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Karleskint
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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.
Key Concepts
• Crustaceans make up a majority of the zooplanton 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
Key Concepts
• Acorn worms are benthic suspension
feeders and deposit feeders.
Molluscs
• Phylum Mollusca
• One of the largest and most successful
groups of animals
• Have soft bodies, usually covered by a
calcium carbonate shell
• Wide range of sizes, lifestyles and
Molluscan Body
• 2 major parts:
– head-foot: region containing the head with its
mouth and sensory organs and the foot, which is the animal’s organ of locomotion
– visceral mass: dorsal body region containing the other organ systems, including:
• circulatory (heart and vessels)
• digestive (stomach, digestive glands, intestine and anus)
• respiratory (gill)
• excretory (nephridium)
Molluscan Body
• Mantle
– protective tissue covering soft parts, extends
from the visceral mass and hangs down on each side of the body, secretes the shell
• mantle cavity: space between the mantle and the body
• Radula
Molluscan Shell
• Secreted by the mantle
• Normally comprises 3 layers:
– periostracum: outermost layer, composed of the protein conchiolin that protects the shell from
dissolution and boring animals
– prismatic layer: middle layer, composed of
calcium carbonate and protein, which makes up the bulk of the shell
Molluscan Shell
• Periostracum and prismatic layers form at
the mantle’s margin as the animal grows,
protects shell from dissolution and boring
organisms
• Nacreous layer is secreted continuously
– nacreous layer of oysters is known as mother of pearl, can become layered over irritating
Chitons
• Class Polyplacophora
• Have flattened bodies often covered by 8
shell plates
• Attach tightly to rocks, usually in intertidal
zone
Scaphopods
• Class Scaphopoda
• Tusk shells (class Scaphopoda)
• Tusk-like shell is open at both ends, with foot,
used for burrowing, protruding from larger end
• Water enters and exits at smaller end
Gastropods
• Class Gastropoda
• May have no shell (nudibranchs), or a
univalve (one-piece) shell
– as the animal grows, whorls of the shell increase in size around a central axis
Gastropods
• Feeding and nutrition
– exhibit wide variety of feeding styles
– herbivores – using their radula, most feed on fine algae; some on large algae like kelps
– carnivores – usually locate prey using its
chemical trail; have evolved various behaviors for capturing/subduing prey
Gastropods
• Naked gastropods
– Nudibranchs (naked gill): marine gastropods that lack a shell
– have cerata: projections from the body that increase the surface area available for gas exchange
– some feed on cnidarians and then use their
stinging cells as defensive weapons in the tips of cerata
Gastropods
• Reproduction and development
– most have separate sexes
– most have internal fertilization
– primitive forms shed their eggs directly into the sea
– 2 types of free-swimming larva
• trochophore (primitive molluscs) • veliger (more characteristic)
– some are hermaphroditic
• e.g. slipper limpets of genus Crepidula
Bivalves
• Class Bivalvia
Bivalves
• Bivalve anatomy
– no head or radula
– laterally compressed bodies
– shell halves attached dorsally at a hinge by ligaments
• umbo: oldest part of the shell, around hinge
• adductor muscles: large muscles which close the 2 valves
– mantle often forms inhalant and exhalant openings to facilitate filter feeding
Bivalves
• Bivalve adaptations to different habitats
– soft-bottom burrowers (infauna)
• siphons: structures formed when the mantle is fused around inhalant and exhalant openings, which project above the surface of sediments • siphons facilitate filter feeding while remaining
Bivalves
– attached surface dwellers
• may lie on one side and cement 1 valve to a hard surface (fusion) • byssus—a tough protein secreted by a foot gland, commonly in the
form of threads (byssal threads), used to attach to the surface – unattached surface dwellers
• live free on bottom or other surfaces
• movement by jet propulsion, used primarily to escape from predators
– boring bivalves
• make burrows in wood or stone • microscopic teeth on the valves
Bivalves
• Reproduction in bivalves
– majority have separate sexes
– fertilization usually occurs in the water column – larvae go through trochophore and then
veliger stage
Cephalopods
• Class Cephalopoda
• Named after the foot, which is modified into a
head-like structure
• Ring of tentacles projects from the anterior
edge of the head, for use in prey capture,
defense, reproduction and sometimes
locomotion
Cephalopods
• Types of cephalopods
– nautiloids
• produce large, coiled shells composed of chambers separated by septa (partitions)
– gas-filled chambers aid with buoyancy
– siphuncle: cord of tissue connecting the nautiloid to uninhabited chambers (it inhabits the last chamber)
which removes seawater from each chamber as it forms
• head has 60-90 tentacles coated with a sticky
Cephalopods
– nautiloids (continued)
• move using jet propulsion
• usually dwell on the bottom during the day and migrate to the surface at night
• eat hermit crabs and scavenge for other food on the bottom
Cephalopods
– coleoids (e.g. cuttlefish, squids, octopods)
• cuttlefish have a bulky body, fins, 10 appendages (8 arms + 2 tentacles), and small internal shells
• squids have:
– large cylindrical bodies with a pair of fins derived from mantle tissue
– 10 appendages (8 arms + 2 tentacles) arranged in 5 pairs around the head and embellished with cup-shaped suckers which are attached by a short stalk and surrounded by
toothed structures
Cephalopods
– coleoids (continued)
• octopods have 8 arms (no tentacles) with suckers without stalks or teeth, and sac-like bodies without fins
• coleoids cloud water with a dark fluid called sepia containing melanin (a brown-black pigment) when disturbed
• swim by jet propulsion by forcing water through a ventrally-located siphon or by fin undulation (in squids)
• octopods – better adapted to crawling over bottom
Cephalopods
• Color and shape in cephalopods
– arm/body movements and color changes are used in communication
– chromatophores: special skin cells containing pigment granules which are concentrated or dispersed to change color
Cephalopods
• Feeding and nutrition
– carnivores – prey is located with highly
developed eyes and captured by tentacles or arms
– a pair of powerful, beak-like jaws in the oral cavity is used to bite or tear tissues; octopods use radula to drill holes in shells
– diet depends on and varies with habitat
• squids are pelagic: fish, crustaceans, squid • cuttlefish find invertebrates on the bottom
Cephalopods
• Reproduction in cephalopods
– sexes are separate
– mating frequently involves some kind of courtship display
– male squid have a modified arm used to
Cephalopods
– eggs are fertilized as they are released into the water, some species lay eggs in shells
secreted by modified tentacles, other species attach eggs to stones or other objects
– females of some octopod species incubate eggs until they hatch
Ecological Roles of Molluscs
• Food for humans and other animals
– snail shells are a calcium source for some marine birds – sperm whales consume masses of squid
• Some snails are intermediate hosts to parasites • Shipworms cause extensive damage to wooden
pilings and boat hulls, but also prevent wood from accumulating in the marine environment
Sipunculids
• Phylum Sipuncula
• Solitary benthic worms that live in burrows in mud or sand, empty mollusc shells, or coral
• Some known as peanut worms – contract into a peanut shape when disturbed
• Either suspension or deposit feeders; have a proboscis and ring of tentacles
• Separate sexes, external fertilization; may either develop directly into worms or have a larval
Annelids: The Segmented Worms
• Annelids—worms whose bodies are divided internally and externally into segments
– segments increase mobility by enhancing leverage – hydrostatic skeleton – compartment contained fluid
providing support to worm body
– setae—small bristles used for locomotion, digging, anchorage and protection
• Types of marine annelids
– polychaetes – echiurans
Polychaetes
• Polychaetes (class Polychaeta) are the most
common marine annelids
• Traditionally divided into 2 groups:
– errant polychaetes (move actively)
• may be strictly pelagic, crawl beneath rocks and
shells, be active burrowers in sand or mud, or live in tubes
– sedentary polychaetes (sessile)
Polychaetes
• Feeding and digestion
– some errant species have mouth equipped with jaws and teeth and are active predators; tube dwellers may partially or completely leave the tube to feed
– many sedentary species are filter or suspension feeders
– digestive tract is usually a straight tube from the mouth to the posterior anus
Polychaetes
– deposit feeders: animals that feed on bottom organic material mixed with mineral deposits
• nonselective deposit feeders ingest both organic and mineral particles, digest the organic particles, and excrete the minerals
Polychaetes
• Reproduction in polychaetes
– asexual reproduction via budding or
fragmentation occurs in some polychaetes – most reproduce only sexually, with the
majority having separate sexes
– gametes are released into the water column – epitoky: the formation of a pelagic
Polychaetes
– epitoky in polychaetes
• swarming: males and females come to the surface in large numbers at night to shed sperm and eggs • swarming of epitokes occurs at specific times of
Echiurans
• Class Echiura (Spoonworms)
• Sausage-shaped annelids resembling sipunculid worms
• Mostly deposit feeders; at least 1 is a filter feeder
– deposit feeders typically have a flat, ribbon-like
proboscis (tube extending from the mouth) to collect particles
• Have separate sexes, shed gametes into the
Pogonophorans
• Class Pogonophora (Beardworms)
• Live in buried tubes and have a cylindrical
body with a ring of tentacles around the
anterior end
• Lack mouth or digestive tract
Nematodes
• Class Nematoda (Roundworms)
• Most numerous animals on earth
• Round, slender, elongated bodies, tapering
at both ends
• Critical role as scavengers
• Some are parasitic
Priapulids
• Benthic worms that bury themselves in sand
and mud in shallow or deep water
• Small species belong to meiofauna; may be
deposit or suspension feeders
• Larger species are thought to be carnivorous
• Have separate sexes; fertilization is external
Ecological Role of Marine Worms
• Nutrient cycling
– through burrowing in sediment, release nutrients buried in the ocean bottom back to the surface for use by producers
• Predator-prey relationships
– Important links in food chains – consume
Ecological Role of Marine Worms
– nematodes are the most abundant members of meiofauna
– echiurans may be significant in the diet of some fishes
– polychaetes are a major food source for invertebrates and vertebrates
• Symbiotic relationships
– tubes and burrows of non-carnivorous
polychaetes provide protected and ventilated retreat for many commensals, e.g., scale
