Copepods

views updated

Copepods

Characteristics of free-living copepods

The parasites

Place in the food chain

Copepods are pale or translucent crustaceans, measuring between 0.04 mm to several millimeters long. They have adapted to many different habitats; while they usually live in saltwater, copepods can live in lakes and ponds as well. Furthermore, they have different modes of locomotion: some can swim purposefully but others are planktonic, floating with the current. Scientists generally distinguish between two basic forms of copepods, free-living and parasitic.

The phylum Arthropoda is the largest phylum in the animal kingdom, containing more than one million identified species. The true number of arthropods may actually be in the tens of millions. Within this phylum, the subphylum Crustacea contains at least 45,000 species. Although the classification of the group has been somewhat fluid, six classes are generally recognized. Copepods belong to the class Maxillopoda and the subclass Copepoda, containing ten orders and more than 14,000 identified species. Three of these orders—Calanoida, Cyclopoida, and Harpacticoida—are primarily free-living and are present in huge numbers.

Characteristics of free-living copepods

Given the incredible number of species, the physical structure of copepods varies greatly. However, the free-living forms of copepods have certain physical traits in common. For instance, the body is usually short and cylindrical, composed of a head, thorax, and abdomen. The lower part of the copepod’s head is generally fused with its thorax; the front of its head often juts forward, like a tiny beak. Its thorax is divided into about six segments; each segment is connected to two appendages.

Generally, a free-living copepod has two pair of antennae and a single eye. The first pair of antennae is larger and has bristles. The male copepod can be distinguished from female because its antennae are slightly different from those of the female, modified for holding her during copulation. The free-living copepods’ limbs are used for movement, sometimes with the help of the antennae. Its thin abdomen lacks limbs, except for the caudal furca—an appendage

akin to a tail. Its tail has bristles similar to those found on its primary antennae. Some tropical forms of copepods actually use their bristles to facilitate flotation.

The parasites

There are over 1,700 species of parasitic copepods. As larva—or nauplia—most look and act like typical copepods. It is only later, when the parasites reach various stages in their development, that they begin to attach themselves to a host creature and radically change in appearance. In fact, many of the adult parasitic copepods are incredibly deviant in physical structure from their free-living relatives. Indeed, it is nearly impossible to find a single trait that is common to all copepods, both free-living and parasitic. One of the only characteristics that they tend to share is that the females have egg sacs which secrete a sticky liquid when the eggs are laid, gluing them together.

Furthermore, parasitic copepods are vastly different in appearance from other crustaceans. In general, adult parasitic copepods are shapeless, having neither limbs nor antennae and sometimes no body segments. Because these creatures start their lives as free-living animals, scientists infer that their ancestors were freeliving and that they only evolved parasitic behavior after their environments dictated it.

Parasitic copepods can inflict severe damage on their hosts. This damage is often worsened by the presence and infestation of accompanying fungi.

Place in the food chain

Free-living copepods form a crucial link in the food chain and are often assigned the role of “primary consumers.” Although some large forms of copepods are predators, free-living copepods are generally herbivores, feeding only on plant plankton which they filter from the water. Specifically, they eat small plant plankton and are, in turn, eaten by larger animals, like herring or mackerel. More fish and other aquatic animals feed on copepods than any other kind of animal in existence. One of the dominant forms of animal plankton, some scientists estimate that there are more copepods on the planet than all other multicellular animals combined.

Order Calanoida

Calanoids are of major importance to the commercial fishing industry. Like other copepods, this species filters minute algae from the water and eats it in large clumps. In turn, these copepods are eaten in large numbers by fish, such as herring and salmon. Calanoids thrive close to the surface of large expanses of water, both seas and lakes. Anatomically, they are easy to recognize. The fused head and thorax of an individual calanoid is oval and clearly separated from its abdomen. The multi-segmented first antennae are long, about the same length as its body. The abdomen is much slimmer than its thorax.

Like other animal plankton, calanoids allow themselves to float with the current, although they can use their first antennae to swim upward in the water. Unlike most other genera of copepods, calanoids move from the surface to the depths of the water each day. At dawn, they sink to several hundred feet in the water; at dusk they rise to the surface again. There are several theories explaining this activity. The most likely reason is that they are escaping the dangerous ultraviolet rays of the sun. Another theory is that they are avoiding predators. In any case, this activity facilitates their fairly even distribution in the sea, since currents at various depths often run in different directions.

Order Cyclopoida

All of the free-living cyclopoida are almost identical to each other in physical appearance. Their antennae are shorter than those of the calanoids, growing about half of the length of their bodies. Their bodies, relatively pear-shaped, have two clearly divided regions: the head and thorax in the front; and the last segment of the thorax fused with the abdomen in the rear. Their front portions narrow slowly into their abdomens. This order contains many marine forms and numerous freshwater representatives. They are rarely planktonic, rather they tend to swim

KEY TERMS

Caudal furca— An appendage on the free-living copepod, resembling a tail, that is attached to its abdomen.

Free-living copepod— The copepods that does not attach itself to a living host but, instead, feeds on algae or small forms of animal life.

Nauplia— Larva of either free-living or parasitic copepods; both kinds of larvae are similar in appearance.

Planktonic— Free-floating; not using limbs for locomotion.

Thorax— The area just below the head and neck; the chest.

near the bottom of the water, never migrating upwards. They thrive in small pools of water with large amounts of aquatic vegetation. Some of the larger species are carnivores, eating insects and other crustaceans.

There are also twelve or more families living in relation to other animals: either as hosts to parasites or as parasites themselves. Some freshwater species are important as temporary hosts to certain forms of worms that are parasitic to man. Other species are parasites on mollusks, sea anemones, or sea squirts. One specific group of parasitic cyclopoids live in the mouths or on the gills of certain fish, like frog-mouths. While the female can grow to 0.8 in (2 cm) long, the male never surpasses about 0.04 in (0.10 cm). The jaws of the female are shaped like sickles, enabling her to cling to her host and eat it.

Order Harpacticoida

Harpacticoid bodies, which rarely exceed 0.07 in (2 mm) in length, are not clearly divided into three distinct regions, and they vary dramatically in shape. Some harpacticoids are long and snake-like, while others are flat. Their antennae are very short and forked.

Harpacticoids are planktonic, generally living in the muddy or sandy areas. Instead of swimming, these copepods hop, using the appendages on their thoraxes in combination with a rocking motion of their bodies. While most harpacticoids feed on organic waste or algae, some species are predators, swarming over small fish and immobilizing them by eating their fins.

Order Monstrilloida

Some of the most advanced species of parasitic copepods are found in this order. These copepods are worm parasites. Their nauplii appear quite typical, but have no stomach. When they find a suitable host, they shed their outer skeleton and all of their appendages and become a mass of cells. In this simple structure, they are able to reach the worms’ body cavity. Once inside their host, these creatures form a thin outer skeleton; the copepods spend most of their lives without a mouth, intestines, or an anus. When they mature, they look like free-living copepods.

See also Zooplankton.