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

Odd Pods, Tanaids in Reef Aquaria

Most aquarists are familiar with some of the crustaceans found in their aquaria. Other than the shrimp or crabs that are added to aquaria by the reef hobbyist, a number of these bugs seem to show up more-or-less by themselves. Although they actually migrate in on live sand or live rock or, on decorative animals such as corals, their appearance is largely unpredictable.

Often the first of these to be seen in reef aquaria are the harpacticoid copepods. These are minute crustaceans often visible only as tiny white dots moving on the aquarium walls. Sometimes they also form swarms in the water, but as these animals are more-or-less natural fish food, such aggregations are very rare in tanks with fish in them.

There are three groups of larger crustaceans that commonly have representatives found in our aquaria. These are the Mysidacea; also known as "the possum shrimps" or "mysids," the Amphipoda; also known as "scuds," "side-swimmers," or "amphipods" and the Isopoda; also known as "pill bugs," "rolly-pollies," or "isopods."

These latter three groups have a lot in common and are actually probably closely related. Taxonomists put them all in the group called the "Superorder" Peracarida, in the great assemblage of animals referred to as the Crustacean Class Malacostraca. Taxonomic "Classes" are groupings that are subdivisions of larger units, each called a phylum. A phylum is easiest conceptualized as a basic and unique way of making an animal, or put another way, "a body plan." Classes are subdivisions of that body plan that are coherent unto themselves. As an example, all birds can be grouped into the Class Aves of the Phylum Chordata. Other groups of Chordate species at the Class level include Mammalia, the mammals, and Reptilia, the reptiles. Likewise there are many other Crustacean classes, such as the Class Cirripedia or the barnacles, and the Class Copepoda or the copepods. However the largest crustacean class, and really the most diverse crustacean group in forms of body structure, is the Class Malacostraca.

About this time in any discussion of crustaceans, as somebody who tries in my feeble way to convey information, I feel the beginnings of dread. Trying to discuss crustaceans, "marine bugs," is hard. It is hard not because they have too few characters for easy identification but because, in a very real sense, they have too many characters, and additionally they are often small and unfamiliar.

Crustaceans have a hard exoskeleton, and it has a lot of architecture. And EVERY little tiny bit of it has a name... And NONE of those names are familiar to anybody other than a specialist in the given group. So, if I am going to discuss some small bug found in our tank, I have to have to try to make sense of a lot of nomenclature for you. This is not an easy task. I can't simply ignore all these names and terms. While there is a LOT of what appears to be "useless" terminology, some of it actually conveys useful information. So bear with me here, things will get worse as I introduce a lot of terms, but then they should get better as I pass into the discussion of the particular small bugs in question, the tanaids.

Figure 1. A male tanaid crustacean crawling on the sediment surface. This is an individual of the NE Pacific species, Leptochelia savignyi, photographed in nature in Barkeley Sound, British Columbia, Canada. The animal is about 5 mm (1/5 inch) long. The large claws characteristic of male tanaids are evident. The whole animal is visible the pair of light dots at the lower left image edge are the terminal ganglia of the animal's nervous system.

Malacostracans, Shrimps and Similar Animals

The variation within this large Crustacean group, the Class Malacostraca, notwithstanding, it does form a real cohesive animal group, readily defined and easily characterized. A bit of background is necessary first, though. Arthropods such as Crustaceans are one of four major animal groups in which the animals are composed of serially replicated body units called "segments." This replication of segments is probably best seen in something like a millipede, but all arthropods are composed of segments. Each segment has a pair of appendages, which may or may not be like those appendages on the segment in front or in back of it. The sequences and groupings of the segments and the modifications seen in them and the appendages are the major characters used to distinguish one small buglet from another. And each and every one of these little bloody whiskers on the side of each bug has a name. Not only that each section of each leg has a name. And often each part of each section has a name or term that describes it. So... you have to look past the fog of names to see the patterns that define the animals.

Malacostracan crustaceans have a body divided into a head region, a middle area called a thorax and rear region called the abdomen. A carapace or shell generally covers the head region and thorax (this is a bit of an "iffy" character; as we will see, many malacostracans don't have this). The thorax consists of eight segments, with male genital opening on the eighth segment, and the female opening on the sixth segment. Additionally, thoracic limbs, often called "walking legs" are very different in shape from the paddle-like appendages of the abdominal region. The abdomen generally consists of six appendage bearing segments, and ends in a tail fan. Finally, most of the animals called Malacostracans have compound eyes, often on stalks. When all of these above characters are assembled in their most generalized aspects, the resulting "animal" is a shrimp-like animal. So malacostracans, are the shrimps and their kin.

The Peracarids, The "Unshrimplike" Shrimps

However, several groups of these "shrimps" are decidedly unshrimp-like. These groups, taken together, are placed in yet another taxonomic group, referred to as the "SUPERORDER" PERACARIDA, (pera = "near or about", carida = "shrimps"). These are the animals similar to, but not quite like, shrimps. This very large group combines several forms of very different appearances, for example, the relatively shrimp-like pelagic mysids and the decidedly not-shrimp-like, flattened, benthonic isopods. That the Peracarida is a successful group is evident in its species richness. There are many thousands of species in the group, and interestingly enough, aside from the rather ubiquitous copepods, crabs and shrimp, virtually all the crustaceans found in marine aquaria are peracarids; and most of them get there uninvited.

Peracarids together share some rather interesting characteristics. Generally, they are small animals; if you wish to examine a peracarid, you will need a good magnifying glass or hand lens. A stereo-microscope, sometimes called a dissection microscope, will work even better. Being good Malalcostracans, the peracarid body is divided into three regions, a head, thorax, and abdomen. However, the degree to which these regions may be differentiated varies significantly between the groups. When viewed from above, the head, bearing compound eyes, is generally noticeable at the front end of the animal and is followed by the thorax. Differentiation between the thorax and the abdomen is often difficult, when viewed from above, as these regions often seem to grade into one another. However, when viewed from the underside, the different shapes of the appendages on the thorax and abdomen makes the determination easy.

Many Peracarida have a carapace, but it is small and may be hard to see, particularly on a smaller animal. A carapace is a fusion and extension of the exoskeleton of the head area and the first one or two thoracic segments to form a "shield like" plate that surrounds and covers the top and sides of the body. In the crabs and true shrimp, the carapace covers the entire head and thorax region, and the segments of those areas are visible only from below. That is not the case with the Peracarids; at best, the carapace involves only the head and first couple of thoracic segments and the remaining thoracic segments are visible from above and the sides. In amphipods and isopods, the carapace is lacking altogether.

No free-living peracarid has a swimming larva. In fact, they have no larval development at all. The larva characteristic of the crustaceans, the nauplius, exists only as a stage in the prehatching embryology of these animals. Instead of a larva, they undergo a complex embryology to develop directly into small juveniles. The females possess a brood-pouch-like structure, termed the "marsupium." This is found on the underside of the thorax, and the fertilized eggs are deposited into it. Additionally and not surprisingly, the females often have specialized behaviors involved with care of the developing embryos. Generally, they have a small number of eggs in each brood, 10 to 30, depending on the species.

The Peracarida may be thought of as being composed of several distinct, probably closely related, groups of similar species. The taxonomic hierarchy progressively includes relatively similar species together. Several similar species may be grouped into a genus. Several similar genera (the plural of genus) may be included into a larger group called a family. Several similar families may be placed into a still larger group called an order. Sometimes several very similar and obviously closely related orders are consider to form a "Superorder." The Peracarida is such a superorder, it encompasses all of the species within seven orders. The three largest of these groups, the ORDERS MYSIDACEA, ISOPODA, and AMPHIPODA have many species, and at least a few species from each one of these orders are commonly found in reef aquaria. Two more groupings, the ORDERS TANAIDACEA and CUMACEA, each have fewer members, and species from them are found in aquaria less commonly than the "big three." Finally, for bookkeeping, there are two groups from specialized habitats each containing very few species, the ORDER THERMOSBAENACEA (found in hot, brackish waters) and ORDER SPELAEOGRIPHACEA (found in one cave in South Africa), which will never be seen in reef aquaria.

The Tanaidaceans or Tanaids

Tanaids (pronounced "ta-nay-ids") are small animals; while the largest get to about 20 mm (3/4 inch) long, most are about one fourth that size. Their bodies are what morphologists would call "sub-cylindrical," that is, their basic cylindrical body is flattened a bit from top to bottom. The head and first two thoracic segment are covered with a carapace. In the species found in aquaria, small compound eyes are present on small bumps on either side of the front of the head. All crustaceans have two pair of antenna or "feelers" arising from the front of the animal. Often, however, one pair is much more evident than the other one. That is the case here, and often only the large first antennae are evident.

Arising from under the carapace, are a pair of pincher-bearing appendages, and in some individuals of most populations these pinchers are positively enormous, compared to the size of the animal, often being more than half the animals total body length. These appendages serve to immediately distinguish tanaids from all other aquarium-dwelling Peracarida. Mysids and cumaceans do not have pinching or grabbing appendages at all. Isopods generally do not have grabbing appendages, but if they do, they are appendages that fold back onto themselves in a manner similar to a jackknife. Amphipods have the first two pairs of appendages formed into grasping or cutting appendages, but they also close like a jackknife. Only in the tanaids are true pinchers or forceps-like appendages found.

Six pairs of smaller, and more normal appearing, walking legs are found on the bottom of the remaining thoracic segments, and smaller flap-like appendages are found on the bottom of the abdomen. These latter appendages serve to ventilate the animal's tube, and in aquaria they move a relatively large volume of water into the sediment, particularly if the population of tanaids is relatively dense.

Figure 2. Diagrammatic representation of tanaids, the male is at the top, the female at the bottom. Note the disparity in the size of the claws (Modified from McLaughlin, 1980).

The natural history and ecology of tanaids is exceptionally poorly known. The links at the end of the article can lead you to a bibliography of every article published on tanaids published in the last 20 years, and there are only a few that are not simply species descriptions. Additionally, ALL of the published works detailing anything at all about the ecological interactions discuss temperate, polar, or deep water forms. There is NOTHING at all about the tropical species. Sigh... we lose again!!

It is evident from some the works on temperate species, however, that they may be very important ecologically. Some species have a very bizarre reproductive biology. Leptochelia savignyi from the North Eastern Pacific (Highsmith, 1984), has been shown to start out life as a sexless neuter, called a "juvenile". Juveniles have no sexual characteristics and are neuter. As a juvenile grows, it becomes a female. At any given time, about 95 percent of the population is composed of individuals that are either neuter or female. As they molt and grow, the females finally reach a full sized condition, and may reproduce several times. If food is abundant they eat a lot, and undergo a final molt (being crustaceans means they must molt their exoskeleton to grow), and change into a male. At this time, a couple of important changes occur: first, their front pincher-claws become very enlarged, and second, the gut degenerates and becomes non-functional. These males don't feed. They have only one function, and that is to reproduce. They become "Roving Romeos" searching out and fighting with other males in their quest to mate with as many females as they can while they live off the stored energy reserves accumulated during the last female stage. Finally, they run out of fuel and die, and the next set of females molts to take their place.

Figure 3. A group of tanaids of the NE Pacific species, Leptochelia savignyi, photographed in nature in Barkeley Sound, British Columbia, Canada. The largest animal is a bit less than 5 mm (1/5 inch) long. Walking legs are visible on the male emerging from his tube. These animals are micro-predators and scavengers.

Ray Highsmith, who did a lot of pioneering work on coral growth as a graduate student, also studied tanaids (Highsmith, 1982, 1984). He worked on these North Eastern Pacific species and found a number of interesting things. No peracarid has dispersal larvae, yet they are often geographically widespread, and one can ask the question. "How do they get to where they are going?" Highsmith found that these animals can disperse great distances as tubes attached to bits of flotsam and jetsam. Additionally, he found that they may form large aggregations or beds, wherein they become very abundant, up to more than 50,000 animals being found in a square meter of substrate. Here they are predators on drifting larvae, small zooplankton, and other small animals. They are very important animals in these assemblages. Not only do they eat a lot of small animals, but they also are able to exclude many larger ones, such as sand dollars, from their beds, presumably by "gluing" the sediment together with their tubes.

They are very ecologically important in many areas, apparently, but also poorly known. They do make an interesting and odd addition to the fauna of a deep sand bed. By their action of moving in and out of their tubes, they undoubtedly help move water through the sand beds. Additionally, they are probably very good micro-scavengers, eating detritus and food particles.

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

Useful or Cited References:

Highsmith, R. C. 1982. Induced settlement and metamorphosis of sand dollar (Dendraster excentricus) larvae in predator-free sites: adult sand dollar beds. Ecology. 63:329-367.

Highsmith, R. C. 1984. Sex reversal and fighting behavior: coevolved phenomena in a tanaid crustacean. Ecology. 64:719-726.

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.

Useful Links:

The Tanaidacean newsletter has many other links to other publications and lots of  interesting information about tanaids. It also lists every publication on tanaids from 1985 to the present.

For much more  information about tanaids, including a lot of line drawings of them, also see the Tanaidacean homepage and the tanaid section on "Crustacea.Net.

This link takes you to an aquarist's website, where he has many good illustrations of some crustaceans and other animals, including a tanaid in a tube in the substrate.

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Odd Pods, Tanaids in Reef Aquaria by Ronald L. Shimek, Ph.D. - Reefkeeping.com