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Marine Biology
Unit 3
MARINE INVERTEBRATES
Unit summary & Lecture notes
Summary
This unit outlines the major groups of marine invertebrates, from protozoans to
echinoderms. Since students are expected to have previous fundamental knowledge of
invertebrate zoology, we will concentrate on some of the adaptations to the marine
environment, and how the particular organisms fit into overall marine environment. If
students do not possess prior invertebrate zoological knowledge, the instructor may
wish to consult a zoological textbook to expand on the presented phyla.
(Table 1: Invertebrate taxonomy and members I)
(Table 2: Invertebrate taxonomy and members II)
(Table 3: Important invertebrate physical characteristics I)
(Table 4: Important invertebrate physical characteristics II)
I.
Protozoans
Protozoans are the most animal-like of the protists. They are eukaryotic and
mostly unicellular. They are heterotrophic and ingest food like true animals.
Some protozoans, however, contain chlorophyll and photosynthesize like plants.
a. Foraminiferans are marine protozoans typically having a shell that usually
consists of calcium carbonate. Most forams live on the bottom, either free
or attached. The shells of bottom-inhabiting forams can be important
contributors of calcareous material in coral reefs and sandy beaches.
Planktonic forams eventually sink to the bottom in such high numbers that
large stretches of the ocean floor are covered by foraminiferan ooze.
Many limestone and chalk beds around the world, like the white cliffs of
Dover England, are products of foram sediments.
b. Radiolarians are planktonic marine protozoans that secrete elaborate and
delicate shells made of glass (silica) and other materials. Typical shells
are spherical with radiating spines to increase surface are for buoyancy.
Most radiolarians are microscopic, but some form bizarre, sausageshaped colonies that reach 3 m in length. Radiolarians inhabit open water
throughout the ocean. The remains of their shells settle to the bottom and
form siliceous ooze known are radiolarian ooze. This ooze is more
extensive in deep water because radiolarian shells are more resistant to
the effects of pressure than those of forams.
c. Coccolithophores aid in the ocean’s overall primary productivity.
II.
Porifera
Sponges are among the simplest multicellular animals, lacking true tissues and
organs. They are mostly marine, living as attached filter feeders. They show an
amazing variety of shapes, sizes, and colors but share a simple body plan.
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Sponges are generally asymmetrical, but some species present a radial
symmetry. Numerous tiny pores on the surface called incurrent pores, allow
water to enter and circulate through a series of canals where planktonic
organisms and organic particles are filtered out and eaten. The water then exits
through the osculum, a larger pore(s) on the top of the sponge. For support in
an aqueous environment, spicules and/or spongin are present. Sponges have a
unique capability to reassociate cells that have been dissociated. Sponges may
also use this regeneration/reassociation to reproduce asexually (called
fragmentation).
III.
Cnidaria
The radially symmetrical cnidarians, also known as coelenterates, exist as
polyps and/or medusae. Nematocysts, stinging structures unique to cnidarians,
are present in tentacles used to capture prey. Some 10,000 species are known,
almost all of which are marine.
a. Hydrozoans (class Hydrozoa) have a wide range of structures and life
histories. Many consist of feathery or bushy colonies of tiny polyps. They
attach to pilings, shells, seaweeds, and other surfaces. Siphonophores
are hydrozoans that form drifting colonies. Some polyps in a
siphonophore colony may be specialized as floats, which may be gas-filled
as in the Portuguese man-of-war (Physalia physalis).
b. Scyphozoans, the jellyfish, are common in all oceans. The medusae are
the dominant stage of the life cycle. The rounded body, or bell, of some
scyphozoan medusae may reach a diameter of 2 m in some species.
Some scyphozoan medusae are among the most dangerous marine
animals known, giving extremely painful and sometimes fatal stings.
c. Anthozoans, the sea anemones and corals, make up the majority of the
cnidarian species. Anthozoans are solitary or colonial polyps that lack a
medusa stage. The anthozoan polyp is more complex than hydrozoan or
scyphozoan polyps.
i. Sea anemones are common and colorful anthozoans that often
have large, muscular polyps.
ii. Stony corals are colonial anthozoans that have calcium carbonate
skeletons that may form coral reefs.
iii. Soft corals such as gorgonians and black corals secrete a tough
branching skeleton made of protein.
IV.
Ctenophora
The ctenophores, or comb jellies, are an exclusively marine group of about 100
species. Their radially symmetrical and gelatinous body resembles that of a
medusa, but a closer look reveals some unique traits. Eight rows of ciliary
combs called ctenes, long cilia fused at the base like combs, beat in waves and
are used in swimming. The continuous beating of the ctenes refracts light,
creating a prism-like multicolor effect. Common both in warm and cold waters,
comb jellies are carnivores with a voracious appetite. Many capture their prey
using two long tentacles armed with sticky cells. Comb jellies lack nematocysts.
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V.
Platyhelminthes
Flatworms are bilaterally symmetrical invertebrates typically flattened in
appearance. They have true organs and organ systems, including a central
nervous system.
a. Turbellarians are the most commonly seen marine flatworm. They consist
mostly of free-living carnivores. Some live inside or on the surface of
oysters, crabs, and other invertebrates.
b. Trematodes, or flukes, are the largest group of flatworms. All flukes are
parasites. Adult flukes always live in a vertebrate. Larvae may inhabit
invertebrates like snails or clams or vertebrates like fish. Flukes are
common in fishes, seabirds, and whales.
c. Cestodes, or tapeworms, are parasitic flatworms that have a long body
that consists of repeated units. These unique worms hang inside the
intestines of most species of vertebrates, including marine ones.
Tapeworms lack a mouth or gut. They absorb nutrients from their host’s
intestinal contents directly across the body wall. Tapeworms may reach a
prodigious length. The record appears to be a species found in sperm
whales that is 15 m long!
VI.
Nematoda
Roundworms are very common inhabitants of marine sediments and are
widespread parasites of most groups of marine animals. Nematodes are known
to inhabit the intestine of seals and dolphins. Their larvae, however, are found in
the flesh of many types of fish and may infect humans when raw or poorly
cooked fish is eaten.
VII.
Annelida
Annelids, or segmented worms, have a body consisting of similar segments and
a coelom.
a. Almost all marine annelids are polychaetes, which are common and
important in many marine environments. There are more than 10,000
species of polychaetes. Each of their body segments has a pair of
flattened extensions, or parapodia, which are provided with stiff and
sometimes sharp bristles, or setae. Many polychaetes crawl on the
bottom, hiding under rocks or coral. These crawling worms, such as most
sandworms, or bristleworms (Nereis), are mostly carnivores. Other
polychaetes burrow in mud or sand.
b. Oligochaetes are small worms found in mud and sand, where they feed on
organic matter. They are the marine relatives of earthworms.
VIII.
Chaetognatha
In terms of number of species the chaetognaths, or arrow worms, rank among
the smallest animal phyla. Only about 100 species, all marine, are known. They
are nevertheless one of the most common and important members of the
plankton. Arrow worms are voracious carnivores with efficient sensory structures
to detect their prey. Their total body length ranges fro a few millimeters to 10 cm,
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but they prey on practically anything that is small and edible. This includes small
crustaceans, eggs and the larvae of fishes and other animals, and other arrow
worms.
IX.
Bryozoa
Bryozoans are invertebrates that form delicate colonies on seaweed, rocks, and
other surfaces. Most of the 4,500 species of bryozoans are marine. Bryozoan
colonies consist of minute interconnected individuals called zooids that secrete
skeletons of a variety of shapes. The colonies may be encrusting or take an
upright form that looks like tufts of crusty lace.
X.
Mollusca
The molluscs constitute the largest group of marine animals. The body is soft
with a muscular foot, and usually possesses an external calcium carbonate shell.
They usually have a shell and most have a radula, a rasping “tongue” unique to
the group. Molluscs exhibit an immense diversity of structure and habit. They
occupy all marine environments from the wave-splashed zone of the rocky shore
to hydrothermal vents in the deep sea. They thrive on practically every
conceivable type of diet.
a. Gastropods (class Gastropoda) are the largest, most common, and most
varied group of molluscs. There are approximately 75,000 species of
gastropods (“stomach footed”). Snails are the most familiar gastropods,
but the group includes limpets, abalones, and nudibranchs. Most
gastropods use their radula to scrape algae from rocks. Some species of
snails, the cone snails, are predatory. Cone snails use a highly venomous
proboscis to inject a toxin into prey. The prey in then ingested whole.
Nudibranchs are gastropods that have lost the shell altogether. They are
predatory, usually feeding on sponges, sea anemones, or other
nudibranchs.
b. Bivalves (class Bivalvia) are the clams, mussels, oysters, scallops, and
similar molluscs. The body is laterally compressed and enclosed in a twovalved shell. There is not head to speak of, and no radula. Bivalves use a
pair of siphons to filter oxygen and food particles out of the water column.
Bivalves such as clams and sand scallops burrow in sand or mud, while
others such as mussels and oysters attach themselves to rocks.
c. Chitons (class Polyplacophora) are a small group of molluscs that can be
identified by the eight overlapping shell plates that cover their slightly
arched dorsal surface. Their internal organs are not coiled as in snails.
There are approximately 800 species of chitons. Almost all are restricted
to rocky shores. Most species use the radula to graze algae from the
rocks.
d. Cephalopods (class Cephalopoda) are “head-footed” creatures. They are
voracious predators that are specialized for locomotion, and include the
octopuses, squids, cuttlefishes, and chambered nautilus. Most
cephalopods are agile swimmers with a complex nervous system and a
reduction or loss of the shell. All 650 living species are marine.
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i.
Octopuses have eight long arms, each lined with suckered discs,
and lack a shell. They are usually bottom dwellers. They range in
size from 5 cm to a record of 9 m in the Pacific giant octopus. They
hunt mostly crustaceans.
ii. Squids are better adapted for swimming than are octopuses. They
posses eight arms and two “tentacles”, all with suckers. The
tentacles can be swiftly shot out to catch prey. Adult size varies
from a few centimeters in length to 20 m in the giant squid
(Architeuthis), the largest living invertebrate. The giant squid is a
deep-water species known mostly from specimens that have been
washed ashore or found in the stomachs of sperm whales.
iii. Cuttlefishes resemble squids in having eight arms and two
“tentacles”, but the body is flattened and has a fin running along its
sides. Cuttlefish feed on the same prey and in the same way as
squid. Their complex language and behavior makes cuttlefish one
of the most studied invertebrates.
iv. The chambered nautilus has an unusual external shell. The shell is
smooth, coiled, and up to 25 cm in diameter. The shell contains a
series of gas-filled chambers that serves as a buoyancy organ.
The body of the nautilus has 60 to 90 short, suckerless arms used
to capture prey such as crabs and fish.
XI.
Arthropoda
More species belong to the arthropods than to any other animal group. There
are more than a million species and several million remaining undiscovered.
Three out of four animals on earth is an arthropod. Arthropods have a
segmented and bilaterally symmetrical body. Their success in adapting to all
types of environments is due in part to a resistant exoskeleton and jointed
appendages.
a. Crustaceans (subphylum Crustacea) make up the majority of marine
arthropods. They include the crabs, shrimp, and lobsters. There may be
as many as 150,000 species of crustaceans. Most are marine.
Crustaceans have specialized appendages used for swimming, crawling,
attaching to other animals, mating, and feeding.
i. Small crustaceans are everywhere: in the plankton, on the bottom,
among sediments, on and in other animals, crawling among
seaweeds. This group includes copepods, barnacles, amphipods,
isopods, and krill, or euphausiids.
ii. The larger crustaceans are known as the decapods. They include
the crabs, shrimp, and lobsters. There are around 10,000 species
of decapods. Many are prized as food and are of great economic
importance. The members of this group play a vital role in the
overall ecology of the ocean: they are the garbage collectors of the
sea. Crabs, shrimp, and lobsters are predators and scavengers,
whose role it is to keep our ocean bottoms clean.
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b. The horseshoe crabs are the only surviving members of a group
(subphylum Meristomata) that is widely represented in the fossil record.
The five living species of horseshoe crabs (Limulus) are not true crabs.
They live on the soft bottom in shallow water on the Atlantic and Gulf
coasts of the US.
XII.
Echinodermata
Echinoderms are radially symmetrical as adults. Most have pentamerous radial
symmetry, that is, symmetry based on five parts. They are characterized by an
endoskeleton and a unique water vascular system. The echinoderms are a large
group of about 7,000 species, all marine. They are important members of the
benthic communities from the poles to the tropics.
a. Sea stars (class Asteroidea), are the most well known of the echinoderms.
Most species have five arms that radiate out from a central disk, although
some species of sun stars have up to 50 arms. Most sea stars are
predators of bivalves, snails, barnacles, and other attached or slowmoving animals.
b. Brittle stars (class Ophiuroidea), usually have five arms, however, the
arms are long, very flexible, and sharply demarcated from the central disk.
The swift, snake-like movements of the arms are used in locomotion. The
tube feet, which lack suckers in the brittle stars, are used in feeding. Most
brittle stars are scavengers. There are more species of brittle stars,
around 2,000, than any other group of echinoderms. They often hide
under rocks and corals, or cover themselves with mud or sand.
c. In sea urchins (class Echinoidea) the endoskeleton forms a round, rigid,
shell-like test with movable spines. Sea urchins graze on attached or
drifting seaweeds and seagrasses. In the process, they also ingest dead
organic matter and encrusting animals such as sponges and bryozoans.
Not all of the approximately 1,000 species in the class Echinoidea have
round tests with prominent spines. Sand dollars are echinoids adapted to
live in soft bottoms by having flattened bodies and short spines. Sand
dollars are deposit feeders.
d. Sea cucumbers (class Holothuroidea) are echinoderms that do not have
spines or the obvious radial symmetry. They are superficially wormlike.
Many sea cucumbers are deposit feeders. Many have evolved novel
defense mechanisms that compensate for the lack of a test and spines.
Some secrete toxic substances. Others resort to a startling response: the
sudden expulsion of the gut and other internal organs through the mouth
or anus. It is assumed that these eviscerations distract the offender while
the sea cucumber, which will eventually grow back the lost organs,
escapes.
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