Bitty Bugs: Copepods in the Reef Aquarium

Humans, in their infinite arrogance, are prone to think of themselves as the masters of creation, and the most important animals on the planet. This may be; but a reasonable argument may be made for considering man as a transitory blip on the radar screen of Earth's life forms, and that the real movers and shakers of the world are those animals that have multiple jointed legs and which wear their skeleton on the outside. Such dominant animals would certainly be the arthropods.

On land, the dominant forms of animal life, in terms of the number of species and in terms of mass of the animals, are certainly the insects. Literally millions of different kinds exist, and the sheer amount of insect flesh is almost incalculable. Arthropods dominate the oceans as well but, interestingly enough, not the insects. For some obscure evolutionary reason, insects are almost totally absent from the seas. There about 30 species of oceanic sea skaters, all in the genus Halobates, and numerous insects living in intertidal habitats, but no insect truly lives a submerged marine existence. This is really rather odd, as insects dominate underwater habitats in fresh water much as they do on the land.

Link to image of Halobates, with information on the life history and reproduction:
Link to information about the evolution of Halobates:

For whatever reason, the arthropod component of the oceanic realm is crustacean. Although insects and crustaceans shared a common ancestor, probably sometime over 400,000,000 years ago, the two groups have diverged significantly since that time. Crustaceans and insects are very different types of animals. Insects are conservative in their basic body plan; they have a consistent body plan throughout the group with little real variation compared to the crustaceans. All insects have, for example, three body regions, three pairs of legs on the middle body region, no appendages on the abdomen, and internal organ systems of the same basic pattern. Crustaceans, on the other hand, come in a very large variety of shapes, sizes, and body forms, some having hundreds of appendages, others having, effectively, none.

The diversity of crustacean shapes and sizes notwithstanding, the numerically dominant animals in the world's oceans belong to one group, the copepods. In fact, one copepod genus, Calanus, likely contains more individual animals over one tenth of an inch in length than any other animal genus. At one time, it was rumored that one marine biology professor put one of his more troublesome grad students to work calculating the number of tons of molts produced annually by one species, Calanus finmarchicus. The student labored mightily and brought forth an answer of 1011 tons per year. As each individual Calanus finmarchicus is quite small, no larger than a small grain of rice, an annual production of one hundred billion tons of molts per year implies huge populations of these animals.

Links to images of Calanus and Calanoid copepods:
Calanus finmarchicus:
Female calanoid showing egg sac:
Beautiful imagery showing appendages in colors, as well as some great information on sensory adaptations:

In fact, copepods are the dominant animals in all the world's seas, and although they may be found in densities as low one per ten or a hundred cubic meters of water, average densities are generally considered to be on the order of one to ten per cubic meter, and often their densities are much greater. This doesn't sound like much until you start to calculate the number of cubic meters of water in the oceans, at which point you realize it sounds like a very big number, indeed.


Copepods are small crustaceans, generally less than a couple of centimeters in length, in their own taxonomic group called, not terribly surprisingly, the Class Copepoda of the Phylum Arthropoda. There are probably close to 7500 species with about 2000 of these being parasitic. Some of these parasites may be quite large, for a copepod; over a foot long, in some cases.

Unlike a crab or shrimp, copepods do not have a carapace or "shell" covering the front part of the body. Also, unlike crabs and shrimp, they have only one eye, and it is on the midline of the body very near the front end. Most other groups of aquarium crustaceans have at least two eyes, and these eyes may (in crabs or shrimps) or may not (in amphipods and isopods) be on stalks.

Link to an image of the front end of a calanoid copepod showing the single pink eye in between the bases of the first antennae:

The copepod body is divided into three parts; a front section called the "cephalosome," a middle section, called the "metasome," and the posterior abdomen, sometimes called the "urosome." As in all arthropods, the body is formed of discrete segments or sections. In primitive arthropods, for example, such as the brine shrimp Artemia salina, many of the body's segments look much like the segments in front of or behind it. In advanced arthropods, such as the crabs, there is much fusion and loss of segmental integrity, and it can sometimes be hard to determine where one segment begins or ends. Copepods fall somewhat in the middle of these extremes. The cephalosome is comprised of the five head segments that characterize all crustacea, but also has one or two "thoracic" segments incorporated into it. The metasome, or thorax, has four or five segments, and the abdomen generally has four or five segments. Except in the abdomen, each segment bears a pair of limbs.

Copepods follow the basic crustacean pattern of limbs or appendages. From the front to the back, they have the following sequence of appendages, one pair per basic body segment:

In the Cephalosome:

  • The first antennae, also known as antennules.

  • The second antennae, also known simply as antennae (if the first antennae are called antennules.

  • The mandibles, or jaws. In some predatory Calanoids, these may be tipped with opal (amorphous silica), presumably to harden the jaws so that they may more effectively crush their prey.

  • The first maxillae, also known as the maxillules,

  • The second maxillae, also known as maxillae (if the first maxillae are called maxillules.

In the Metasome:

  • Four or five pairs of thoracic appendages, all of more-or-less similar architecture, except in the male, where the last pair of thoracic appendages is modified as copulatory organs.

In the Abdomen:

  • There are no appendages. This is different from the situation found with the crabs and shrimps, where there are paired appendages under each abdominal segment.

Figure 1. Side of view of a harpacticoid copepod showing the body regions.

The internal morphology of all copepods is relatively simple. The mouth, located in front of the mandibles on the animal's bottom surface and facing toward the rear, is covered by a moveable flap called the labrum. The esophagus is short and passes from the mouth forward a short distance in the body, and then curves up toward the back and enters the stomach. The stomach, or foregut, is relatively capacious, and it joins to the midgut which extends through most of the animal. The hindgut is short and found only in the last segment or so of the body. At the juncture of the stomach and midgut are found a pair of digestive sacs, or caeca.

Feeding and digestion in copepods is an interesting process, and totally unlike that found in most other animals. Movement of the food through and within the gut is done by muscular control, not by the action of cilia, as in many other animals. There is no mucus produced in the gut of copepods. As food enters the gut, it is completely encased in a thin, flexible, and permeable bag made of chitin secreted by the foregut. This bag looks rather like a thin cellophane sack. This bag defines, at first, a food pellet, and then a fecal pellet. As food passes into the midgut, digestive fluid is released, and the gut contents become acidic. There is a wide array of enzymes together in the fluid: proteases to breakdown proteins; lipases and esterases for fat digestion, along with fat emulsifiers; and carbohydrases of both disaccharides and polysaccharides, for the digestions of sugars and starches. No chitinases are found in any arthropod, and cellulases are absent in most of them as well. This digestive fluid bathes the food pellet in its bag, and digested food is squeezed out of the bag by the muscular contraction of the gut walls. This digested food mixes with other fluids in the midgut. Periodic contractions of the midgut force this gut fluid into to the digestive caeca where absorption occurs. When the digestive process is complete the food pellet is moved along to the hindgut where it is compacted into a fecal pellet and expelled.

Figure 2. Harpacticoid copepod from an aquarium. Note the red eye between the bases of the antennae. The gut is faintly visible as a tube in the center of the animal.

Link to a page showing the acidity of the midgut of a harpacticoid using a pH indicator dye. Nice photos! :

The tubular digestive caeca are bathed in blood as copepods have an open circulatory system with a clear blood, and respiratory pigments are generally absent. This blood carries the digested food molecules throughout the body. The large pelagic calanoid copepods have a small heart which pumps blood to the head, but the harpacticoid copepods common in reef aquaria are truly heartless animals. In these small bugs, movements of the body and gut suffice to move the blood around.

The copepod life cycle is basic and similar in all the groups. The adults copulate and this is often followed by deposition of the eggs into one or two egg sacs carried by the female. In some groups, particularly the parasitic forms, the eggs may be shed into the sea. The eggs hatch to release the typical crustacean larva called a nauplius. The nauplius undergoes four or five molts, becoming larger and adding segments and appendages with each molt. Consequently, there are five (in harpacticoid copepods) or six (in pelagic calanoid copepods) naupliar stages. After the last naupliar stage, the subsequent molt remodels the animal into a juvenile copepod, called a copedidite.

Link to an image of a copepod nauplius:
In this image, the gut is visible as the dark tube in the center of the animal. The mouth is evident as the dark spot at the top of the gut tube, in between the bases of the middle pair of appendages (the second antennae).
Link to a different nauplius image, with the single red eye visible:

Copepidites have distinct segmentation, which is lacking in the naupliar stages, and the body regions become apparent. There are four more copepidite stages, after which the animal molts into an adult, and becomes reproductive. There are no further molts after this stage. For the larger pelagic calanoid copepods, the eggs are shed in the spring, and the first half of the life, until the first copepidite stage, occurs in the first summer. The animal overwinters as this juvenile stage. When the spring plankton bloom occurs, the animal rapidly grows and passes through the remaining molts to reach a functional adult within a few weeks. These animals typically live about two years, although they may live longer. In our tanks, the small harpacticoid copepods may pass through all of these molting stages within a few days. This short adult to adult period, coupled with the basic high reproductive capacity of the group means that these harpacticoids have a truly amazing capability for rapid population explosions.

Copepod Diversity

There are four distinct groups of free-living copepods, and several types of parasitic ones. The parasitic copepods are bizarre animals living in or on all sorts of animals. Some of them are rather well-studied as they parasitize salmon and other important food fish. Rarely, one or more of these parasites makes its way into an aquarium on a wild caught fish or within some other organism, such as a tunicate. Those copepods parasitizing fishes are usually very evident and easily removed. Those in other animals generally remain unnoticed.

The four groups of free-living copepods are the Calanoida, the Cyclopoida, the Harpacticoida, and the Misophrioida. The last group is very uncommon and represented by only a few species, and I will not discuss it further. The calanoids are quite distinctive. They are generally relatively large animals; the body may be the size of rice grain or larger. The first antennae are very large, with more than 15 segments, and are often highly elaborated with large bristles and other structures, thought to increase water resistance and thus retard the animal's sinking rate. Calanoids are exceptionally important ecologically in that they are major components of almost all marine food webs. Although they may be kept in aquaria, they need specialized plankton tanks, such as plankton kreisels, to keep them from impacting against the walls. Generally, their culture is limited to a few progressive commercial aquaria such as the Monterey Bay Aquarium.

Cyclopoids and Harpacticoids both may be found in reef aquaria, but cyclopoids are not particularly common. They may be quite difficult to distinguish from one another. Generally harpacticoids have a smoothly tapering body, whereas the calanoids have a pronounced constriction just before the last thoracic segment. Often, as well, the front end of a harpacticoid has a small pointed projection, the rostrum, which is lacking in most cyclopoids. Cyclopoids are typically planktonic and are very common in freshwater ecosystems.

Link to a diagram of a Cyclopoid copepod:

Harpacticoid copepods are more commonly benthic, living on the bottom, or epibenthic swimming just above the bottom. Often they are referred to as demersal zooplankton, which means pretty much the same as epibenthic zooplankton. Demersal or epibenthic - the terms really don't matter. What matters is that these are some of the most important food items in natural reefs and in aquaria. They are the food of many small-mouthed corals, small fishes, and some other benthic animals, such as zoanthids, and small sea anemones.

In reef aquaria, harpacticoids are commonly the first small "bugs" seen on the walls of the aquarium shortly after it is set up. Often dense swarms of them may occur in the water, particularly if fish have not been added. Once the other benthic fauna starts to become abundant, the numbers of the harpacticoids drop as predators on them become more abundant and common. Nevertheless, unless something becomes highly amiss in the aquarium, they never disappear. They can be very fecund, and their life cycles are short, often taking only a few days to go from adult through eggs and immature forms to adult. They are major constituent animals of the detritivore guild, and will eat small particulate debris. Additionally they eat bacteria and microalgae that they scrape off sand particles. In aquaria, in addition to the adults being eaten, their eggs and larval stages are food for small-mouthed corals such as Acropora.

While the vast majority of harpacticoids are free-living and, in aquaria, beneficial, a few species are found living on and, presumably, parasitizing corals. These bugs are sometimes seen by aquarists living on some species of small mouthed corals, such as Acropora species. They are generally bright gold, often with a bright red patch, and with this coloration they closely mimic some stenothoid amphipods found on similar corals. The stenothoids are probably harmless commensals of cnidarians in all seas, while the harpacticoids appear to be parasitic. The significance of the similarity of color pattern is unclear, and it really appears that virtually no scientific research is being done with regard to either group or their effects on the corals.

Figure 3. Acropora bugs. The upper right image is a harmless stenothoid amphipod, found on Acropora. The other three images are of a harpacticoid copepod, probably Tegastes, which parasitizes coral. Note the close similarity of shapes; and although distinct differences may be seen, they are subtle. Probably the best character to distinguish the two crustaceans is the number of eyes. Copepods have only one, amphipods have two. These animals are about 0.01 inch long.

click here for full sized picture
Figure 4. Blood sucking copepod parasite (Lepeophtheirus sp.) fastened to a sculpin just behind the eye. The white strands are egg sacs.

Figure 5. Small copepods, probably harpacticoids, found living on a sea star, Linckia. They were about 1/32 of an inch long.


In both the natural reef and our artificial ones copepods are common, and important animals. The large planktonic copepods characteristic of the open ocean are lacking in our systems, but many of the other types of copepods are commonly found. Their populations in our tanks may be immense; in a large tank, the harpacticoids probably number in the millions. Their contribution to the tank's energy and nutrient flux is considerable and of great importance to the well being of our aquaria. This notwithstanding, we must also be aware that, as with most large animal groups, not all of the animals likely to be found will be desirable. Some copepods are very well adapted to the parasitic mode of life and these animals, as well as their beneficial brethren, often are found in our aquaria.

If you have any questions about this article, please visit my author forum on Reef Central.


More information on crustaceans, in general, and copepods, in particular, may be found in the following references:

Bliss, D. E. (Ed.): 1982-1985. Biology of the Crustacea. 10 volumes. Academic Press, New York.

Kozloff, E. N. 1990. Invertebrates. Saunders College Publishing. Philadelphia. 866 pp.

McLaughlin, P. A. 1980. The Comparative Morphology of Recent Crustacea. W. H. Freeman and Co. San Francisco. 177 pp.

Ruppert, E. E. and R. D. Barnes. 1994. Invertebrate Zoology. Saunders College Publishing. Philadelphia. 1056 pp.

Schmitt, W. L. 1971. Crustaceans. University of Michigan Press. Ann Arbor. 204 pp.

Schram, F. R. 1986. Crustacea. Oxford University Press. New York. 700 pp.

ReefKeeping Magazine™ Reef Central, LLC. Copyright © 2002

Bitty Bugs: Copepods in the Reef Aquarium -