Arthropods: Animals with Jointed Appendages

Arthropods: Animals with Jointed Appendages• Phylum Arthropoda = 75% of species• Have exoskeleton—a hard, protective exterior skeleton composed

of protein and chitin (a tough polysaccharide)– molting—shedding and replacement of exoskeleton to permit

animal’s growth• Body is divided into segments• Usually, each segment has a pair of jointed appendages, for

locomotion, mouthparts, sensation, ornamentation• Have highly developed nervous systems

– sophisticated sense organs– capacity for learning

• 2 major groups of marine arthropods:– chelicerates – have a pair chelicerae (oral appendages) and lack

mouthparts for chewing food– mandibulates – have appendages called mandibles that can be

used to chew food

Arthropods: Animals with Jointed Appendages

• Have highly developed nervous systems– sophisticated sense organs– capacity for learning

• 2 major groups of marine arthropods:– chelicerates – have a pair chelicerae (oral

appendages) and lack mouthparts for chewing food

– mandibulates – have appendages called mandibles that can be used to chew food

Chelicerates• 6 pairs of appendages; 1 pair are chelicerae for feeding• Horseshoe crabs

– 3 body regions• cephalothorax – largest region with the most obvious

appendages• abdomen – contains the gills• telson – a long spike used for steering and defense

– body is covered by a carapace—a hard outer covering• Horseshoe crabs (continued)

– locomotion by walking or swimming by flexing the abdomen– mostly nocturnal scavengers– smaller males attach to females to mate, and eggs are laid in a

depression on the beach; larvae return to the sea to grow

Chelicerates

• Horseshoe crabs (continued)– locomotion by walking or swimming by flexing

the abdomen– mostly nocturnal scavengers– smaller males attach to females to mate, and

eggs are laid in a depression on the beach; larvae return to the sea to grow

Chelicerates

• Sea spiders– have small, thin bodies with 4 or more pairs of

walking legs– only marine invertebrate known where the male

carries the eggs (with an extra pair of appendages)

– palps—appendages used in sensation– feed on juices from cnidarians and other soft-

bodied invertebrates, using a long sucking proboscis

Mandibulates• Crustaceans—marine mandibulates • Crustacean anatomy

– 3 main body regions:• head• thorax• abdomen

– appendages:• 2 pairs of sensory antennae• mandibles and maxillae used for feeding• walking legs, swimmerets (swimming legs), legs modified for

reproduction, chelipeds (legs modified for defense)– gas exchange

• small crustaceans exchange gases through their body surface• larger crustaceans have gills

• Molting– Crucial part of the life cycle– Frequency of molting decreases with age– Controlled by specific hormones produced in a gland in the head,

and initiated by environmental conditions

Mandibulates

– gas exchange• small crustaceans exchange gases through their body

surface• larger crustaceans have gills

• Molting– Crucial part of the life cycle– Frequency of molting decreases with age– Controlled by specific hormones produced in a

gland in the head, and initiated by environmental conditions

Decapods• Order decapoda; includes animals with 5 pairs of walking legs:

– crabs– lobsters– true shrimp

• 1st pair of walking legs are chelipeds—pincers used for capturing prey and for defense

• Wide range in size• Nutrition and digestion

– chelipeds are used for prey capture– appendages are used for scavenging– predation and scavenging are usually combined – some decapods are deposit or filter feeders

Decapods

• Specialized behaviors– hermit crabs inhabit empty shells– decorator crabs camouflage carapaces with

bits of sponge, anemones, etc.– common blue crabs are agile swimmers

Decapods

• Nutrition and digestion– chelipeds are used for prey capture– appendages are used for scavenging– predation and scavenging are usually

combined – some decapods are deposit or filter feeders

Decapods

• Reproduction– sexes usually separate– males have appendages modified for clasping

females and delivering sperm• spermatophores—sperm packages• copulatory pleopods—2 pairs of anterior abdominal

appendages that deliver sperm

– most brood their eggs in chambers or modified appendages

Decapods

• Reproduction (continued)– larval stages:

• zoea larval stage—initial stage in crabs, recognized by the very long rostral spine and sometimes lateral spines (thought to reduce predation)

• nauplius larva—initial stage in shrimp

Mantis Shrimp

• Order Stomatopoda

• Highly specialized predators of fishes, crabs, shrimp and molluscs

• 2nd pair of thoracic appendages– enlarged– has a movable finger that can be extended

rapidly for prey capture/defense– used to spear or smash prey

Mantis Shrimp

• Reproduction– some mantis shrimp pair for life and share a

burrow– zoea larvae hatch from an egg mass– retain planktonic form for 3 months, then

settle and take up adult lifestyle

Krill

• Order Euphausiacea• Pelagic, shrimp-like, 3-6 cm long• Filter feeders that eat zooplankton• Most are bioluminescent

– photophore—specialized light organ– swarms—large masses of individuals;

bioluminescent is thought to signal swarming behavior

• Food source for some whales, seals, penguins, and many fishes

Amphipods

• Order Amphipoda• Shrimp-like, with posterior 3 pairs of

appendages directed backward• Many are burrowers; some construct tubes

which they inhabit• Most are detritus feeders or scavengers, some

are herbivores– gnathopods—special appendages for picking up plant

and animal remains

• Eggs fertilized in female’s brood chamber; young resemble adults upon hatching

Copepods

• Class Copepoda – the largest group of small crustaceans

• Usually the most abundant member of the zooplankton

• Mostly suspension feeders; some rely on detritus, some are predators

• Males fertilize females with spermatophores; eggs are shed into the water column where they hatch

Barnacles• Class Cirripedia – the only sessile crustaceans• Most have calcium carbonate shell• Attach directly to a hard surface, or have a stalk for

attachment• Filter feed using cirripeds—feathery appendages which

extend into the water when the shell is open• Reproduction

– hermaphroditic– cross-fertilized using a long, extensible penis– brooded eggs hatch into nauplius larvae– nauplius larvae develop into cyprid larvae, which have

compound eyes and a carapace of 2 shell plates– cyprid larvae attach using adhesive glands in

antennae, then metamorphose into adults

Barnacles

• Reproduction– hermaphroditic– cross-fertilized using a long, extensible penis– brooded eggs hatch into nauplius larvae– nauplius larvae develop into cyprid larvae,

which have compound eyes and a carapace of 2 shell plates

– cyprid larvae attach using adhesive glands in antennae, then metamorphose into adults

Ecological Roles of Arthropods• Arthropods as food

– important food sources for marine animals and humans– copepods form a link between phytoplankton they eat and many animals

that use them as a major food source– krill are consumed in large quantities by whales and other organisms

• Arthropods as symbionts– cleaning shrimps remove ectoparasites and other materials from

reef fish– some copepods are ectoparasites for fish– some copepods are endoparasites or commensals within

polychaete worms, echinoderms, tunicates, bivalves or cnidarians

– amphipods carry sea butterflies– barnacles are commensal with many hosts

• Role of arthropods in recycling and fouling– grass shrimp feed on detrital cellulose material, and so helps break

down algae and grasses in tidal marsh ecosystems– barnacles are a serious fouling problem on ship bottoms

• attached barnacles can reduce ship speed by 30%• special paints and other anti-fouling measures

Ecological Roles of Arthropods

• Arthropods as symbionts– cleaning shrimps remove ectoparasites and

other materials from reef fish– some copepods are ectoparasites for fish– some copepods are endoparasites or

commensals within polychaete worms, echinoderms, tunicates, bivalves or cnidarians

– amphipods carry sea butterflies– barnacles are commensal with many hosts

Ecological Roles of Arthropods

• Role of arthropods in recycling and fouling– grass shrimp feed on detrital cellulose material, and so helps

break down algae and grasses in tidal marsh ecosystems– barnacles are a serious fouling problem on ship bottoms

• attached barnacles can reduce ship speed by 30%• special paints and other anti-fouling measures

Lophophorates

• Lophophorates are sessile animals that lack a distinct head

• Possess a lophophore—arrangement of ciliated tentacles that surround the mouth, used for feeding, gas exchange

• 3 phyla of lophophorates:– Phoronida (phoronids)– Ectoprocta (bryozoans)– Brachiopoda (brachiopods)

Phoronids

• Small, worm-like animals• Secrete a tube of leathery protein or chitin

that can be attached or buried in bottom sediments

• Catch plankton and detritus with mucus-coated tentacles

• Can reproduce sexually or asexually (budding, transverse fission)

• Have a planktonic larval stage

Bryozoans

• Small, abundant, colonial animals• Most live on rocks, shell, algae, mangroves,

etc. in shallow water• Colonies are composed of zooids (tiny

individuals), each inhabiting a box-like chamber it secretes

• Most are hermaphroditic brooders• Larvae are planktonic; they settle to form

new colonies

Brachiopods

• Most lamp shells are benthic and live in shallow water

• Have mollusc-like, bivalve shells– valves differ in size and shape, and are dorsal

and ventral– a pedicle (fleshy stalk) attaches the shell or is

buried

• Gather detritus/algae with lophophore• Generally have separate sexes; larvae are

planktonic and settle in 24-30 hrs.

Ecological Roles of Lophophorates

• As a group, they are filter feeders

• Food for many invertebrates, especially molluscs and crustaceans

• Largely responsible for fouling ship bottoms

Echinoderms: Animals with Spiny Skins

• Phylum Echinodermata

• Larval forms exhibit bilateral symmetry but most adults exhibit a modified form of radial symmetry

• Mostly benthic, and found at nearly all depths

• Sea cucumbers and brittle stars are commonly found in deep-sea samples

Echinoderm Structure

• Endoskeleton—internal skeleton that lies just beneath the epidermis– ossicles—plates of calcium carbonate– endoskeleton is composed of ossicles held

together by connective tissue

• Spines and tubercles project outward from the ossicles– pedicellariae—tiny, pincer-like structures around

the bases of spines that keep the body surface clean in some echinoderms

Echinoderm Structure

• Water vascular system—unique hydraulic system that functions in locomotion, feeding, gas exchange and excretion– water enters by the madreporite– passes through a system of canals– attached to some canals are tube feet—

hollow structures with a sac-like ampulla within the body and a a sucker protruding from the ambulacral groove

Sea Stars

• Class Asteroidea

• Typically composed of a central disk + 5 arms or rays

• On underside, ambulacral grooves with tube feet radiate from the mouth along each ray

• Aboral surface—the side opposite the mouth, which is frequently rough or spiny

Sea Stars

• Feeding in sea stars– most are carnivores or scavengers of

invertebrates and sometimes fish– prey are located by sensing of substances

they release into the water– sea stars envelope and open bivalves, evert a

portion of the stomach, and insert it into the bivalves to digest them

• digestive glands located in each ray provide digestive enzymes

Sea Stars

• Reproduction and regeneration– sea stars can regenerate rays; some can

regenerate themselves from a single ray plus part of the central disc

– asexual reproduction involves division of the central disk and regeneration of each half into a new individual

– most have separate sexes, which shed eggs and sperm into the water for fertilization and hatching into usually planktonic larvae

Ophiuroids

• Class Ophiuroidea– e.g. brittle, basket and serpent stars

• Benthic with 5 slender, distinct arms, frequently covered with many spines

• Lack pedicellariae and have closed abulacral grooves

• Tube feet lack suckers and are used in locomotion and feeding

• Brittle stars shed arms if disturbed

Ophiuroids

• Feeding in ophiuroids– carnivores, scavengers, deposit feeders,

suspension feeders, or filter feeders– brittle stars usually filter feed by lifting their

arms and waving them in the water– deposit feeders use their podia to gather

organic particles from the bottom into food balls and pass them to the mouth

– basket stars suspension feed by climbing onto corals/rocks and fanning their arms toward the prevailing current

Ophiuroids

• Reproduction and regeneration in ophiuroids– autotomize—to cast off, as of an arm, when

disturbed or seized by a predator– asexual reproduction by division into 2 halves

and regeneration of individuals– mostly separate sexes– may shed eggs into water or brood them in

ovaries or a body cavity– planktonic larvae metamorphose into adults

within the water column

Sea Urchins and their Relatives

• Class Echinoidea – echinoids• Body enclosed by test—a hard exoskeleton• Benthic on solid surfaces (sea urchins) or in sand

(heart urchins, sand dollars)• Regular (radial) echinoids—sea urchins; spheroid

body with long, moveable spines• Irregular (bilateral) echinoids—heart urchins and

sand dollars; have short spines on their tests

Sea Urchins and their Relatives

• Echinoid structure– tube feet project from 5 pairs of ambulacral

areas– spines project from the test

• aid in locomotion and protection, and may contain venom

– sexes are always separate– regular echinoids have 5 gonads; irregular

echinoids, 4– sperm and eggs shed into the water; fertilized

eggs hatch into planktonic larvae

Sea Urchins and their Relatives

• Feeding in echinoids– feeding in regular echinoids

• mostly grazers which scrape algae and other food materials from surfaces

• Aristotle’s lantern—a chewing structure of 5 teeth

– feeding in irregular urchins• irregular urchins are selective deposit feeders• some sand dollars are suspension feeders

Sea Cucumbers

• Class Holothuroidea• Have elongated bodies, and usually lie on 1

side• Respiratory trees—a system of tubules

located in the body cavity which accomplish gas exchange

• Sexes are generally separate• Eggs may be brooded or incubated; larvae

are planktonic

Sea Cucumbers

• Feeding in sea cucumbers– mainly deposit or suspension feeders– oral tentacles—modified tube feet coated with

mucus which are used to trap small food particles

• Defensive behavior– Cuvierian tubules—sticky tubules released from

the anus of some species– eviscerate—to release some internal organs

through the anus or mouth

Crinoids

• Class Crinoidea – sea lilies and feather stars• Primitive, flower-like echinoderms• Most are feather stars, which seldom move and

cling to the bottom with grasping cirri• Suspension feeders• Can regenerate lost arms• Separate sexes shed eggs/sperm into the water;

larvae have fee-swimming stage, then attach to the bottom and metamorphose into minute adults

Ecological Roles of Echinoderms

• Spiny skins deter most predators• Predators of molluscs, other echinoderms,

cnidarians, crustaceans– crown-of-thorns sea star eats coral– sea urchins destroy kelp forests

• Black sea urchins control algae growth on coral reefs

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

Tunicates

• Subphylum Urochordata• Mostly sessile, widely distributed• Named for their body covering

– tunic—body covering, largely composed of a substance similar to cellulose

• Types:– sea squirts– salps – larvaceans

Sea Squirts

• Class Ascidiacea

• Name derived from tendency to expel a stream of water when disturbed

• Round or cylindrical bodies with 2 tubes projecting from them:– incurrent siphon that brings in water and food– excurrent siphon that eliminates water and

wastes

Sea Squirts

• Lifestyles: solitary, colonial, compound– compound—organisms composed of several

individuals (zooids) that share a common tunic

• Filter feed on plankton in the water passing through their pharynx– some have symbiotic algae or bacteria

• Can regenerate lost body parts

Sea Squirts

• Asexual reproduction (by budding) occurs in colonial ascidians

• Most are hermaphrodites that release gametes into the water column for fertilization

• Tadpole-like larvae are free-swimming for 36 hrs., then settle and metamorphose into the sessile stage

Salps and Larvaceans

• Salps– class Thaliacea– free-swimming tunicates with incurrent and

excurrent siphons on opposite ends of their barrel-shaped bodies

• pump water through to swim

• Larvaceans– class Larvacea– free-swimming; produce delicate enclosures of

mucus used in feeding

Cephalochordates

• Subphylum Cephalochordata- lancelets• Fish-like chordates; slender, laterally

compressed and eel-like in form and behavior

• Benthic; burrow in coarse sands• Suspension feed by projecting their heads

above the sand• Separate sexes practice internal fertilization

Cephalochordates

• Have complex life cycles with benthic adults and planktonic swimming larvae

• Important as food in parts of Asia

• Used as chicken feed in parts of Brazil

Arrowworms

• Phylum Chaetognatha• Common planktonic animals with a torpedo-

shaped body• Grasping spines (large curved hooks) hang

from the head and flank the vestibule (chamber leading to mouth)

• Carnivorous; seize other planktonic prey animals with grasping spines and inject tetrodotoxin