Amphipods by Ronald L. Shimek, Ph.D.

Several groups of predominantly marine animals are hard to discuss without resorting to the overuse of superlatives. Adjectives such as "greatest," "most successful," and "most important" tend to dominate discussions about these animals. Primarily, such superlative animals tend to be either large, evident, ecologically successful or the particular author's favorites. Many of them are abundant, and some of them draw our attention because of their activities. In many ways, the animals I will discuss in this column, the amphipods, should fit into those animal groups discussed with the use of superlatives. Amphipods are certainly "ecologically successful;" additionally, they are very successful evolutionarily, a fact reflected in their abundance in almost all marine environments. They are also, however, generally small, obscure, and often cryptically colored. These latter "attributes" have worked together to keep their natural history poorly known, especially in the tropics where there are many other more beautiful or striking animals to study. Nevertheless, even in these areas the amphipods are diverse, abundant, and ecologically important. It should be evident, then, that some of the most common inhabitants of coral reef aquaria are amphipods. It turns out that in most cases they are also some of the most desirable animals to have in those aquaria.

So, what is an amphipod? Well, according to tradition, the answer to the old English riddle, "Why is a duck?" is, "Because one of its legs is both the same." Following the same logic, the answer to, "Why is an amphipod?" should probably be, "Because two of its legs are both different." The name "amphipoda" is derived from "amph" (from the Greek amphi=amphis meaning "both sides of, double; also apart, asunder, or round about"), and "pod" (from the Greek pous or podos meaning "foot," as in podion, "a small foot," or podotēs, “footed”), and refers to the superficial appearance that these animals have two distinct types of appendages or feet (Jaeger, 1955).

Taxonomically, amphipods belong to the Order Amphipoda, of the Superorder Peracarida, of the Class Malacostraca, in the Subphylum Crustacea of the Phylum Arthropoda. To an invertebrate zoologist, this listing of terms gives a great deal of information. I suspect, however, that after reading that list of multisyllabic words the average reef aquarist will be pretty much left thinking that the old English riddle about ducks makes more sense.

Breaking all of the taxonomic terms down into their meanings with regard to amphipods does tell us a lot about the animals, however. Amphipods are arthropods, and as such they possess the arthropodan features of a segmented body with jointed, segmented, appendages and an external skeleton, or integument. The integument is composed of the outer epidermis or skin of the animal, plus many chemicals secreted by that epidermis. This exoskeleton is flexible, but not stretchable, and in order to grow, the animal must frequently molt or shed its "skin" and grow a new one. In reality, the skin itself is not shed, but rather the old external chemical layers, or cuticle, are partially broken down chemically and resorbed. What remains, a thin remnant of the original integument, is shed. The skin or epidermis underlies the cuticle, and remains with the rest of the animal. A new, larger, cuticle is formed under the old one as part of the molting process, and becomes exposed only after the integumental remnant is shed. Although at first the new cuticle is soft, it hardens up in a short while.

Each of the major arthropodan groups, the Crustaceans (shrimp, crabs, amphipods), Uniramians (insects, centipedes), and Chelicerates (spiders, mites, horseshoe crabs), is very different from the others, with each exhibiting many unique characteristics. While they all share a common ancestor, probably something like a trilobite, that ancestor lived about half a billion years ago, so much divergence and many modifications of the animals in each lineage have occurred since then. Many of the differences that separate them are obvious. For example, while crustaceans, including amphipods, always have two pairs of antennae, uniramians have only one pair and chelicerates lack them altogether. Additionally, crustacean appendages generally consist of two separate parts or branches. For example, each of a crab's walking legs consists of two branches, the first being the visible part of the leg, and the second being the gill found branching off the leg's base. No arthropods other than crustaceans have branched appendages. Crustaceans are predominantly animals of the marine realms, and that group is moderately successful on both the land and in fresh water. Insects and chelicerates are predominantly terrestrial or fresh water. Although some chelicerates are found in marine habitats, few insects are found there.

Even though they superficially appear to be quite different from crabs and shrimp, amphipods are considered to be relatively closely related to both groups. As such they are placed in the group called the Peracarida, or "near shrimps." All of these types of animals share many similar structural characteristics such as the same number of appendages found in each body region, and the general body form. However, the amphipods and several other groups of mostly small animals lack the distinctive carapace or shell found in the crabs and shrimps. Another important difference is that amphipods have a brood pouch on the female's ventral surface while true crabs and shrimps both lack this structure. After copulation the amphipod female places the eggs in the brood pouch and cares for them until they are ready to leave the pouch as fully functional little amphipods. This means that amphipods lack free-living larvae and have to disperse by actual migration of the adults, primarily the females.

There are three major, and one very minor, groups of amphipods, but representatives of only one of them are likely to be commonly found in our tanks. The common type of amphipod is called "a Gammaridean amphipod," and belongs to a grouping named after one of the more common fresh-water amphipodan genera, Gammarus. Such animals are characterized and differentiated from shrimps by the absence of a carapace or shell over the front part of the body. The separate anterior body segments are easily seen. This condition may be contrasted with that seen in the shrimps or crabs, whose separate segments are covered by the carapace or fused to it.

The amphipods are typically flattened from side-to-side and they also usually possess large compound eyes on either side of the head. Unlike the eyes of shrimps or crabs, these eyes are not on stalks, but are imbedded in their head. To make things a bit more interesting as well, although the name "amphipod" refers to two types of legs, these animals actually have a variety of leg structural types on any given animal. Most of the visible segments are found on the middle body region, referred to as the thorax, and each of these segments bears one pair of appendages. The two most anterior pairs of visible thoracic segments have modified appendages called gnathopods. "Gnath" means "jaw" and the gnathopods, literally "jaw feet," are thought to be used in feeding, but relatively little close and careful observation has been done on animals in this group. What these appendages actually are used for is largely open to question. In any case, gnathopods typically, but not always, end in recurved jack-knife-like claws called "subchelae" (See Figures 2, 6, and 7). Behind the two pairs of gnathopods are two pairs of shorter legs followed by three pairs of longer legs to the rear. The longer legs tend to splay outward, giving the animal a recognizable and typical posture, rather like a bicycle supported by training wheels.

The internal anatomy of a typical amphipod is shown in Figure 3. As in all arthropods, the nervous system lies along the middle of the ventral surface. There is a ganglionic swelling in each segment. The large supraesophageal ganglia are located above the esophagus and together with the nerves running around the esophagus, constitute the brain. The eyes communicate directly with these ganglia by large optic nerves. The antennae, or feelers, are sensory and large nerves run out to them as well.

The mouth is located near the base of the head, and leads to a short esophagus passing vertically to the stomach, located just behind the head. Inside the stomach are some chitinized plates lined with ridges that serve to grind food. Because their mouth size is constrained by the more-or-less rigid exoskeleton, most of the food that enters is either liquid or is torn into small bits by the appendages around the mouth. A long midgut passes though the majority of the body. A series of pouches or caecae arise from the midgut. One very short caecum rises from the top of the gut and runs forward a short distance toward the head. Two to eight pouches arise from the sides and bottom of the midgut, right behind the stomach. These pass to the rear and extend almost to the abdomen. Both digestion and the secretion of digestive enzymes and "juices" occur in the various caecae. A single similar pouch arises from the end of the midgut and runs forward up over the midgut caecae and gonads. Its function is unknown. Posterior to the origin of this caecum, the gut is referred to as the hindgut.

A long tubular heart is suspended from the dorsal body wall in the thorax. Three pairs of valved openings, called "ostia," allow blood to flow one way into the heart from the surrounding space. When the heart contracts blood is pumped forwards and backwards through the anterior or posterior aortae, respectively. Lateral blood vessels lead from the heart to the body wall. The blood leaves these blood vessels and flows around and through the tissues in channels. The blood flow is rapid and completes a complete circuit in just a few seconds in a small amphipod. The blood is loaded with various types of corpuscles, but we really don't know how most of them function. Gammaridean amphipods have no specific respiratory organs, and gas exchange probably takes place over the entire body surface.

Amphipods have a rather straightforward life cycle that allows them to reproduce well in our systems. They generally have separate sexes and hermaphrodites are rare. The genders are easily distinguished. The oviduct opens at the base of the legs of the sixth thoracic segment, and the vas deferens opens at the base of the legs of the eight thoracic segments. The males have a pair of penes, modified thoracic appendages, and fertilization is internal. Shapes of the eyes, antennae, and second gnathopods may also vary between the sexes. Additionally, females have the brood pouch discussed earlier. It is formed under the female's bottom surface by a series of plates originating from the inside edge of each of the thoracic legs. The plates extend over the ventral midline and overlap with plates from the other side, creating a chamber between them and the ventral surface. After copulation, the fertilized eggs are deposited in the chamber and are held there as they develop. When the larvae finally mature to juveniles, the female releases the brood and they disperse in the local area. Amphipods lack a free-swimming or free-living larval stage. There is not much chance of larval mortality in this type of reproduction unless, of course, the female is eaten. Normally, when the populations build up and become dense, pregnant females emigrate to establish a new population.

Generally, gammaridean amphipods are regarded as looking pretty much like the diagram in Figure 2 above, or the photo below (Figure 4). This is, however, a large group, with probably well over 5,000 species, some of which are weird and spectacular oddballs. The group displays a significant amount of variety of shapes and sizes (Figure 5). It goes without saying that these animals occupy a wide variety of ecological niches. Gammaridean amphipods may be predators, herbivores, detritivores or bacteriovores. A few are omnivorous. Many are commensal and are found living in or on other animals, and a few are parasites.

Figure 4. Typical Gammaridean amphipods. These two specimens, both about 4 mm (5/32") long,
are both found in aquaria and show the characteristic amphipod body shape.

The most important part of any animals' natural history is their need to obtain food, and the variety of ways in which amphipods feed may be reflected in differences in their basic feeding appendages. In general, the gnathopods are considered to be involved with prey capture, food manipulation or feeding. The basic morphology of the gnathopods is shown in the diagrams in Figures 2 and 7 and exemplified in the photo, Figure 6, but there is a significant amount of variation in what the actual structures are. Some of the various shapes of gnathopods are shown in Figure 7. Each of these changes from the simple subchelate appendage shown in Figure 2, reflects a difference in diet and lifestyle. Consequently, it should be obvious that the many types of Gammaridean amphipods have radiated into a great many niches, and occasionally we might encounter some rather odd ones in our aquaria.

Figure 6. The subchelate first gnathopod of an amphipod. This appendage is transparent,
and the relatively massive muscles used to close the dactyl against the propodus are
labeled, as are blood corpuscles in the blood channels.

Figure 7. Some of the many modifications seen in the shape of the first (mostly) gnathopod. In the "simple" form the appendage is basically a walking leg. In all other forms the appendage bears a flexible pinching or cutting claw on its end. The pink segment, the propodus, is the basal part of the claw. The yellow segment or dactyl is the moving part of the claw. Modified from Staude, 1987.

One very small group of oddball amphipods is called the Ingolfiellidea. These animals normally live between and on sand grains, and to the best of my knowledge, they have never been seen in aquaria. The other two groups of amphipods are both ecologically important and diverse. They are the Hyperiidea and the Caprellidea. The hyperiids live in the plankton, and many are predatory or parasitic on gelatinous zooplankton. Some hyperiids are rather bizarre, and one, Phronima, , has been rumored to have been the model for the Alien in the movie of the same name. Phronima is pelagic and lives in pelagic tunicate houses.

The other group of amphipods, the Caprellids, are fascinating animals. One lineage of caprellids live on whales and are called "whale lice." The second lineage is called skeleton shrimp, which is an apt name. They look like small multi-armed skeletons and are found clinging to algae, the backs of sea stars or gorgonians. They are really very common, but seldom make it into our reefs.

To a person used to considering the Gammarids as typical amphipods, the first time a Caprellid is examined it is very difficult to consider it an amphipod. At first glance, and probably second and third glances as well, Caprellids look NOTHING like a Gammarid. They have a long tubular body terminating in a small but bulbous head that sprouts two pair of long antennae, which may be as long as the rest of the body. They have relatively large and pronounced appendages that look like they are made of sticks, but that end in claws that close like a jack-knife. The name of skeleton shrimp is apt. They look like some sort of weird crustacean skeletons that have come to life.

Although large ones may be several centimeters long, most of the ones that make it into reef tanks are on the order of a centimeter in length or less. They often can extend their anterior appendages to a span as wide as they are tall. They have the habit of sitting on a rock promontory or a piece of algae, which they grip with their hind legs. The body extends vertically up into the water and the front appendages are spread wide as they wait for something to pass them in the water. They will reach out and grab food drifting by, or they may work along an algal frond, gorgonian branch, or some other substrate gleaning food from it.

As in the other amphipods, females have a brood pouch. In this case, it is located in the middle of the body as the animal stands up, and is often visible as a small white spot in the middle of some of the animals. They are typically pale amber or white, but may display other colors as well. Many of these are harmless or beneficial herbivores or scavengers, but some are carnivorous, and they could eat small, soft coral polyps, and other small animals.

As with so many animals, caprellids may enter our systems as hitchhikers on live rock, algae, corals, or in live sand. Generally, they are fairly harmless, and are so much fish food. Occasionally, some are found that will cause some minor problems. Desirable and generally herbivorous, caprellids may be cultured in a refugium, sump, or occasionally in the main tank, provided there is an appropriate food, generally some alga. Culturing consists of providing light and algae and letting the animals do their thing. Carnivorous forms may be removed with forceps, if they are causing problems. Generally, in the main tank, fishes remove them before the aquarist was aware that they were even there.

It is generally very easy to recognize the majority of amphipods in our aquaria; they lack the carapace, or shell, of crabs and shrimps, and tend to be laterally compressed. The appendages of the posterior thorax that extend laterally are also distinctive. Most aquarium species are small, seldom reaching more then a few millimeters in length. They normally vary little in gross structure from the illustration shown in Figure 2.

These bugs are also often referred to simply as gammarus shrimp, which is a misnomer as most of the species found in our tanks are not in Gammarus proper. Occasionally, as well, we hear various vernacular corruptions of Gammarus, such grampus or gramus, used to describe these animals. Most aquarists think that all amphipods are pretty much like the standard gammarid amphipod that they see in their tanks, and that all of these animals are herbivores or detritivores. Within the limited world of reef tanks, this is more-or-less true; however, not all gammaridean amphipods are herbivorous, some are very decidedly carnivorous.

Nonetheless, the most common amphipods found in aquaria are either herbivores or detritivores. They tend to eat either plant or algal material preferentially and either graze on algae or eat debris of plant or algal origin. Generally, they don't eat much in the way of animal flesh, although occasionally we do get some predatory amphipods in our systems. It is difficult to distinguish between any of these species without specific microscopic examination, so the only way most hobbyists have of differentiating between the two types (and keep in mind there are several hundred potential species in each type) is to watch them feed. In our aquaria amphipods are typically part of the cleanup crew. In addition, they are good food for whatever fish can catch them. Altogether they are a beneficial and interesting component of our systems' fauna.

Carnivory in amphipods is not uncommon, and occasionally some truly carnivorous forms hitchhike into marine aquaria. In the deep sea, or even in many shallow water areas below the photic zone, carnivorous amphipods are either dominant members of the scavenging guild or predatory in their own right. I even know of one species that on occasion is a man eater, being the man it was documented eating! This particular amphipod species, Chromopleustes pugettensis, is strikingly colored (Figure 10). The body is a rich dark brown with a brilliant white saddle and longitudinal gold stripes. The eyes are lavender and the legs are blue. Regular readers of this column will probably recognize this coloration as warning or aposematic coloration. This is coloration of animals that are dangerous, and this amphipod is decidedly dangerous - both to its predators and its prey. Additionally, this is an amphipod species that does not hide. The animals are highly visible on the bottom in daylight and do not flee when approached. This behavior is also an indication that the animal is protected from predation.

Chromopleustes pugettensis is patchy in its distribution. It is generally uncommon, but when it is found in an area, it tends to be found in large aggregations; I have observed several swarms of more than 20,000 individuals on a single dive. During most of the year Chromopleustes pugettensis appears to be associated with sea cucumbers. Detailed ecological information is lacking, but it is likely eating small cucumbers or portions of larger ones. Sea cucumbers, like many echinoderms, contain toxic chemicals called saponins. Saponins are thought to confer freedom from predation on many echinoderms, and generally predators on echinoderms are very rare. It has been presumed that somehow, Chromopleustes has adapted to eating sea cucumbers and has concentrated their saponins in its body. This has never been tested; however, in a series of trials it was evident that most fish in the area will not eat the amphipod. Many years ago, I did some trials trying to feed the amphipod to various predators. In only one case did a fish eat the bug, which it promptly spit back out. The next morning the fish was dead. Anecdotal evidence, to be sure, but certainly interesting anecdotal evidence.

On April 2, 1983, I was diving in an area called Pole Pass, in the San Juan Islands of Washington. During this dive, my dive partner and I came across a large sea star, Pycnopodia helianthoides, that was spawning. It was absolutely covered by a swarm of the amphipods and they were tearing off pieces of its upper surface. As we attempted to get close, the swarm rose and a portion of it settled on my face and before I knew what was happening the bugs were biting my face and lips. I rapidly "back pedaled" and managed to brush them all off, but by the time I had done this, they had managed to break my skin in several places and I was bleeding quite profusely. NASTY LITTLE BUGS!!!

Fortunately, most reef aquarium amphipods are much more benign. The above experience, however, ought to convince most aquarists that they cannot take the non-predatory nature of amphipods for granted. On the other hand, amphipods that are reclusive and which lack warning coloration are probably quite safe and beneficial to our systems.