, used to dig burrows

Invertebrate Chordates

•  4 key anatomical characteristics

1) notochord

2) pharyngeal gill slits

3) post anal tail

4) dorsal, hollow nerve tube

Tunicates

•  Subphylum Urochordata

•  Mostly sessile, widely distributed

•  Named for their tunic

•  Types:

–  sea squirts

–  salps

–  larvaceans

Sea Squirts

•  Class Ascidiacea

•  Tends to expel a water when disturbed

•  Filter feeders

•  2 projecting tubes

–  incurrent siphon

–  excurrent siphon

•  Lifestyles: solitary, colonial, compound

•  Filter feeders

•  Budding occurs in colonial ascidians

•  Most are hermaphrodites

•  Short lived, tadpole-like larvae

Salps and Larvaceans

•  Salps

–  free-swimming tunicates with incurrent and excurrent siphons

•  Larvaceans

–  free-swimming; produce delicate enclosures of mucus used in feeding

Cephalochordates

•  Fish-like chordates

•  eel-like in form and behavior

•  Benthic

•  Suspension feed

•  Separate sexes

•  Internal fertilization

•  Complex life cycles