There are more reef fishes available to aquarists now than ever. Some of these are amazingly colorful, others are very hardy, while still others are unusual in their appearance and/or their behavior. But few match the comely appearance, the interesting behavior, and overall durability of the comet. This “old timer” in the marine hobby, has long been a favorite of beginning and advanced aquarists alike.

The comet, which is also known as the marine betta, is unmistakable. It is brown overall, with white spots on the head, body, and fins. There is also an ocelli on the posterior portion of the dorsal fin. Although it was once thought that there were two distinct species in this genus, only C. altivelis is currently recognized as valid.  The second species, known scientifically as Calloplesiops argus, was thought to differ from C. altivelis in its coloration. It had more numerous, smaller spots on the body, lines rather than spots on the median fins, and a white patch at the end of the tail. However, further investigation has shown that these color differences are a function of the individual’s size. As C. altivelis grows larger, the spots on the body become smaller and more numerous, the spots on the median fins elongate and join, and the white patch becomes more pronounced at the tip of the tail. Since smaller individuals are most frequently seen in aquarium stores, you are less likely to encounter the argus phase of this fish. However, an altivelis may turn to an argus in your aquarium if you keep it long enough. Occasionally, individuals that are in the middle of this color transformation are seen in the hobby. These fish display characteristics of both color phases. Some particularly attractive C. altivelis have a considerable amount of yellow under the ocelli, on the adjacent caudal fin and the upper caudal peduncle.

The comet is a wide-ranging species, having been reported from the Red Sea east to the Line Islands, north to southern Japan and south to the Great Barrier Reef and Tonga. It is a medium-sized fish, attaining a maximum length of 20 cm (7.9 in.).

Batesian mimicry is defined as a relationship where a harmless animal resembles a harmful one. It serves as a defensive strategy. By appearing to be something harmful, the mimic is often ignored by experienced predators.

One interesting comet characteristic is the apparent mimetic relationship that exists between this fish and the whitemouth moray ( Gymnothorax meleagris ). When threatened, a comet will raise all of its median fins and swim into a hole or crevice. But rather than disappearing completely, it typically stops in the entrance of its sanctuary and leaves the posterior part of its body exposed.  With its large ocelli and the white-spotted body, the comet's “back end” is similar in appearance to the head of G. meleagris. By mimicking an aggressive moray, the comet may intimidate or scare off would-be predators. A similar type of mimicry exists in certain butterflies. Some of these vulnerable insects have ocelli on their wings that when viewed from the right vantage point look like the eyes of an owl. Comets will regularly perform this display in the aquarium. Sometimes they will lower one pelvic fin, spread the dorsal and anal fins and swim backwards toward the observer. 

The comet's eye-spots might also serve another function.  Because many predators go for the head in order to incapacitate their prey, the eye-spots on the posterior part of the body may serve to deflect such attacks to the less vulnerable tail region.    

In its natural habitat, the comet is a secretive fish that spends most of the daylight hours in caves and crevices. They have been found on submerged coastal cliffs, lagoon patch reefs and pinnacles, on the reef face, and fore reef slopes at depths from 3 to 45 m (10 to 149 ft.). It occurs singly, in pairs, or in small aggregations. For example, Kuiter and Debelius (1984) report finding three individuals in an area of 1 m² (10.8 ft.²). At night, they move from cover and apparently stalk shrimp, crabs, and small fishes. It sometimes shares the caves and crevices in which it hides with shrimps (e.g., Stenopus hispidus ), the spotfin lionfish ( Pterois antennata ), and pygmy angelfishes.

This fish exhibits a series of unique motor patterns when stalking its prey. When it hunts benthic prey, it tips its body forward, erects its huge pelvic fins, and curls its tail to one side. The comet then propels itself towards its potential victim by undulating the pectoral fins. This hunting behavior appears awkward and conspicuous to the human observer, but it may be that this exaggerated approach distracts the comet's victim and the extended pelvics and laterally directed tail form a barrier to impede the prey items escape (similar in function to the lionfish’s enlarged pectoral fins). 

Other reef fishes (e.g. soapfishes, trumpetfishes) are also known to employ unusual swimming patterns when approaching their quarry, possibly for the purpose of distraction.  Whatever the function, the comet often succeeds in closing the distance between itself and its prey, and when within striking range it lunges forward to ingest the food item.  

"The comet is an incredibly hardy aquarium fish. I have had several individuals that survived otherwise total tank wipe outs caused by parasitic infections. In fact, I have yet to see a comet with a severe case of saltwater ich ( Cryptocaryon irritans ), even in aquaria where every other fish was covered with cysts!"

The comet is an incredibly hardy aquarium fish. I have had several individuals that survived otherwise total tank wipe outs caused by parasitic infections. In fact, I have yet to see a comet with a severe case of saltwater ich ( Cryptocaryon irritans ), even in aquaria where every other fish was covered with cysts!   During attempts to extract several of these fish from aquariums, I have had them tear their fins up and scales off after they wedged themselves between pieces of live rock. These wounds healed quickly without any signs of bacterial infection. Yes, I am convinced these fish are almost indestructible! The only malady I have seen comet’s suffering from, and only on rare occasions, is lateral line and fin erosion. This can typically be prevented or even reversed by feeding a varied diet, soaking the fish’s food in Selco or by using a Ecosystem filtration system (a.k.a. Miracle Mud filter).  

Although these fish are very durable, comet keeping is not without some drawbacks. When initially introduced into an aquarium, these fish are very shy. Your new comet may hide for a week or more before you even catch a fleeting glimpse of it. Their timid nature can present a problem when feeding time rolls around, especially when included in a tank with more aggressive tankmates. The comet is a rather meticulous stalking predator. If it is with fish that quickly dash in and ingest introduced food, the comet may never have a chance to fill its belly. This can be particularly problematic if the comet is kept in a tank that lacks live substrate (more on this later).

When first acquired, the comet will usually only eat live food. Try adding feeder mollies, guppies, ghost shrimp, or brine shrimp to entice you comet. If this does not elicit a feeding response try dimming the lights and then adding the food. Acquaint yourself with your comet's preferred hide outs and attempt to present the food near these areas. Although these fish may only accept live food at first, they can be weaned onto frozen preparations (e.g., Lifeline foods), frozen mysid shrimp, and chopped seafoods. One way to dupe your comet into accepting these substitutes is to place the food in the current produced by a water pump. 

With time and conditioning, your comet will spend more time in view. Some individuals will even beg for food when you approach the aquarium. In general, a comet is more likely to spend time in the open in tanks with dim-lighting or when light levels are reduced (this makes sense when you consider that they are nocturnal fish). But do not expect your comet to constantly swim about at the front of the aquarium and entertain your guests -  this is not the nature of this beast. 

Adults of this species can be housed in a species tank as small as 30 gallons, although an aquarium of 55 gallons or larger would be preferable. Calloplesiops altivelis is a good candidate for the reef aquarium. However, beware! - your comet may thin out your crustacean community. Although comets may ignore resident crustaceans, newly introduced shrimp often become the targets of a hungry comet’s hunting efforts. I have seen them eat peppermint shrimp ( Lysmata wurdemanni ) and cleaner shrimps ( Lysmata amboinensis ), and once had a large individual bite the antennae off of a banded coral shrimp ( Stenopus hispidus). Because a newly released shrimp is most vulnerable to attack when it is drifting in open water, the best way to prevent it from being eaten is to transport the shrimp to the security of the reef structure with your hand or a net. Shrimps that have recently molted are also more prone to becoming comet food.

Small fish tankmates are also at some risk when introduced in with a resident comet. I had a large individual that persistently stalked small dottybacks and shrimp gobies. Fortunately, it never succeeded in capturing either of its more diminutive tankmates. Usually comets are a minimal danger to piscine tankmates that are established residents. However, any fish that can be ingested is potential prey.

I once thought it was necessary to frequently feed comets in a reef aquarium (see Michael 1991). However, I have since found out that this is not the case. This fish apparently can find enough worms and small crustaceans associated with the live rock to adequately supplement its diet. I once had a comet survive for four months without having had any food introduced to its tank. The aquarium was full of live rock and I had not seen the fish (which was the only piscine inhabitant of the tank) for some time so I decided it must have died when I was on a trip. Four months later, when I was taking the rock out of the tank, I found the comet alive and well! Except for being a little bit thinner, the fish was fine. At the time of this writing, this fish is still alive (it now resides in a reef aquarium at the Henry Doorly Zoo, Omaha, Nebraska) after being kept in captivity for over nine years. (Longevity records like this one are not at all uncommon for comets.) While it can supplement its diet with invertebrates that associate with live substrate, it is a good idea to target feed this fish once every week or two.

Although it is possible to keep more than one comet in the same tank, two individuals of the same sex may quarrel. If you want to attempt to keep two C. altivelis together, it is best to add them to a larger aquarium with plenty of hiding places. Fortunately, comets are rarely combative with heterospecific tankmates. In contrast, the passive C. altivelis may be harassed and physically damaged by larger more aggressive fishes such as larger dottybacks (e.g., Ogilbyina spp.), more pugnacious hawkfishes (e.g., Paracirrhites spp.), surgeonfishes, and triggerfishes. As we mentioned earlier, this fish may also have difficulty competing for food with more cantankerous species in a fish-only aquarium that lacks live rock.

The comet has been known to spawn in captivity. Courtship and spawning behavior has not been described because it occurs in the caves and crevices in which the adults hide. Males are reported to be more secretive than the females, which are often found hanging near the entrance of their nesting site. It is not uncommon for one member of the pair (apparently the female) to have torn fins as a result of spawning activity.

A captive female comet will deposit a golden brown egg mass on a cave or crevice wall. The egg mass usually measures about 2.5 cm (1 in.) in length and 0.5 mm (0.25 in.) in width and is comprised of 300 to 500 eggs that measure about 1 mm in diameter. The eggs are attached together and to the substrate by sticky threads. The male comet ceases to feed and guards the eggs, spreading its fin and shielding them with its body. The male will even chase its partner away from the nesting site if she gets to close. Wassink (1990) reports a time period of 10 days or longer between spawnings, while Baez (1998) reports they are not predictable, cyclic spawners like other fish species that they raise. Our limited observations of the wild comet suggest that spawning may occur slightly more frequently than every 10 days (e.g., after an 8 day period).

Captive studies have shown that at a water temperature of 26 º C (79 º F), the eggs hatch in about 5 or 6 days. (The incubation period seems to correspond with the limited data we have from the field.) Most of the eggs hatch at dusk or after dark, although they will also hatch during the day. The fry are about 3 cm (1.2 in.) long when they emerge. They are remarkably well developed and feed immediately. They double in length in the first 14 days and on the 16th day the dark colored youngster develops a white spot on each side of the body, due to a loss of pigmentation in this area. The pigment cells continue to disappear until the body is white overall. At the same time, the free-swimming fry begin to lead a more reclusive life-style, hiding in reef crevices. As night falls, the fry come out of hiding to feed. At two months of age, the spots begin to appear on the head, but the sides remain white until they are about seven months old. Spots also begin to appear on the head, but the sides remain white until they are about seven months old.

This is a great candidate for captive breeding programs, in fact at least one ornamental marine fish hatchery (Sea Quest Hatcheries, Puerto Rico) was selling captive raised stock at one time. Of course, getting a pair is always easier if you can tell the sexes apart. In the case of the comets, sexing the fish is apparently difficult to do. One reference suggested that the only possible difference between the sexes appears to be the size of the spots – those on males are reported to be smaller than those of females. But this seems to be more a function of the size than the sex. (It maybe that the males tend to be larger than the females and therefore they usually have smaller spots –see comments about “C. argus” above.) Although information on their sexuality is lacking, they are probably protogynous hermaphrodites (that is, they change sex from female to male). If this is the case, placing a smaller and a larger individual together will increase the chances of getting a heterosexual pair. 

Happy fish-watching!