Marine Animals Without a Backbone

Animals without a backbone are known as invertebrates. Those with a backbone are called vertebrates. About 97% of animals are invertebrates. All major animal groups have representatives in the marine community. Several animal groups are exclusively marine.

GENERAL CHARACTERISTICS OF ANIMALS

Multicellular, diploid organisms with tissues, organs or organ systems in most

Heterotrophic

Require oxygen for aerobic respiration

Reproduce sexually, or asexually, or both

Most are motile at least during some portion of the life cycle

Animal life cycles include a period of embryonic development

GENERAL CHARACTERISTICS OF ANIMALS

Basic body structure:

- Symmetry (body plan)

Radial- round; equal parts radiate out from a central point (like that seen in a sea star)

Bilateral- organism can be divided into right and left halves that are more or less equal (ex: marine mammals)

Sponges have no symmetry and are known as asymmetrical.

MAJOR PHYLA OF MARINE INVERTEBRATES

01. Sponges

02. Cnidarians

03. Flatworms

04. Ribbon worms

05. Nematodes

06. Annelids

07. Sipunculans

08. Echiurans

09. Molluscs

10. Arthropods

11. Bryozoans

12. Phoronids

13. Brachiopods

14. Arrow worms

15. Echinoderms

16. Hemichordates

17. Tunicates

18. Lancelets

Porifera: The Sponges

Characteristics of Phylum:

All sponges are sessile (non-motile and living attached) and some encrust on hard substrate.

Numerous tiny pores (ostia) exist in the body to allow water to pass through the sponge.

This water movement is required to allow sponges to filter feed (a type of suspension feeding) on plankton and dissolved organic matter in the water.

Water flow is also essential to carry metabolites (waste) away from cells and to carry gametes.

Asymmetric body with no true tissues or organs.

Mostly marine

Varying size, shape, and color

Solitary or colonial

Regeneration capability

Filter feeders

Types of Cells:

Choanocytes (or collar cells) –

line interior canals of the body;

flagella on the choanocytes create a water current that brings more food particles into the body

“Collars” on choanocytes traps food particles.

Pinacocytes- these flattened cells cover exterior of body.

Porocytes- cells with a pore to allow water to pass into body

Structural support:

Spongin is a support protein

Spicules are support structures made of silica or calcium carbonate.

These spicules have a variety of shapes from simple rods to star-shaped

Modes of Reproduction –

Asexual budding- fragmentation of a cluster of cells from original sponge can begin growth in a new area.

Sexual- sperm are released into surrounding water (broadcast spawners) to be picked up by a nearby sponge and directed to egg .

Most sponges are hermaphroditic (possess male and female reproductive parts).

Cnidarians: Stinging animals

Characteristics of Phylum:

Radial symmetry

Mostly marine organisms, about 10,000 species known

Two body forms exist :

Medusa – free floating form which is transported by water currents, mouth with surrounding tentacles are positioned downward

Polyp – sessile, attached form with mouth and tentacles are positioned upward

Two tissue layers are present-

Epidermis covers body surface

Gastrodermis lines internal body cavity and is specialized for digestion

Cnidocytes (stinging cells) with nematocyst capsules located on tentacles. These are used for protection and for feeding

Digestive system is incomplete (sac-like with mouth only)

Nerve net throughout body coordinates movements

Some jellyfish also have sensory cells and contractile cells

Reproduction in Cnidarians:

Sexual-

Medusa is normally the sexual stage with epidermal gonads – eggs and sperm released from medusa

Fertilized egg results in zygote

Zygotes develops into a swimming larva called a planula

Planula “settles” on bottom to form colony

Eventually, new medusa are formed by this mature colony

Asexual-

Polyps normally reproduce by budding

Classification of Cnidarians

1. Class Scyphozoa -

True Jellyfish

Free swimming large medusa forms with polyp only in reproductive life

All marine

They move by rhythmic contractions, but cannot fight against prevailing water currents

Many with powerful stings

2. Class Anthozoa –

Corals, Anemones, Sea Fans/Whips, Sea Pansy
Colonial polyps that normally lack a medusa stage
Corals secrete calcium carbonate “shells”
Most coral species possess symbiotic algae within body tissues called zooxanthellae
All marine

3. Class Hydrozoa

Mostly polyp forms with reproductive medusa

Physalia is unusual species with a colony of polyps carried by a gas-filled float

Freshwater and marine species

Ctenophores – also known as comb jellies

Characteristics of Phylum:

About 100 species, all marine

8 rows of thick cilia that beat continuously

No nematocysts unless they harbor some from eating cnidarians

Long sticky tentacles used to capture prey

Most species are planktonic

Can be found in warm and cold waters

Flatworms

Characteristics of Phylum:

Consist of flukes, tapeworms and turbellarians

About 20,000 species exist

Some parasitic, others free-living

Well developed reproductive system in most

First brain- clusters of nervous tissue in head

Three distinct tissue layers

Bilateral symmetry

Types of Marine Flatworms

Turbellarians

Mainly free-living carnivores

Eye-spots present to determine light/dark patterns

Most small

Some with striking coloration patterns

Flukes

All parasitic

Amazing reproductive abilities

Have suckers to attach to inside of blood vessels near intestines of host or other tissues

Vertebrates are the host of the adult fluke

Larval stages may be harbored in shellfish or fish

Tapeworms

Parasitic

Scolex with suckers and hooks for attachment in host’s intestines

Body is made of repeated segments

Specialized cuticle surrounds body - allows absorption of nutrients

No digestive system of their own – they get all their nutrients from their host

Adult tapeworms live in vertebrates, larvae are found in invertebrate and vertebrate species

Ribbon Worms

Characteristics of Phylum:

About 900 species, mostly marine

Prefer shallow, coastal waters

Complete digestive system with mouth and anus

Ciliated epidermis.

Circulatory system

Proboscis, a long fleshy tube, is used to entangle prey

These marine predators eat other invertebrates such as worms and crustaceans

One species reaches a length of 100 ft making it the longest invertebrate known

Nematodes, or Round Worms

Characteristics of Phylum:

Bilateral symmetry

Complete digestive system

Cuticle present to guard against drying

Some species are important decomposers on the sea floor

Some species are parasitic (Female can lay up to 200,000 eggs/day)

Estimates on the actual number of species range from 10,000 to 25,000 - real numbers probably higher

Larval stages often seen in fish – these larval stages can be passed to humans if raw (or undercooked) fish is consumed

Annelids, Segmented Worms

Characteristics of Phylum:

About 20,000 species

Live in salt water, freshwater or moist terrestrial

Well developed nervous system (with brain)

Segmented internally and externally

Closed circulatory system

Bilateral symmetry

Setae in most (except leeches) – these are bristle-like structures that extend from the sides of each segment which help the annelid stay in place

Types of Annelids:

Polychaetes:

Largest group of annelids, also most diverse

About 10,000 species

Nearly exclusively marine, some in freshwater and brackish water

Distinct head

Some build calcareous tubes or tubes of sticky proteins

Some are carnivorous, others are deposit feeders

Many polychaetes have a planktonic larval stage called a trochophore; this type of larvae is also seen in other invertebrate groups

Leeches

Most found in freshwater, some tropical species in moist terrestrial environment, few marine

Marine species are parasitic and may prey on other invertebrates or fish

Anterior and posterior suckers to hold prey

Dorsoventrally flattened

Segmentation not as distinct as in other annelids

No setae like in other annelids

Oligochaetes:

Terrestrial earthworms are also in this group

The marine species are like terrestrial earthworms in that they burrow in soft sediments where they are deposit feeders

Mainly found in shallow coastal waters

Have fewer setae than seen in polychaetes

Pogonophorans or beard worms

Highly specialized group of about 135 species

Lack a digestive system, which is highly uncommon in the animal world

They appear to use tentacles located at the interior end to absorb nutrients directly from the seawater

As a group, they are largely restricted to the deep sea

Range in size from 4 inches to 7 feet or longer

Sipunculans or Peanut Worms

Characteristics of Phylum:

About 320 species

All marine, found mainly in shallow coastal waters

Deposit feeders

Their soft unsegmented bodies possess a retractable multi-lobed or tentacled anterior used for feeding

They normally burrow into soft sediments

Sometimes found in corals or in empty shells of other animals

Echiurans

Characteristics of Phylum:

135 species

Non-retractable spoon-shaped or forked proboscis (feeding structure)

Some taxonomists classify them with annelids

Deposit feeder

Some burrow in soft bottoms

Others live in coral

Molluscs

Characteristics of Phylum:

More than 200,000 species

Name means "soft body"

Basic body plan – head, muscular foot and visceral mass in most species

Mantle- secretes shell, waste disposal, sensory reception, respiration

Many have a shell of calcium carbonate

Radula for grazing is unique to this group

Some are deposit feeders, others carnivores, some use radula for scraping algae, encrusting animals, etc. off substrates

Well developed nervous system

Open circulatory system

Complete digestive system

Trochophore larvae develops into a planktonic veliger larvae complete with shell (miniature version of adult)

Types of Molluscs:

Chitins

800 species

All marine

Dorsal shell of 8 plates.

Ventral muscular foot.

Ventral mouth with radula

Mostly found in shallow water, coastal environments of hard substrate

Many graze on algae & small animals in marine intertidal zone (area between high and low tides)

Types of Molluscs:

Bivalves

Clams, Oysters, Scallops, Mussels, etc.

Two shells or “valves”

Oldest part of the shell is called the umbo

Shell grows out from the umbo in concentric rings

No head present

No radula present

Adductor muscles secure valves together

Muscular foot used for burrowing in bottom and other locomotion

Water circulated with siphons

Gills for respiration & food gathering (filter feeding)

Some species burrow, others attach to hard substrates via byssal threads, or grow attached to each other

Gastropods

Largest class of molluscs, about 75,000 species

Name means "belly-footed"

Coiled shell on most species

No shell on sea slugs (nudibranchs)

Radula for grazing on plants in most, some are deposit feeders

Some species are carnivorous and use radula for prey capture (some will even prey on members of the same species)

Cephalopods

Squid, Octopus, Nautilus, & Cuttlefish

All 650 species marine

Fast swimming predators due to water jet propulsion

Well developed eyes

Thick mantle covers the body

Use beak-like jaws and radula to crush or rip prey

Adapted tentacles

Cephalopods

Shell internal or absent in most

Most advanced invertebrates

In octopus, the shell has been replaced by a beak-like jaw which can deliver a powerful bite

Some octopus have toxic bites

Ink sac is also seen in octopus to allow escape from predators

A stiff internal “pen” is seen in squid is a modified shell

Arthropods

Characteristics of Phylum:

About 1 million species known, mostly marine

Most marine species are in a group of arthropods called crustaceans

About 75% of all animals on earth are arthropods

Chitin exoskeleton- hard, but light and moderately flexible

Since the skeleton is external, an arthropod must shed the shell to be able to grow – this process is called molting. There is a soft new shell underneath.

Jointed appendages

Many divided into sections called head, thorax and abdomen

Specialized segmentation- segments combined for specific functions

Specialized eye & sensory organs- wide angle of vision

Specialized respiratory structures, gills, used for gas exchange

Some are filter feeders, some scavengers, others carnivores

Male transfers sperm directly to female to ensure reproductive success

In some species, female will house eggs for a time until they are further developed

Females can store sperm for fertilization at a later time

Many arthropods have complex behaviors including mating rituals

Types of Marine Arthropods:

Crustaceans

68,000 species

2 pairs antennae

Gills for respiration

Head and thorax fused into a single unit called a cephalothorax; entire external body is called the carapace

Large array of appendages specialized for different functions; ex: pinchers on crabs, swimmerettes on the underside of shrimp hold developing eggs, etc.

Types of crustaceans – copepods, barnacles, amphipods, isopods, crabs, shrimp, lobsters, etc.

Horseshoe crabs-

5 pairs of legs, first pair modified in males for reproduction

Females larger than males

Mating pairs come onto beaches each spring to breed and lay their eggs in wet sand

Among the oldest creatures on earth – they have remained virtually unchanged for millions of years

They live and borrow in soft sediments, normally near shore where they feed on other invertebrates and scavenge.

Sea Spiders:

Four of more pairs of jointed legs

Not insects or true spiders

Possess a mouth and proboscis for feeding

Mainly feed on sea anemones and hydrozoans (they are voracious predators!)

More common in cold waters, but can be found worldwide

Insects:

Very few marine insects exist

Many insects feed in the intertidal zone at low tide, but these are just temporary visitors

Bryozoans

Characteristics of Phylum:

4500 species, all marine

Sometimes called moss animals

Small, colonial animals

Some are encrusting forms, others form a branching network that looks like a small tree

Suspension feeders

Bryozoans (plus phoronids and brachiopods) possess a unique feeding structure called a lochophore. The lochophore is an extension of the body wall that ends in a tentacled structure that surrounds the mouth

Phoronids

Characteristics of Phylum:

20 species

Found in shallow, coastal waters

Phoronids make tubes where they live

Somewhat resemble polychaete worms externally

Lochophore present

Brachiopods

Characteristics of Phylum:

300 species, exclusively marine

Many live in deep sea or at polar regions

Two shells or “valves” enclose the body

Possess lochophore

Well known from fossil record

Fossil record indicates there were many more species in geologic past

Arrow Worms

Characteristics of Phylum:

100 species, all marine

These organisms are planktonic, but slightly larger than most plankton

They are voracious predators that are widely distributed in the marine community

They will swim in rapid, darting movements to capture prey

Echinoderms

Characteristics of Phylum:

Name means "Spiny Skin"

Endoskeleton

Water vascular system with tube feet important in feeding and locomotion

Skin gills for respiration.

Radial symmetry in adults- larvae are bilaterally symmetrical

Nervous system is decentralized- no brain- this allows any portion of the body to lead

Can regenerate lost body parts

All 7000 species exclusively marine

Types of Echinoderms:

Sea stars-

Move with tube feet

Have a central disc in center of body surrounded by five arms (or multiples of 5 arms – some species have 50 arms)

Internal organs extend through the entire body, including the arms

Calcium carbonate plates are loosely embedded in spiny skin making them slightly flexible

Carnivores that normally consume shellfish and coral

Brittle stars

Like sea stars, they have a central disc surrounded by arms

Five arms seen in brittle stars are thin and covered in numerous spines

Internal organs are restricted to the central disc

The tube feet present in brittle stars are without suckers and used for feeding on detritus and small animals

Sea Urchins, Sea Biscuits, Sand Dollars

Elongated, movable spines much longer than those sea in other groups

Rigid plates are fused into a solid structure called a “test”

Move with tube feet

Mouth on the bottom, anus on top of body

Biting mouth for grazing– aristotles lantern is the feeding structure of muscles and mouthpieces

Feed on detritus, encrusting organisms, algae or anything else they can scrape off surfaces

Crinoids

Represented by feather stars and sea lilies

600 species are typically found in deep water

Sea lilies live attached while feather stars are mobile

These organisms have 5 or more arms that branch out for suspension feeding

Some use a mucous net to aid in food capture

Sea cucumbers

Five rows of two feet are restricted to one side, where the animal lies

The plates found in the sea cucumbers are loosely embedded in the thick skin

They are deposit feeders

Sea cucumbers have a interesting predator escape plan called evisceration, where they expel the internal organs; it is assumed this allows escape for the sea cucumber.

Since all echinoderms have regenerative capabilities, these internal organs will grow back.

Hemichordates

Characteristics of Phylum:

Although a rare group, these worms seem to span a gap between invertebrates and more advanced chordate animals

Acorn worms share some features with chordates including pharnyngeal gill slits, nerve cord and a stomocord that is similar to the chordate notochord

Acorn worms are deposit or suspension feeders and use proboscis for feeding

Acorn worms have larvae that resemble those seen in echinoderms

85 known species

The Phylum Chordata is a phylum that contains two invertebrate groups, tunicates and lancelets, as well as many other, more familiar animals such as fish, amphibians, reptiles, birds and mammals. Chordates have several features that are seen at least during some portion of the life. Lancelets are the only chordates that possess all the features as adults.

Characteristics of Chordates:

Notochord - nerve cord support

Tubular nerve cord

Muscular pharynx

Gill slits

Post-Anal Tail

Ventral heart

Tunicates

Characteristics of Phylum:

Commonly called “sea squirts” because they filter feed via an incurrent siphon and “squirt” water out an excurrent siphon after the water has been filtered

Larvae has chordate characteristics that are not seen in adults- only pharynx remains

Called tunicates because of thick outer covering called a tunic

Larvae are free swimming after fertilization occurs in open water- mass reproduction

Adults normally live attached to boats, docks, reefs, or other hard substrate

All 3000 known species are marine

Lancelets

Characteristics of Phylum:

23 species

Very small, only up to 3 inches long

Live in shallow marine waters as filter feeders

Body shows segmented muscle tissue

Notochord attached to the muscles

Gills are used to filter food, not in respiration

Possess all chordate features as an adult