A Spineless Column by Ronald L. Shimek, Ph.D.

Pills, Parasites, and Predators; Isopods in the Reef Aquarium

Ah... Reef aquarium keeping, the hobby where beautiful animals live in artful simulations of nature and provide enjoyment and pleasure for their owners, as long as those beautiful animals are well fed... by feeding on the life blood of those same reef keepers.

Here are links to a couple of accounts of aquarists becoming food: 1 2

Recently, a number of reef aquarists have discovered the "delightful" situation wherein THEY have become the food of choice for some of their pets. This was not a situation that they planned for, nor was it one that they anticipated, but it occurred nonetheless. When we bring some reef rubble into our systems, we drag along any of the animals in that rock at the same time. Most of these animals are either helpful or at least benign. A few cause problems.

And, those problems may be very serious, indeed, as will be illustrated by examining representatives of one of the most successful groups of marine animals, the isopods. Isopods form a group of about 5,000 species, in what taxonomists call the Order Isopoda, of the Superorder Peracarida, of the Class Malacostraca of the Subphylum Crustacea within the Phylum Arthropoda. Whew...

Commonly called isopods, pill bugs, fish lice and rolly-pollies, these animals are found in all parts of the marine environment. While many isopods are free living and harmless or beneficial to reef tank denizens, a large number are either predatory, or parasitic, or dangerous to other reef aquarium animals.

Some Characteristics:

To put the group in its proper perspective and to comment on it in a meaningful manner, I must present some information about Isopods. Taxonomists place these particular animals within the animal group characterized by having an exterior skeleton and jointed legs. This large group of animals is called The Phylum Arthropoda, and includes such varied animals as crabs, insects, and spiders. This is an immense group with well over 1,000,000 species, and as result of the human desire to put everything into pigeonholes, quite an impressive amount of nomenclature has been developed so that various researchers may discuss these animals without having to relate to cumbersome descriptions. Instead, we all have to use a lot of cumbersome terms...

Our sea-going bugs are placed within the Arthropod subdivision called the "Subphylum" Crustacea because they are animals that have two pair of antennae, a particular kind of larva called a nauplius, and at least some of their legs have two main branches. Insects with only one pair of antennae and spiders with no antennae at all, for example, are not Crustaceans.

Like insects, isopods have a body divided into three major regions, the front region or head, a middle region called the thorax, and a posterior region referred to as the abdomen. However, unlike most insects, these regions may not be easy to distinguish, particularly when looking down on the animal from above. Isopods often appear to be relatively similar from front to back, and often it is only by examining the animals from the underside that the regions become clearly evident.

The head is a single structure, but it is composed of the fusion of several segments and bears six pairs of appendages. The often-fearsome jaws and the two pair of sensory antennae are the most obvious of these. Most shallow-water marine isopods have a pair of compound eyes located at either side of the head, on the upper surface; unlike the compound eyes found in shrimps and crabs, these eyes are not on stalks. Very deep-water forms, or species that live in caves, are often blind.

The thorax, or middle body region, consists of eight segments, with the male's genital opening on the eighth, and the female's on the sixth segment. The limbs of these thoracic segments are very different from those of the last body region, which is called the abdomen. Thoracic appendages are generally narrow legs used only for walking, and food manipulation, not for swimming or other forms of locomotion. The abdomen has six appendage-bearing segments. The appendages found under the abdomen are typically used for swimming and as gills, and often are fan or paddle-shaped. The abdomen has six segments, all of which may be more-or-less fused with the final "extra" segment called the telson, depending upon the particular subgroup of isopods. Gills are typically found under the abdomen, and the heart is located in the abdomen above the gills. The basic segmentation patterns, including the number of appendages and segments in the various body regions, and the position of the gonadal openings, are the same in isopods as in shrimps and crabs, and serve to unite the isopods with them and several other groups in the taxonomic group called the "Class Malacostraca."

The Class Malacostraca is subdivided yet further, and the isopods belong in the group called "The Superorder Peracarida." This very large group combines several forms of very different appearances; for example, the shrimp-like mysids, the laterally flattened amphipods, the cylindrical tanaids, and the isopods, which are flattened from top to bottom, are all peracarids. None of these animals has free-living larvae. They all develop directly from eggs into small juveniles. The females possess a brood-pouch-like structure, called the marsupium, located on the bottom of the thorax. Additionally, unlike shrimps or lobsters, most of the animals within this group lack a carapace or shell-like extension covering the head and thorax region.

For comparison, here are links to information about...

Mysids, Amphipods and Tanaids.

The isopods are grouped to form their own group of peracarids, called the ORDER ISOPODA. The name "Isopoda" means "similar feet' (iso = alike, podus = foot) and refers to the walking legs and feet of the thorax, and indeed there is very little variation between all walking legs on any given isopod. This characteristic alone can be used to separate them from the other peracarids, but there are other differences as well.

Within this general description, the isopods have radiated into all sorts of odd shapes and species with all sorts of life styles. Isopods are one of the few crustacean groups to become successful in terrestrial environments as well as marine and freshwater ones, and many aquarists will recognize them as "pill bugs" or "rolly-pollies." Nonetheless, most of the Isopod diversity is within the marine realm, and as reef aquarists, we occasionally run across a bit of that diversity

The Isopoda is a huge and diverse assemblage including many parasitic and terrestrial forms, as well as an amazing diversity of free-living marine species. Although most are minute, some very large isopods are found in the deep ocean. Individuals over 30 cm long are routinely collected from the Gulf of Mexico, and rumors persist of sightings of animals over 1 m in length.

More information on Isopods may be found by following this link.

Reef Aquarium Isopods

Figure 1. A composite diagram of the common reef aquarium species of free-living isopods, shown from above, without their legs. Sand skaters are not included. The relative sizes are correct, although many of the Munnids and Idoteids seen in tanks are smaller relative to the other types. The largest of these would be about 2 inches long, but smaller individuals of even the largest species are occasionally found. Identifying characteristics are indicated by the arrows. These are the shapes of the body segments in the Munnids, the size of the eyes and shape of the final segment in the Idoteids, the size and shape of the eyes in Cirolanids and Aegids, the two pairs of large and evident antennae in the Aegids, and the body shape and male tail appendages in the Sphaeromatids. Drawings modified from Brusca, 1980; and Kozloff, 1996.

At least seven different general kinds of isopods are found in coral reef aquaria, and if the frequency of reports is any indication, their abundance has increased drastically within the past couple of years. Biologists often classify similar species together in a genus, and similar genera together in a family, and each of the main types of isopods is from a different family. The isopods in the family Idoteidae of the suborder Valvifera are characterized by having a long narrow body, often with a long tapering final segment or telson. Idoteids are generally considered to be algae-eating animals, and many are often colored to match the algae they are found on. In reef aquaria they are benign or helpful algal grazers. Large Idoteids are rare in aquaria. The specimens I have seen in aquaria have been very small, most likely juveniles of larger forms, around 1/10 of an inch or less in length. Adults of most reported species are much larger, up to a couple of inches long. It is possible that these small aquarium specimens are introduced with live rock, and most simply do not survive in our systems.

More information and pictures about Idoteids may be found by following this link.

Some very tiny isopods appear to be relatively common in aquaria, and possibly are very widespread, although unreported in the hobby. These are members of the family Munnidae of the Suborder Asellota. Some Asellotids get large, and intertidal ones that act much like non-flying cockroaches are found in the high intertidal zone of areas in Florida and along the Pacific Coast of Vancouver Island. However, the ones found in aquaria seem to be tiny little bugs, and are quite likely mis-identified by many aquarists as amphipods, even though the body is quite characteristically flattened in the isopod manner. They are distinguished from other isopods by having both pairs of antennae enlarged and evident and by rather "separate-appearing" segmentation when viewed from above.

Some detailed diagrams of Munnids are found here.

Individuals from another group of tiny isopods are sometimes found, but they are very hard to see, and may be fairly common. These are members of the Family Serolidae. They are tiny flattened bugs that are often about the size of a large grain of sand. They appear to flit effortlessly over the sand surface and, in fact, almost appear to be skating on it. This locomotion gives the group its common name of Sand-Skaters.

More information about sand skaters is found here and here.

Figure 2. Female Epicaridean isopods, Bopyroides hippolytes on the sides of two candy striped shrimp, Lebbeus grandimanus, from the N. E. Pacific. The large white lump on each animal's left side is the female parasitic isopod fastened in the shrimp's gill chamber sucking its blood.

The rather odd epicaridean isopods are rarely, but regularly, reported from aquaria. "Epi" means "on" and "carid" means shrimp, so these animals are aptly named, as they are isopods most frequently seen as parasites on shrimp. The males are very small, look like normal isopods, and are almost never seen. The parasitic females may be quite large, and look like large lumps or tumors found on the side of a shrimp. The female lives fastened onto her host's gills or upper leg segments and sucks the host's blood. It lives under the carapace, which becomes deformed over the parasite. The parasite is generally not recognizable as an isopod, but rather looks like a large white lump on the side of a shrimp.

Epicarids appear to be rather well-adapted parasites, and do not seem to harm their host much, in spite of their rather ghastly appearance. They may be found occasionally on the sides of all of the various shrimp imported for the hobby. They appear to be most frequently seen in peppermint shrimp, Lysmata wurdemanni.

More information on Epicaridean isopods is found by following this link.

The three remaining types of isopods are all highly mobile, very active animals. One group is comprised of the harmless scavengers in the group called the Family Sphaeromatidae. One very large group of isopods, the Family Cirolanidae, is comprised of carrion-eating scavengers and parasites. The parasites may prey on and suck the blood of some fishes. Both types of Cirolanids may be found in aquaria, and actually they are quite common. The final type of isopod that we need to be concerned about will kill and eat fishes. These are animals from the Family Aegidae.

Sphaeromatids are small bugs, generally less than a centimeter in length. They are common scavengers in many shallow marine environments, including coral reefs, and they are harmless to reef aquarium inhabitants. They can be recognized immediately by a couple of distinctive characters. First, each individual has the capability to roll into a ball-like terrestrial pill bug. None of the other isopods likely to be found aquaria will be able do that. Second, when examined with a hand lens or magnifying glass, the last pair of appendages of the males are expanded and extend to the rear, like small rudders or the fins on a 1959 Caddy. Females lack these extensions, but if some of the isopods are seen with them, that is usually a good indication that the rest of them are also Sphaeromatids.

More information about, and some gorgeous pictures of, Sphaeromatids may be found by following these links: 1 2 3 4

Figure 3. Comparison of Sphaeromatids (top) Cirolanids (bottom), views from the side and top. The easiest distinguishing characters are the large obvious eyes on the Cirolanids, and the large obvious tail appendages on the male Sphaeromatids.

These final two groups of isopods, the Cirolanids and Aegids are so similar in appearance that it typically takes an expert to distinguish them on the basis of a few minor structural details. Not being an isopod expert, I will treat them together. These are flattened, streamlined crustaceans with a smoothly rounded or tapered and pointed front end. Their very large eyes are found laterally on the head and may occupy as much as half of the space of the head. They are strong, very fast swimmers and have no obvious trailing appendages. The thoracic legs are robust and end in very sharp gripping claws. Aegids differ from Cirolanids in several small details; probably the most evident is that both pairs of antennae are often evident and visible, IF you can find one that holds still long enough to observe that trait.

The taxonomic Family Cirolanidae is huge, consisting of several dozen genera, and probably many hundreds of species. Many of them are generally benign animals that are obligatory carrion-feeding scavengers. Some of these scavengers have been found in aquaria, and appear to be very well adapted to being part of "the clean-up crew." Many of the rest appear to be capable of scavenging when such food is available, but they will occasionally swim up into the water and attack fish, fastening on and sucking their blood. Finally, several species appear to be more-or-less obligate blood-sucking parasites of fishes, although some may be able to live for extended periods by scavenging some dead food. Within the last couple of years, some of these latter species have been seen in aquaria with alarming frequency. In many cases, these infestations appear to be the result of a pregnant female that enters the aquarium and then drops her brood of 10 to 30 young, all of which are immediately hungry for a nice meal of fish blood. A hobbyist will see the alarming sight of one or more fish with from one to twenty blood-sucking parasites on it. Often the isopods are nocturnal, and unless the aquarist is alert, they may not notice the parasites, as the bugs drop off the fish shortly after the lights go on and find shelter in the rocks. Prolonged exposure to such densities of blood suckers WILL kill fish.

The only way to rid an aquarium of these animals is to catch them all, which although tedious, is possible. Generally, this involves using a sacrificial fish, usually something easy to catch and moderately large. A yellow tang is a good choice for this because the fish's color pattern allows easy determination of the presence of the parasite. The other fish in the tank are collected and removed to a quarantine tank, and the "bait" fish is introduced. This fish is checked periodically and, if the parasites are seen on it, it is netted and removed to a flat surface where it may be immobilized with a wet paper towel. The parasite is removed with a pair of tweezers or forceps. The fish is then returned to the tank, and the procedure repeated. Often the parasites are nocturnal; consequently, the fish will have to be examined before the lights come on. The aquarium is probably free of the parasites if none are seen on the fish for a month or so after the last one has been collected.

More about Cirolanids may be found by following these links: 1 2

A nice picture of a scavenging Cirolanid is found here.

Aegids are "bugs from hell" as far as the aquarist is concerned. They are like predatory Cirolanids, only more so. Large Aegid isopods in the North Eastern Pacific have been seen to wait on the bottom until an acceptable fish, such as a small salmon, swims overhead. The isopod then swims rapidly up and fastens on to the fish, and proceeds to eat its fins and tail. The bug then slices open the fish and eats all its blood, proceeding then to eat the lateral muscle bands and, when they are done, they discard the guts and skeleton.

Parasite, indeed!

Figure 4. This is an Aegid similar to those collected in aquaria photographed in nature in the waters of northern Puget Sound, Washington, USA. This animal, Rocinela belliceps, was about an inch long and is capable of killing small salmon, and making the lives of some of my students miserable (see below).

The same species will fasten onto larger fish and eat its way into a major blood vessel where it will remain for some time sucking blood and eating tissue. When sated, it will excavate its way out of the host and swim away.

Tropical species show up somewhat frequently in reef tanks either riding on a fish or in a piece of live rock. Often the first the aquarist knows of them is when they see the isopod on a fish. Murphy's Law is active here; the bug will never be on a cheap or expendable fish. The problem is how to remove the isopod from the aquarium. If the bug stays on the fish, the fish needs to be captured. This happened to me several years ago, and the fish it was on was a Mandarin dragonet. Imagine trying to catch this fish in a fully set up 100 gallon aquarium! Of course, I couldn't catch the fish until the next day, and the isopod was still on him. There wasn't much left of the fish. Even I, with my notable lack of coordination and dexterity, can catch a fish that is half eaten.

If you can catch the fish, the isopod may be removed with a pair of forceps. Carefully!!! Upon removal, the fish should be isolated in a hospital tank, and treated with antibiotics until the wound heals. The bug may be disposed of. Carefully!!! About 15 years ago, I had a student who was holding in her clenched hand a 1.5 inch long Aegid. The bug cut through the flesh of her palm, dug in, and started to eat HER. Her vocal response was rather impressive. So was the tenacity of the isopod, it was HARD to remove!

If you notice one of these animals in your tank, and it leaves the fish, there is almost nothing that may be done to catch it. They are very fast and quite capable of avoiding a net. And if it is a pregnant female (and remember, all female isopods have brood pouches), and the brood hatches, you have REAL problems. There are only three courses of action in this situation; and I truly am not jesting about these responses. The first is to remove all the fish from the tank and wait the two or three months until you are certain that all the isopods have died from starvation. The second solution is to effectively nuke the tank. Remove all live rock and discard it as the isopods may hide in it and, as some of the isopods bury in the sand, you should also remove and discard the sand.

You may, of course, take the third option and do nothing. The most likely outcome in this situation will be that the isopods will kill your fish one by one. These isopods are masterfully designed predators. Hope fervently that you never have to deal with them.

Here are some more data on Aegids.

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

Selected References:

Brusca, R. C. 1980. Common Intertidal Invertebrates of the Gulf of California. University of Arizona Press. Tucson. 513 pp.

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

Kozloff, E. N. 1996. Marine Invertebrates of the Pacific Northwest. University of Washington Press. Seattle. 539 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.


All photos and images courtesy of Ronald L. Shimek.

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Pills, Parasites, and Predators; Isopods in the Reef Aquarium by Ronald L. Shimek, Ph.D. - Reefkeeping.com