By Hugh R. Williams
Of the more than one million described species of animals in the world, approximately 95% fall under the heading of invertebrates. There are more anatomical differences between jellyfish and insects than there are between insects and elephants. The invertebrates comprise an extremely diverse assemblage of animals, from single celled protozoans to insects, crustaceans and starfish. There are more than thirty major groups, or phyla, of invertebrates. For our purposes here, we will discuss the general characteristics of just a few groups common along our coast and at Gray's Reef. A companion Gray's Reef poster to this section entitled "Invertebrates of the Reef," can be obtained from Gray's Reef National Marine Sanctuary, 10 Ocean Science Circle, Savannah, GA 31411.
SPONGES
Sponges belong to the phylum Porifera, which means "pore bearer," and constitute the simplest of multicellular animals. They are sessile organisms, which means that they live attached to objects, and therefore are not able to move about. They have no true tissues or organs, rather various individual cells perform the life functions of the sponge. Most sponges are marine; only 150 or so freshwater species have been identified out of some 5,000 species within the group.
Structurally, sponges are supported by a matrix of fibrous material and interwoven spicules (tiny spikes), usually made of calcium or silica. This matrix is what remains behind when sponges are found on the beach or prepared and sold for market. The living part of the sponge exists within this matrix. Water enters the sponge through many pores, and is helped along by specialized cells which sweep the water through interior chambers and canals. The water circulating through the sponge supplies oxygen and food, and removes wastes as well. Sponges pump a surprising amount of water in a day. For example one type of sponge that measures 5 inches by 1/2 inch (10cm. x 1cm.) will pump as much as six gallons (22.5L) of water per day through its chambers.
Sponges eat very fine detritus particles and organisms such as bacteria and plankton. Sponge beds provide a significant amount of habitat for other animals, and they also may be responsible for filtering, i.e., cleansing, vast quantities of water in given areas.
JELLYFISH, ANEMONES AND CORALS
These animals, along with the less familiar hydrozoans, belong to the phylum Cnidaria. They are the first group of animals to display differentiated tissues, limited organ development, and nerve cells. They possess an internal digestive cavity and a mouth, which permit a greater range of food sizes than exists with the sponges. The mouth is encircled by tentacles which aid in capturing and ingesting food.
Cnidarians may either be free-swimming, jellyfish-like animals, or solitary, sessile polyps, such as anemones. The hydrozoans and corals are colonial cnidarians with many individual polyps sharing a common structural form. Anyone who has encountered a sea nettle or man-o-war in the ocean is aware of the stinging properties of these animals. The tentacles are lined with special pressurized cells which eject a harpoon-shaped barb which embeds itself in the tissue of its prey. This barb contains a toxin which immobilizes and kills small creatures, so they can be eaten by the jellyfish. Most jellyfish toxins are, at worst, irritating to creatures as large as humans, but those of the Pacific sea wasp and the Portuguese man-o-war can be fatal. The jellyfish and anemones feed on crustaceans and small fish, whereas the corals and hydrozoans feed principally by filtering plankton from the water.
FAN, TUBE AND BRISTLE WORMS
This group of common marine worms known as polychaetes is related to the common earthworm. Both belong to the phylum Annelida. Unlike the earthworms, their body segment's possess lateral "legs" called parapodia. The fan and tube worms construct burrows in the sediment or construct rigid limy tubes on the surface of solid objects, including the shells of other animals. Many of these worms possess mucous-covered tentacles which surround the mouth for capturing food. The bristle worms, which resemble caterpillars, live either on the ocean bottom, inside other animals (sponges, clams), or they swim or burrow through the sediments.
Polychaete worms feed in many different ways. Some are filter feeders, others are predators, detritus feeders, scavengers or deposit feeders. The latter consume sediment (sand and mud), take nutrition from whatever organic material is present, and eject the castings (cleansed sediment). On some beaches, deposit feeders occur in densities of up to 2,500 worms per square foot, and process as much as 14,000 tons of sand on a mile of beach in a year!
MOLLUSKS
Mollusks contain the second largest number of species of invertebrates after the arthropods. Seashells are among the most familiar remains of marine invertebrates. Mollusks include snails (gastropods), clams (bivalves), squid and octopus (cephalopods), and less familiar chitons and tusk shells. Most mollusks possess hard shells which remain long after the actual animal dies and are the delight of many beachcombers.
Gastropods are found in marine waters, fresh water and on land. Included in the group are limpets, snails (whelks, conchs, olive shells) and slugs. Nearly all possess a rasp-like feeding organ called a radula and have a single curled shell (except for slugs which have no shell). They may be herbivores, carnivores, scavengers, deposit feeders or filter feeders. Many have a discernible head, antennae and eyes. Along our shores we commonly find lightning, channeled and knobbed whelks, augers, moon snails and lettered olive snails. The deeper waters of Gray's Reef are home for the tulip whelk and helmet snails.
The bivalves include the oysters, clams and mussels. Their shells consist of two parts hinged at the back. They can completely close to protect the soft parts of the animal's body. Nearly all are filter feeders. Clams, angel wings, cockles and their close relatives bore into the substrate (sand. mud. wood) and are able to move about. Oysters cement themselves into place and become permanently fixed in one spot (sessile). Mussels are also sessile using a thread-like material to anchor themselves to roots of plants or other surfaces. Scallops sit on top of the substrate, and are able to swim away from predators.
The cephalopods, which include the squid, octopus and nautilus, possess the most highly developed nervous systems and are the largest of all invertebrates. Most cephalopods utilize jet propulsion to swim by expelling water from the body cavity through a siphon. The octopus spends most of its time crawling along the bottom and uses jet propulsion only to escape predators; squids have fins for swimming but can jet propel themselves as well. Cephalopods are predators and use their tentacles to capture and manipulate prey. Squid possess ten tentacles whereas octopus have eight. They both possess a parrot-like beak and inject a poison to subdue prey. Giant squid may attain a length of 16 meters (over 50 feet).
ECHINODERMS
Sea stars, sea urchins, sand dollars and sea cucumbers all belong to the group called echinoderms (meaning spiny skin.) They are found only in marine waters and most are bottom dwellers. Their bodies are divided into five parts arranged in a disc, sphere or cylinder. They also possess an internal water-vascular system (water-filled chambers and canals) typically used for locomotion. Most are scavengers or carnivores and feed on worms, mollusks, corals, sponges, crustaceans and even fish.
The sea stars possess sucker-like tube feet to help move them across the bottom and aid in capturing prey. The tube feet are located on the bottom of the five arms radiating out from a central disc. A sea star can exert enough prolonged suction on a clam or oyster to open its shell. It then inverts its stomach inside the clam and digests the soft tissue.
Sea urchins and sand dollars are, respectively, spherical or flattened animals that possess movable spines. Urchins usually live on firm bottoms and scrape food off the surface. Sand dollars are burrowers and feed on detritus found in the sediments.
ARTHROPODS
This is the largest group of all animals, containing some three-quarters of a million described species. The major unifying trait within the group is the presence of jointed legs. Included within the group are crustaceans, spiders, insects and several less familiar organisms. Spiders and insects are found primarily on land and in fresh water. Crustaceans, on the other hand, are most common to marine and estuarine environments.
The most common marine arthropods are the decapod crustaceans, which include the crabs, shrimp and lobsters. The prefix deca means ten, and pod refers to legs, so these are ten-legged animals. They also possess a calcified exoskeleton, and have two pairs of antennae. Some decapods are predators; Some, however, are scavengers, and others are herbivores and filter feeders. True crabs differ from other decapods in that their abdomen folds up under the carapace rather than extending like those of shrimp and lobsters. Hermit crabs are an exception because they lack a hard calcified abdominal shell. They utilize the abandoned shells of snails to house themselves for protection. As they grow they must continually move to larger and larger shells to accommodate their increased size.
Barnacles are also crustaceans. Similar to shrimp in many ways, they differ from most other crustaceans by living within a hard, cone-shaped, calcareous shell. As larvae they drift in ocean currents and eventually cement themselves head first onto a hard object. Their feet are modified and feather-like, and are used to sweep fine particles into their mouths.
The horseshoe crab is not really a crab at all. It is more closely related to spiders and scorpions. Horseshoe crabs live in shallow water and plow through the loose sediment while feeding on mollusks, worms and algae. They are also capable of swimming and use their page-like gills to propel themselves through the water.