Marine Invertebrates

Look ma, no bones!

Marine Invertebrates

Animals that lack backbones are known as invertebrates. Over 98% of species on Earth are invertebrates that rely on other strategies than a backbone for support such as hydrostatic pressure, exoskeletons, shells, and in some, even glass spicules. Some invertebrate phyla have only one species, while others like Arthropoda include more than 83% of all described animal species with over a million species. The most common marine invertebrates are sponges, cnidarians, marine worms, lophophorates, mollusks, arthropods, echinoderms and the hemichordates.


There are between 9,000 and 15,000 species of sponges classified under the Phylum Porifera. Sponges are relatively simple animals that originated with the first animal life in the Precambrian times.

The anatomy of a typical sponge is organized so that flagella inside the sponge pull water into small holes (ostia) in the body and expel waste through larger holes (oscula). Sponge species have a variety of body plans that provide structure, including support by organic fibers (Class Demospongiae - 90% of sponge species), calcareous spicules (Class Calcarea ~400 species), and siliceous spicules (Class Hexactinellida) or combinations of these.

The body plan of a sponge has adapted to filter small food particles from the passing water allowing them to reside in most habitats, including polar shelves and submarine caverns that often contain very few nutrients.

Sponge, HondurasLike other animals, sponges were found to also grow extremely slowly in cold waters such as those of the Antarctic. Age estimates based on growth rates of one glass sponge (Scolymastra joubini) in the Ross Sea were between 15,000 and 23,000 years, which means that specimen appears to be the longest-lived animal on earth yet recorded. Sponges are often studied by scientists to find clues about the first life forms on Earth with more than one cell.

Sponges are hermaphroditic and are able to reproduce both sexually and asexually. Most sponges usually reproduce by sexual reproduction, where sperm cells (spermatocytes) develop from choanocytes (collar cells) and eggs develop from oocytes. When environmental conditions are favorable, spermatocytes are ejected in out-going currents and the eggs, once fertilized inside the sponge in some sponges, develop into flagellated larva that swim about as plankton until they find a suitable place to settle and grow into adults. Asexual reproduction occurs when favorable environmental conditions deteriorate and includes both regeneration (regenerating from fragments), budding (groups of cells differentiate into small sponges that are then released externally or expelled through the central canal (oscula)), or the formation of gemmules ("survival pods" of unspecialized cells that remain dormant until conditions improve and then either form completely new sponges or re-colonize the skeletons of their parents).

Sponges are eaten by chitons, snails, nudibranchs, turtles, fish, and insects. They provide a home to sea anemones, polychaetes, octopuses, copepods, zoanthids, shrimps, brittle stars, amphipods, barnacles, and fish. There are numerous symbiotic relationships between animals and sponges.

Sponges that are composed of organic fibers (demosponges) have been used by humans for thousands of years for cleaning and other purposes. Sponge diving has declined significantly due to overfishing and most sponges these days are now synthetic.


The Phylum Cnidaria ( (“Ny-DARE-eeya”) consists of about 10,000 species of "simple" animals found only in marine habitats and includes Class Anthozoa (corals and sea anemones), Class Hydrozoa (hydrozoans), Subphylum Medusozoa: Class Cubozoa (box jellyfish), Class Scyphozoa (jellyfish), and Class Staurozoa which contains Order Stauromedusae (stalked jellyfish). Phylum Cnidaria may also contain Family Polypodiidae and Family Tetraplatidae. Species in cnidaria have special stinging cells called cnidocytes (see figure). Cnidarians evolved during the Precambrian era and are some of the earliest multicellualr life forms known.

Most cnidarians have a very basic body plan which includes a digestive cavity with one opening. This opening functions as both the mouth and anus for the organism. The only true organs in cnidarians are the gonads. Most cnidarians are symmetrical, an observation referred to as “radial symmetry.” Cnidarians also have an ectoderm (tissue that covers the outer body surfaces) and an endoderm (inner layer of cells forming the gastrointestinal and respiratory tracts, and inner organs). The ectoderm is connected to the endoderm by a gel-like substance known as the mesoglea. Cnidarians use a nerve net and very basic receptors for impulses to move. Oxygen is taken in directly from the water through the tissues.

Organisms in cnidaria capture and kill their prey using cnidocysts, or stinging cells, around their mouth which send out stinging barbs which immobilize their prey and help protect against predators. Once prey is captured the tentacles move it into the central gastrovascular cavity where it is digested. Waste is then expelled back through the mouth.

The four classes of cnidarians are the Anthozoa, the Hydrozoa, the Scyphozoa, and the Cubozoa. Anemones, corals, and sea fans are in class Anthozoa, which was the first to diverge throughout evolution. Portuguese man-o-wars and obelia are examples of animals in Hydrozoa, jellyfish are in class Scyphozoa, and box jellies are in class Cubozoa.

Cnidarian species have a variety of life-cycles. Some alternate between being free-swimming medusae and asexual polyps depending on their environment. In some groups like Anthozoa, organisms never make it to the free-swimming medusae stage, but instead live their whole lives as a non-moving polyp. Organisms in the groups Scyphozoa and Cubozoa spend most of their lives in the medusal stage. Medusae can measure anywhere from a few millimeters to 30 meters long including the tentacles. Some, like the Siphonophores, are individuals but can live in colonies and appear as one organism.

Marine Worms

Marine worms can be placed into more than ten different phyla and come in a variety of colors, shapes, and sizes. Marine worms are often confused with other animals with thin and long bodies. Most marine worms are grouped into the Annelids, a group that includes the Polychaetes (bristle worms), Oligochaetes, Hirudinae, and the Eunice aphroditois. Polychaetes are most often found near the shoreline and swim or crawl using a pair of legs found on each segment of their body. The Oligochaetes, which include earthworms, are mainly found on land and the subclass Hirudinae include leeches that usually live in freshwater environments. Some marine worm species, such as the bearded fire worm, can deliver a nasty burning sting to humans when handled.

The body structure of an annelid consists of a front end with a prostomium, also referred to as a significantly defined head. Most annelids have two pairs of eyes, three antennae, a pharynx or proboscis used to eat food and tentacle-like cirri for probing the surrounding area. An example of the biodiversity of worm species is the Sipunculid also known as the peanut worm. This worm digs itself into a hole underneath rocks, eats organic material, has no segments and looks like a peanut when it pulls its proboscis into itself.

In general, marine worms live underneath rocks near the edge of the ocean, in algae, or anywhere there is mud or sand. Species of marine worms can be ringed, segmented, or flat and include tube-digging worms, burrow-dwelling worms, ribbon worms, and peanut worms.

Some common annelids include the tube-making Galeoloaria, the stinging fireworm, the short scale worm, and the huge Eunice aphroditois. Tube worms actually make a tube with a hard shell and retreat into the shell when threatened. The Christmas tree worm has many brightly colored feather-like tentacles shaped somewhat like a Christmas tree that is used to filter food from the water.

Last modified: Wednesday, 13 January 2016, 9:49 AM