By Shannon OBrien
Copepods are small, microscopic crustaceans that make up the subclass Copepoda. They are found in both marine and fresh water environments, but the majority of the 10,000 species exist in marine environments (Britannica Online 2000). Copepods are able to survive in a variety of different places from the surface down throughout the depths of the ocean to the plankton, benthic, and littoral zones of freshwater lakes (Pennak 1953). Some can even survive in damp moss found in forests, moisture in the base of leaves and in humans (Britannica Online 2000). Within the subclass Copepoda there are 10 different orders, which are Calanoida, Cyclopoida, Harpacticoida, Monstrilloda, Mormonilloida, Siphonostomatoida, Gelyelloida, Poecilostomatoida, Misophrioida, Platycopioida (Staton 1999). When discussing Copepoda as a group it is hard to generalize about their characteristics because there are so many variations among them. However, in most areas, basic conclusions about copepods behavior and structures can be reached.
Although the structure is not the same in all species, most are a variation on the basic accepted structure. Most copepods are between .5 mm and 2mm long. In general the body of a copepod is divided into two sections: the mestasome, the anterior end which is often wider and the urosome, the thinner and usually shorter posterior end. According to Pennak (1953) most free-living copepods are clearly segmented, and can be divided into the two sections but the same is not true for some parasitic copepods, which are greatly modified and specialized.
The mestasome consists of the head, which is usually fused to the first and/or second thoracic segment, and the remaining 4 to 6 thoracic segments. The head and the fused appendages are usually referred to as the cephalothorax (Hickman 1973). The common feature uniting all the copepod orders is a single simple eye in the middle of the head, at least in the larval stage (Museum Victoria, 2000). The free living forms have a cephalosome, a shield over the head and some thoracic segments. The majority of the appendages are found in the mestasome. Attached to the head are five appendages, the antennules, antennae, mandibles, first maxillae, and the second maxillae. The antennules are usually the largest and their function varies depending on the species. In most cases it has sensory organs, aesthetasks, attached to it, making the antennules important for sensory, but in other orders like Calanoida, Cyclopoida, and Harpacticoida the antennules on the males have knee-like joints that allow for movement important for grasping onto females during mating (Fitzpatrick 1983) (Davis 1955). The second antennae are smaller and important in swimming and balancing. The remaining appendages on the head are important for food collection, grinding, and processing (Fitzpatrick 1983). The other important appendages located in the metasome are the swimming legs, usually 5 pairs, which are attached to each of the unfused thoracic segments (Davis 1955).
The urosome, which is made up of the abdomen usually has 1 to 5 segments and no appendages except for the structures off the posterior end of the body called the caudal rami. The caudal rami have setae that may act as balancers and stabilizers while moving (Pennak 1953). The other important structure included in the urosome by some authors is the genital segment. According to Fitzpatrick (1983), the actual section that the genital segment is included in varies between authors because it is actually the last thoracic segment fused to the first abdominal segment. But Fitzpatrick (1983) refers to the entire fused unit as the genital segment and includes it in the urosome. Generally, although there are many similarities among the body structures of all the different orders, there are many variations of the fusion of the segments that distinguishes between different species (Hickman 1973).
Among all the copepods, marine and freshwater, there are both free-living ones and parasitic ones. Among the free-living copepods, they are both filter feeders, grazers, and predators. They all have the same basic mouth parts but they have been modified for biting, seizing, filtering and scraping. Their food consists of unicellular plants and animals, bacteria, and organic debris (Pennak 1953). In copepods that are filter-feeders, the setae on their front appendages create a current of water around the body which is then directed into the filter chamber by the maxillipeds, where the food is filtered out. While feeding, if food is abundant, they store excess amounts of fat within their bodies in case of there being a shortage of food in the future as well as helping with the floatation of their bodies in the water (Davis 1955). DeMott and Moxter (1991) after performing experiments with copepods and Cyanobacteria also concluded that copepods can distinguish between what they eat. When copepods were put with the Cyanobacteria, they were size-selective eaters and avoided eating any that had toxins. These results also suggest that they have an important impact on the continued existence of toxic Cyanobacteria in aquatic environments.
The parasitic copepods, on the other hand, attach themselves to other organisms and obtain nourishment from the hosts tissues. Different parasitic copepods affect a lot of different organisms ranging from marine ostracods, fish, to whales (Britannica Online 2000). However, they are rarely abundant enough in nature to cause problems, but where they do have a large effect is in fish hatcheries. Because the fish are abundant in such a small area, it is easier for the parasitic copepods to find hosts and quickly infest the whole hatchery pond (Pennak 1953). They also pose a threat to humans because they can transmit the guinea worm (Britannica Online 2000).
Reproduction in copepods is usually sexual reproduction and does not vary that much for the different free-living copepods. The main differences among different genera are the breeding periods and behaviors. Most species of copepods, both male and female, have paired genital pores. The pores are located on the genital segment for both male and females. The male and female clasp together with the aid of the males antennules and in some species the fifth leg. During this period of clasping which varies among species, the male transfers the sperm in a packet called the spermatophore. The female holds the spermatophore in the seminal receptacles, off the ventral side of her genital segment. Fertilization of the eggs occurs while the eggs leave the females reproductive tract after male and female have separated (Pennak 1953). At this point the female copepod either releases the eggs or carries them in one or two ovisacs that are attached to the genital segment. In some orders of Copepoda, however, males are rare or do not exist like in the common genus Mormonilla and sexual reproduction does not happen. Instead all the eggs develop into new females through parthenogenesis, which is when unfertilized eggs develop into adults.
Once the eggs are released, there are a series of stages until a sexually mature adult develops. The general life cycle of a copepod includes the egg, six nauplius stages, five copepodid stages, and the adult. In the first nauplius stage after the egg hatches, the nauplius is a small active larva with 3 appendages, which resemble the antennules, antennae, and mandibles. During the later nauplius and copepodid stages, the larva gets bigger, acquires more appendages and starts to resemble an adult copepod until finally emerging as an adult. This cycle that includes 5 nauplius stages and 5 copepodid stages is for copepods in general, but it has been determined that in many parasitic species especially but also free-living species the cycle is abbreviated (Pennak 1953). The time it takes to complete the cycle also varies depending on the species and environmental conditions (Pennak 1953). In some species adaptions have been seen to help survive unfavorable environmental conditions that increase the length of the life cycle. One example is the production of resting eggs in some species. These resting eggs have especially thick walls so they can stand up to harsh conditions and stayed preserved and unhatched until the conditions improve (Pennak 1953). The other adaptation made by some species, which draws out the length of their life cycle is the formation of encystments usually in the copepodid stages to protect themselves from harsh conditions (Hickman 1973).
Some factors that seem to impact freshwater copepods are temperature and altitude. Temperature seems to affect the distribution and activity of copepods. Some species are more apt to be found in colder, deeper lakes whereas some are only found in warmer lakes (Pennak 1953). Pennak (1953) also cited Coker (1933) and Aycocks (1942) experiment that suggested the size the copepods grew to be was directly related to the temperature they were exposed to. Altitude was also suggested to have an impact on the coloring of the species. Hickman (1973) stated that most species are grayish and transparent but some located in higher altitudes were orange, red, or other colors. Byron (1982) supported this in his investigation in which he concluded that much of the variation of pigmentation is due to environmental conditions along with metabolic stimulation. However, many of the variations can be explained just by observing the difference in water temperatures. He stated that copepods had the darkest pigmentation in colder water.
Finally copepods are an important ecological part of both marine and freshwater environments. They are very important sources of food because of their large numbers and high nutritional value. In freshwater they are not the primary source of food for fish and other organisms because Cladocera are also present (Pennak 1953). However, in marine environments they are the predominant source of food for organisms like jellyfish, basking sharks, and whales (Davis 1955).
Copepods are microscopic crustaceans found in many different environmental niches including marine, freshwater, and even some moist terrestrial areas. In both marine and freshwater environments they are important to the structure of the food chain. There is a lot of variation in characteristics between the different species. Copepods generally are free-living and make-up a large amount of the zooplankton in bodies of water but parasitic species also exist.
Byron, E. 1982. The Adaptive Significance of Calanoid Copepod Pigmentation: A comparative and Experimental Analysis. Ecology 63(6):1871-1886.
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Davis, C. 1955. The Marine and Fresh-water Plankton. Michigan State University press.
DeMott W. and F. Moxter. 1991. Foraging Cyanobacteria by Copepods: Response to Chemical Defense and Resource Abundance. Ecology 72(5): 1820-1834
Hickman, C. 1973. Biology of the Invertebrates. The C.V. Mosby Company, St. Louis.
Fitzpatrick Jr., J. 1983. How to know freshwater Crustacea. Wm. C. Brown Company Publishers, Iowa.
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