EcoSystem Aquarium Commensal Critters: Animals that Live on Gorgonians and Black Corals : Page 1
Space is at a premium on coral reefs. As was the case in the trench-bordered fields of France during World War I, there is a battle going on for almost every inch of “ground” on a coral reef. These competitive living circum-stances have favored the evolution of inter-esting relationships between very different creatures to increase their chances of survival. Gorgonians and black corals are common in coral reef habitats prone to food-carrying currents. The polyps of these sessile animals capture planktors that collide with their stinging cell-armed tentacles. In an ecosystem in which places to live are in short supply, these cnidarians would seem to provide a suitable home for animals to live on. In this article, I will briefly examine some of the unique associations that exist between gorgonians and black corals, and certain crustaceans and fishes. Before we begin our examina-tion of some of these critters, we need to define some terms related to symbiosis. So, let’s get started. Symbiosis:The Breakdown Symbiosis literally means “living together.” It was coined by a French naturalist to refer to the relationships he observed between dif-ferent animal species. The individuals involved in these associations are known as symbionts. If one of the organisms Saw blade shrimp ( Tozeuma armatum ) will move to the opposite side of a sea whip when approached by a predator. Photo: J. Cairns-Michael in a symbiotic association is a passive participant, it is known as the host. For example, the whip coral that provides a home for a whip coral crab is known as the host. We can break down symbiotic relationships into several different categories: Parasitism, Coral 47
Commensal Critters: Animals That Live On Gorgonians And Black Corals
Scott W. Michael
Space is at a premium on coral reefs. As was the case in the trench-bordered fields of France during World War I, there is a battle going on for almost every inch of “ground” on a coral reef. These competitive living circumstances have favored the evolution of interesting relationships between very different creatures to increase their chances of survival.
Gorgonians and black corals are common in coral reef habitats prone to food-carrying currents. The polyps of these sessile animals capture planktors that collide with their stinging cell-armed tentacles. In an ecosystem in which places to live are in short supply, these cnidarians would seem to provide a suitable home for animals to live on. In this article, I will briefly examine some of the unique associations that exist between gorgonians and black corals, and certain crustaceans and fishes. Before we begin our examination of some of these critters, we need to define some terms related to symbiosis. So, let’s get started.
Symbiosis literally means “living together.” It was coined by a French naturalist to refer to the relationships he observed between different animal species. The individuals involved in these associations are known as symbionts. If one of the organisms in a symbiotic association is a passive participant, it is known as the host. For example, the whip coral that provides a home for a whip coral crab is known as the host.
We can break down symbiotic relationships into several different categories: Parasitism, Commensalism and mutualism. In a parasitic relationship, one of the partners harms the other (in many cases, the parasite feeds off the host). An example would be the copepod crustaceans that live and feed on the body tissues of sharks and rays. The species that harms the host is known as a parasite.
In a commensal relationship, one organism benefits from the relationship, while the other partner is neither harmed nor helped by the relationship. For example, the crinoid shrimp that live in among the arms of a feather star (crinoids) — they use the feather star as a place to hide and feed, and while they do not harm the echinoderm, the star doesn’t benefit from their presence (or at least at this time there is no benefit known for the host). It should be pointed out that the term commensalism was originally used to refer to feeding relationships between two species (the word actually means “at table together.”) However, the term now has taken on the much broader meaning described above.
In a mutualistic relationship, both organisms benefit from their partnership. One of the best known examples of mutualism are the anemonefishes, which use sea anemones as places in which to refuge. In turn, the anemonefishes defend their hosts from predators like butterflyfishes. The anemonefish’s excretions also provide a source of nutrients (nitrogen) for the sea anemones.
We can breakdown commensal relationships even further. For example, “inquilinism” is an association where the host serves as a source of protection. Some good examples of commensal relationships that would fall into this category would be the cardinalfishes that shelter among the spines of sea urchins, and the hairy squat lobsters that live between the ridges of barrel sponges or among the arms of feather stars. The hosts are not harmed, nor do they benefit from these commensal associations. Mutualistic relationships that would be classified under the heading of inquilinism would include the anemonefishes that live among the stinging tentacles of sea anemones, Dascyllus damsels that live among the branches of staghorn corals, and coral crabs that also live between stony coral branches.
“Endoecism” is a relationship in which one species uses the burrow constructed by another organism as a place to live. For example, some of the dart gobies (Ptereleotris spp.) Live in burrows constructed by pistol shrimps (Alpheus spp.), some boxing shrimp (Stenopus spp.) Live in the tunnels dug by large mantis shrimps, and there are juvenile surgeonfishes that live in the homes constructed by the orange-spotted sleeper goby (Valenciennea puellaris). These are all examples of commensal relationships.
“Phoresis” is a symbiotic relationship in which the host provides a means of transport for its asso- Ciate. An example of this would be the remoras -— or sharksuckers, as they are often collectively called — that ride on sharks and rays. (In the case of the remora, their hosts may also provide them with a source of the parasitic crustaceans that some of these fishes feed on.) The relationship between barnacles and the whales or sea turtles that they live on is also an example of a phoresistic relationship.
Gorgonian/Black Corals and their Symbionts — What Type of Relationship?
Okay, now that we have unpacked the term symbiosis and its many components, we need to try and categorize the relationships that exist between these cnidarian hosts and their crustacean/fish symbionts. Unfortunately, detailed studies on the relationships between these animals are few and far between, so most of what we know (or think we know) is speculative.
The relationship between crustaceans/fishes and their gorgonian/black coral hosts would appear to be commensal — that is, where the former benefit, but do not harm, the host appreciably. The benefit for the crustacean or fish is that the host provides a place to live and hide. As you will see in the accompanying photographs, these symbionts often blend in very well with their cnidarian hosts, often mimicking the color and even the surfaces of the gorgonian or black coral. The most amazing example of this is the pygmy seahorse (Hippocampus bargibanti), which sports bumps on the body surface that look like closed Muricella polyps. There is no doubt that certain “free loaders” sometimes remove patches of polyps and may compete for food particles. For example, the whip coral gobies will remove a section of polyps to provide substrate on which they will lay their eggs. This is obviously not beneficial to the host, but probably does not do enough damage to the gorgonian or black coral to cause any long-term harm.
But what type of commensalism is it? It would be classified as inquilinism — that is, associations in which the host serves as a source of protection and as a surface to live on. For example, the whip coral crab (Xenocarcinus tuberculatus) is always found on black corals and gorgonians. These hosts provide a needed refuge, and a “perch” on which these animals grow and capture their prey. Because their hosts have a predilection to settle and grow in current-prone areas, it makes sense that an animal that relies on the current to bring its food would seek out a host that prefers these types of conditions.
Although there are a number of soft corals that act as hosts to various other marine organisms, we will limit our coverage to the gorgonians (Subclass Octocorallia; Suborders Scleraxonia and Holaxonia). The gorgonians are unique among the octocorals in having skeletons that are comprised of gorgonin and/or calcite spicules. This proteinaceous material gives the colony a rigid, or stiff, quality. Gorgonians vary in shape. They can be a complex of interconnected branches, which form a net that is effective for collecting plankton swept by them in the water currents. Some are simple, unbranched whips. The “fanlike” group of gorgonians includes members of the genera Acanthogorgia, Annella, Melithaea, Muricella and Subergorgia. Many species in these genera are more colorful and usually live at moderate depths.
The whip-like species belong in the genera Junceella. Members of this genus often protrude from the ocean bottom or reef walls, and are sometimes found in dense aggregations that conjure up images of an alien forest from a science fiction film. Many of the Junceella species are found at the great depths of fore reef slopes.
While at first glance the black corals may look like gorgonians, upon close examination you will find they are quite different. In fact, the black corals are often included in the subclass Cerianthipatharia, along with the tube anemones. The black corals have a thorny, black or brown hard skeleton that surrounds a central coenenchyme (also called a coenosarc). They lack sclerites, and the polyps have six finger-like tentacles (gorgonians have eight) and cannot be completely retracted. The black corals are more characteristic of deep reef habitats (they are most common at depths in excess of 328 feet; 100 meters). Those species that inhabit shallower depths tend to occur in more shaded microhabitats, like steep reef walls, under overhangs or in caves.
Black corals can be bush-, whip- or spiral-like. The Antipathes spp. Are bush, bottle-brush or fan-like, with bright-yellow, rusty orange, or tan polyps. These corals are usually found in deeper waters on steep walls. The wire, spiral and whip corals (Cirripathes and Stichotpathes spp.) Are elongate corals that form long whips or coils. The polyps are arranged around the long stem and can be dull yellow, brown, creamy or white. The wire corals are usually found on reef faces or drop-offs, but they are occasionally seen on sand slopes.
Some Crustacean Symbionts
There are a variety of crabs in the family Majidae (known as spider or decorator crabs) that are associ- Ates of gorgonians and/or black corals. The genus Xenocarcinus contains the most well-known of these. The whip coral crab (Xenocarcinus tuberculatus) is an obligatory black coral dweller — that is, it is always found in association with black corals. They come in a variety of colors, depending on the color of the back coral whip they are on, with horns on the carapace (the chitonous shell that covers the back of a crab) that accentuate their amazing camouflage even more. The gorgonian crab (Xenocarcinus conicus) is a smooth-bodied species that is less finicky about the host it selects — it may be found on sea fans, black corals or hydroids. This crab usually mimics the color of its host (which can be red, orange or even blue). It has a white stripe down the middle of the carpace and sometimes has small spots and appendages on the rostrum (the beak-like process at the front of the head).
It is thought that this and the other species of Xenocarcinus feed on detritus and plankton that is trapped by their cnidarian host, but little information exists on their biology. It is most likely that the larval X. tuberculatus settles on the black coral whip and spends its entire life on the same individual host. But like so much about the lives of many crustaceans, little is truly known about its life history.
There are a variety of shrimps that are obligate black coral associates. These include members of the genera Dasycaris, Miropandalus, and Pontonides. They cling tightly to the coral branch and are often indistinguishable from their hosts. Harding’s black coral shrimp (Miropandalus hardingi) has yellow spines on its grayish- green body, which look like the polyps of its black coral host. The Zanzibar whip coral shrimp (Dasycaris zanzibarica) is transparent, with white or yellow bands (depending on the Cirripathes spp. It is living on), and has humps on the thorax and abdomen that enhance its camouflage. A pair of these shrimp will usually occupy the same black coral whip, with the female being up to twice the size of the male. Pontonides unciger is a similar animal, but lacks the humps on the body and also has spines on the rostrum (the rostrum is smooth in D. zanzibarica). This animal also lives on Cirripathes spp. And is usually found in pairs. The diets of the shrimps in these three genera are not known. Speculation is that they feed on zooplankton that is captured by their Hosts. They may also feed on their hosts’ mucus (however, this has yet to be documented).
At least some members of the genus Tozeuma are found on whip-like gorgonians and black coral whips, and among bushy black corals. (There are species in the genus that are also found on algae, seagrass and hydroids.) These shrimps have a long, sharp rostrum with a serrate ventral edge (as a result they are sometimes called needle-nose or saw blade shrimp) and elongate bodies. The long rostrum and body helps it blend in with the black coral branches it often refuges among. Some Tozeuma have transparent bodies, with irregular bands that are similar to the polyp color of the host (they can be white, orange or yellow). One anti-predation response these shrimp exhibit is to move around the black coral or gorgonian branch when approached by a potential predator, thus placing themselves out of the “shrimp eater’s” field of vision. This behavior can also drive an underwater photographer nuts as he or she tries to line up one of these shrimp for a shot! Anecdotal evidence indicates that they are sexually dimorphic (i.e., there are distinct differences between the sexes), with the females being as much as three times the size of the male. The life history of the coral-dwelling species is poorly known. Like their crabby cousins, it is suspected that they feed on detritus and plankton that are trapped by the slime of their hosts.
Some Fish Symbionts
The most celebrated piscine-gorgonian associates are the two pygmy seahorses, Hippocampus bargibanti and H. denise. H. bargibanti is almost always found on the sea fans, Muricella plectana (red or pink individuals) and Muricella paraplectana (yellow or orange specimens). A single gorgonian may be home to a pair or group of these 3/4-inch (2 centimeter) seahorses. In fact, as many as 28 pairs have been reported from a single fan. From their sea fan host, they pick off passing zooplankton or planktors that adhere to the sea fan’s slime.
Denise’s pygmy seahorse (Hippocampus denise) is a similar species that is found on the gorgonians Annella reticulata, Muricella sp., Acanthogorgia sp. And Echinogorgia sp. It was only recently described (in 2003) and some authors have suggested it is simply a variant of H. bargibanti. Unlike H. bargibanti, it is smooth-bodied, lacking the pseudo-polyps we see on its close relative (if “warts” are present, they are very small). According to the researchers who described H. denise, it is also distinguished by its more slender body, smaller size, more rings in the tail and predilection for different hosts. Denise’s pygmy seahorse is also more active than its close relative, spending more time moving about its host during the day. This is one of the smallest bonyfishes, reaching sexual maturity at the tiny size of only 1/2 inch (13.3 millimeters).
The most common fish found on gorgonians and black corals are the gobies, in particular members of the genus Bryaninops. There are eight species in the genus, four of which are found living on these cnidarians. One of these has been studied in some detail. This species, the wire coral goby (Bryaninops yongei), is found only on its namesake — the wire coral, Cirripathes anguina. This fish is transparent, with diffuse bars on the upper part of the body that help it blend in with its host. The young of B. yongei typically live on smaller whips, while larger C. anguina are occupied by a monogamous adult pair. Occasionally, a larger whip may be home to an adult pair and one, or more, juvenile. This goby is capable of bidirectional sex change (i.e., it can change sex both ways). Because of their very patchy distribution and reliance on this very specialized micro-habitat, individuals must have the ability to change sex so that it is easier to find a potential mate. When they spawn, the male removes polyps from the host black coral and the female deposits her eggs on the exposed skeleton.
Other species known to live on gorgonians and black corals include the large whip goby (Bryaninops amplus), the Loki seawhip goby (B. Loki), and the black coral goby (B. Tigris). The large whip goby is found on a variety of gorgonians, sea whips and fans. The Loki whip goby is found on soft corals in the genera Junceella, Ellisella and Subergogia. Finally, the black coral goby is found on black coral bushes in the genus Antipathes. The social biology of the other species is probably very similar.
This concludes our brief survey of some of the fishes that are found on gorgonians and black corals. While relatively few of these species are available or suitable for the home aquarium, I hope that this article will help the diving aquarist spend more time examining these potential hosts for these amazing little commensals. Happy fish watching!
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Lourie, S.A. and J. E. Randall. 2003.A new pygmy seahorse, Hippocampus denise (Teleostei: Syngnathidae), from the Indo-Pacific. Zool Stud 42(2):284-29.
Munday, P. L., S. J. Pierce, G. P. Jones and H. K. Larson. 2002. Habitat use, social organization and reproductive biology of the seawhip goby, Bryaninops yongei. Mar Fresh Res 53:769-775