Invertebrates

Invertebrates comprise a richly diverse group of organisms, with many forms exhibiting brilliant colors and remarkable adaptations. The greatest diversity can be found in the ocean, but many species, especially insects, occur on the land as well. Here, we survey a number of major groups that might be seen by the casual observer.

Members of the phylum Porifera are commonly known as sponges. The name comes from the Latin words ferre and porus, "to bear" "pores". Sponges are sessile (don't move) animals which look like a bag full of holes. The bag wall is made of two layers. The area between the layers is called the mesohyl and the cavity inside is called a spongocoel. They can be from 1cm to 2m in height and either one long tube or a number of branching tubes.

The sessile nature of these animals restricts their ability to catch food, consequently they are filter feeders. Choanocytes on the inside of the sponge are like little fingers that make a current to draw water and food into the animal through the porocytes which make the pores in the wall. Amoebocytes, cells which travel in the mesohyl, take food that has been captured by the choanocytes, digest it and tranport the energy and material to other cells.

Sponges are hermaphrodites which means they produce both sperm and eggs. Sperm is made from choanocytes and eggs are made from the amoebocytes. Most of the eggs produced by a sponge are fertilized by it's own sperm but some sperm may float into a neighboring sponge and fertilize it's eggs. The eggs are contained in the mesohyl where they develop into flagella covered larva. These larva then swim out of the sponge searching for a hard place to attach to. Most larva do not survive to become adult sponges.

Members of the phylum Cnidaria are the sea anemones, corals, jellyfish and others. There are two forms of Cnidarians which can be thought of as a sac with tentacles. There is a single opening to the sac which serves as the mouth and anus, the tentacles are around this opening. There are two forms of the Cnidarians; polyps and medusas. The polyps are sessile and have the their opening facing up while the medusas float through the water with there openings facing down.

Anemone
Photo Floyd Hayes

There are about 10,000 species of Cnidaria.

The tentacles of the cnidarians are used to capture prey from the water. These tentacles are armed with cells called cnidocytes containing nematocysts which are used to sting prey.

Three classes of animals are in the Cnidarian phylum and are classified according to their life cycles. The class Hydrozoa alternate between the polyp form and the medusa from. Scyphozoa are jellyfish which exists primarily as medusas. Finally, the class Anthozoa include sea anemones and corals which are primarily polyps. The calcium carbonate external skeletons of corals are what we see and call coral.

The common name for members of the Phylum Ctenophores is comb jellies. Ctenophores get their name from the comblike cilia plates that they use for locomotion. They are the largest animals to use cilia for movement. They are not very large in size or number.

Only about 100 species are known ranging in size from 1 to 10 cm. Most are spherical but a few elongate forms can reach up to 1 m in length. If you get the chance to see one closely you might observe a rainbow of colors shimmering in the cilia plates. This is cause by the refraction of light when the cilia move. Most ctenophores are also bioluminescent and can be seen at night to glow blue or green.

In spite of their small size and delicate appearance the comb jellies are a force not to be reckoned with. An accidental introduction of one species in the Black Sea during the 1980's devastated the fisheries industry within 10 years. Ctenophores are carnivores that eat zooplankton. They do this by dragging long retractable tentacles and capturing prey out of the water. In the Black Sea they successfully out competed the larval fish by eating most of the zooplankton before the fish eggs had hatched.

Crustacea is a subphylum of the phylum Arthropoda and contains about 55,000 species. Members include the crabs, lobsters, shrimps, and barnacles all of which can be seen around San Salvador. Crustaceans have exoskeletons and are segmented like insects. They have many appendages which come in pairs and can be regenerated. Two pairs of these appendages are antennae and three pairs form the mouth parts.

Five species of crustaceans are endemic to Lighthouse Cave on San Salvador. One species of Isopod, one species of Ostracod and three species of copepods. The Isopod Bahalana geracei lives in the substrate in the caves 2 meter deep marine water. The females are 15mm long and the males are 8mm. They can't roll into a ball like the familiar relatives, the pill bugs. The Ostracod Spelaeoecia barri are much smaller, from 1.17mm to 1.34mm and live in a foot thick layer of bat guano in the cave. The three Copepods are; Enantiosis cavernicola, Speleoithona salvadorensis, Paralaophonte echinata.

The fireworm, seen at right, might look soft to touch but be careful. The bristles can create a burning on your skin.

Another form of polycheates are the tube dwelling worms. They build tubes from mucus and sand to live in. Below you can see the fanworms emerging from their tubes to trap microscopic food particles from in their feathery tentacles.

There are over 50,000 species in the phylum Mollusca. Most mollusks have hard calcium carbonate shells for protections but some, like octopuses and squid, have internalized their shells or lost them altogether. Other mollusks include clams, snails, slugs and chitons. There are three main parts to every mollusks; the foot used for movement, the visceral mass contains most of the internal organs, and the mantle secretes the shell and folds over the visceral mass for protection.

The flamingo tongue seen at right, is a mollusk that can be seen in San Salvador corals in water down to 30 feet. These animals are about an inch long and have white shells. The bright, colorful spots that you see are actually tissue used for breathing that the snail wraps around its shell. Flamingo tongues depend on healthy coral for survival because it is their main food.

Bark Scorpion
Photo William K Hayes

Scorpions are invertebrate members of the phylum Arthropoda and class of Arachnida which includes spiders and ticks. As arachnids they have 8 legs.

The bark scorpion is present in the Bahamas. The species found on San Salvador is probably Centruroides guanensis. It can be found under bark or stones in lowland forest and arid coastal plains. The sting is sharp and can last 1-5 hours.

Insects belong to the class Insecta which is in the phylum or Arthropoda. They are distinct from the arachnids because they have only 6 legs. Insects are the most diverse and numerous kind of animals containing more individuals than all other classes of animals combined.