Marine Invertebrates

Porifera

These are sponges, characterized by being aquatic, porous, and filter feeders. They lack true tissues and organs, primarily reproducing asexually or sexually. Common example: the common bath sponge.

Cnidaria

This group includes jellyfish, corals, and sea anemones, characterized by radial symmetry, a central mouth surrounded by tentacles, and specialized cells called cnidocytes. Examples include the moon jellyfish (Scyphozoa) and the sea anemone (Anthozoa).

Hydrozoa

A class within Cnidaria, often forming colonies; includes species like the Portuguese man o' war. Physiology includes both polyp and medusa forms.

Scyphozoa

This class includes larger jellyfish, characterized by their predominantly medusa form and ability to swim. Example: Aurelia aurita (moon jelly).

Anthozoa

Consists of corals and sea anemones, which exist only as polyps and can form symbiotic relationships with zooxanthellae. Example: the staghorn coral.

Ctenophores

Also known as comb jellies; they have eight rows of cilia and are mostly bioluminescent. They typically have a complete digestive system.

Platyhelminthes (Flatworms)

These are bilateral, unsegmented worms that can be free-living or parasitic. Common examples include planarians (Turbellarians) and liver flukes.

Turbellarians

A class of flatworms that are mostly free-living and aquatic. They have a soft body and are often predatory or scavengers.

Flukes

Parasitic flatworms that have complex life cycles; often affect the liver or blood of their hosts. Example: the sheep liver fluke.

Annelids

Segmented worms that have a closed circulatory system and can be aquatic or terrestrial. Common examples include earthworms and leeches.

Polychaetes

A class of annelids characterized by having many bristles and often living in marine environments. Examples include bristleworms.

Mollusks

A diverse group of invertebrates with soft bodies, often protected by shells. They include gastropods, bivalves, and cephalopods.

Gastropods

A class of mollusks that undergo torsion during development, includes snails and slugs. Example: the common garden snail.

Bivalves

Mollusks with two shells, primarily filter feeders. Examples include clams, oysters, and mussels.

Cephalopods

Highly intelligent mollusks known for their complex behaviors and advanced nervous systems. Examples include octopus, squid, and cuttlefish.

Chitons

Mollusks with eight overlapping calcareous plates on their back, which can cling to rocks in intertidal zones.

Arthropods

Invertebrates with exoskeletons, segmented bodies, and jointed appendages. They are the most diverse animal group. Examples: insects, arachnids, and crustaceans.

Crustaceans

A subgroup of arthropods, primarily aquatic, with two pairs of antennae and usually a hard shell. Examples: crabs, lobsters, and shrimp.

Copepods, amphipods, isopods

Different orders of crustaceans; copepods are small planktonic crustaceans, amphipods are shrimp-like, and isopods include terrestrial and marine species like pillbugs.

Barnacles

Sessile crustaceans that attach themselves to surfaces and filter feed with their specialized appendages.

Krill

Small, shrimp-like crustaceans important in the marine food web, especially in polar ecosystems.

Decapods

Crustaceans with ten limbs, including crabs, lobsters, and shrimp; most are important in commercial fisheries.

Hermit crabs

Crustaceans that inhabit shells of other organisms for protection. Example: coenobita.

True crabs

A group of crustaceans known for a short, wide body and a significant variety of species. Example: blue crab.

Lobsters

Crustaceans characterized by long bodies and tails, primarily found in freshwater and marine environments. Example: Homarus americanus.

Merostomata (Horseshoe crabs)

Ancient marine arthropods with a hard shell, primarily living in shallow coastal waters.

Pycnogonids (Sea spiders)

Arachnids adapted to marine environments, characterized by long legs and a small body. They often feed on soft-bodied invertebrates.

Echinoderms

Marine invertebrates with radial symmetry and a water vascular system, such as sea stars and sea cucumbers.

Sea stars

Echinoderms known for their star shape, they have regenerative abilities and tube feet for movement.

Brittle stars

Similar to sea stars but have long, flexible arms and can move more quickly.

Sea urchins

Round echinoderms characterized by a hard shell covered in spines; they graze on algae.

Sea cucumbers

Soft-bodied echinoderms that can be found on the sea floor and often play a role in nutrient recycling.

Invertebrate

An organism that lacks a backbone or vertebral column; includes all animals except for species in the subphylum Vertebrata.

Radial symmetry

Body plan in which body parts are arranged around a central axis; seen in cnidarians and echinoderms.

Bilateral symmetry

Body plan in which the left and right sides are mirror images; seen in most animals including arthropods and vertebrates.

Sexual reproduction

Reproduction involving the fusion of gametes from two parents, leading to offspring with genetic diversity.

Asexual reproduction

Reproduction without the fusion of gametes; common methods include budding and fragmentation.

Complete digestive system

A digestive system with a separate mouth and anus; allows for more efficient processing of food.

Incomplete digestive system

A digestive system with a single opening that serves as both mouth and anus; found in some cnidarians.

Closed circulatory system

A circulatory system where blood is contained within vessels; allows for efficient circulation and higher metabolic rates.

Open circulatory system

A circulatory system where blood is not always contained within vessels and bathes the organs directly; characteristic of many invertebrates.

Exoskeleton

A hard outer structure that provides protection and support; typical in arthropods.

Endoskeleton

An internal skeleton that supports the body; present in vertebrates and some echinoderms.

Hydroskeleton

A fluid-filled cavity that maintains shape and support; characteristic of soft-bodied invertebrates like jellyfish.

Herbivory

Feeding on plants; common in many marine and terrestrial herbivores.

Carnivory

Feeding on other animals; typical behavior in many predator species.

Parasitism

A relationship where one organism benefits at the expense of another; includes many types of flatworms and some arthropods.

Planktonic larval development

A life stage where many marine organisms have free-floating larvae that develop into adult forms, often settling on substrates after metamorphosis.