Not Your Average Betta: Meet the Marine Betta

Here's a species that I have seen cause some confusion in my time in the trade, and a prime example of why people shouldn't rely too heavily on common names. The marine betta is not a relative of the Siamese fighting fish (Betta splendens) that happens to live in the sea. Nor is it a grouper, despite what another common name I've seen used suggests: 'comet grouper'. The more accurate common name here is simply the 'comet'. 

Instead, it's a member of the longfin, or roundhead, family Plesiopidae, and is known scientifically as Calloplesiops altivelis. This is a group of fishes quite separate from the groupers.

It gets mixed up with groupers thanks to its predatory nature and somewhat similar body shape. 

All photos by Shutterstock

Long fins and contrasting markings

The comet, or marine betta, has relatively long, flowing fins (hence the connection to the freshwater Betta genus), which makes it a very striking fish to look at. Its body and fins are predominantly dark, covered with small white spots, while the fin margins can have a faint yellowish-green hue.

Finally, the comet also displays an eyespot, or ocellus, at the base of the dorsal fin.

This is a fish that certainly couldn't be labelled colourful, but the contrasting pattern and flowing fins certainly make up for what it lacks in that department. 

Natural habitat and behaviour

The comet has a wide distribution in the Indo-Pacific, from East Africa all the way across to Tonga. It lives on coral reefs and drop-offs down to about 45 m (148 ft) where it usually hides away in crevices, caves and rocky overhangs. 

Calloplesiops altivelis is nocturnal by nature, emerging from its hideout in the cover of night to hunt small fish and crustaceans. 

Growing to around 20cm (7.9 in) in length, it is a relatively modest-sized predator, but it isn't without its own threats. Larger predatory reef fishes may see the comet as a potential prey item, and this is where its unusual eyespot comes into play.

Eyespots are found in many fish species and can help confuse potential predators by making it difficult to distinguish which end of the fish is the head. However, the Comet takes this defensive strategy a step further.

Its contrasting spotting, prominent eyespot and unusual defensive posture combine to mimic a species of moray eel. When threatened, the comet will retreat headfirst into a suitable cave or crevice, leaving its rear end exposed. By spreading its fins, it creates the impression of a moray eel peering out from the reef, with the prominent eyespot resembling the eel's eye.

The species being mimicked is the white-spotted moray (Gymnothorax meleagris), a much more formidable-looking reef predator, pictured below. 

Aquarium Setup and Requirements

While comets can reach a reasonable size, they aren't particularly active fish. A tank of around 250 litres (55 gal) could be considered a starting point, although, as always, bigger is better. I'd favour a larger aquarium where possible, particularly if you're planning a community setup.

Related article: Choosing the right marine aquarium.

Their natural behaviour gives us a good indication of how to aquascape their aquarium. Plenty of live rock, arranged to create caves, crevices and overhangs, will provide the comet with the shelter it needs during daylight hours. Ideally, there should be several suitable retreats, allowing the fish to feel secure without being confined to a single hiding place.

As a reef-dwelling species, standard marine aquarium conditions are suitable, with stable water quality being more important than chasing particular figures:

• Temperature: 24–27°C (75–81°F)

• pH: 8.1–8.4

• Specific gravity (salinity): 1.023–1.026

However, their carnivorous diet and eventual adult size mean they can produce a reasonable amount of waste, so effective biological and mechanical filtration is important. Good water movement and regular maintenance will also help maintain water quality.

Reef tank compatibility

Although the comet is a reef fish in the wild, keeping one in a home reef aquarium does come with a few considerations. Its predatory nature means that small fish and ornamental crustaceans, particularly shrimp and small crabs, may not be safe tankmates.

Corals themselves should generally be left alone, but the comet's cryptic nature and nocturnal lifestyle can make it something of a challenge to observe. It is often more active around dusk and after dark, and tends to favour shaded retreats during daylight hours.

This can present a challenge in brightly lit reef aquariums, particularly those housing corals with high lighting requirements. It's not necessarily a reason to rule out keeping one in a reef setup, though. Providing plenty of shaded areas and caves within the rockwork will give the comet somewhere to retreat while allowing the rest of the aquarium to receive the lighting it needs.

Ultimately, you may not see much of your comet during the day, but that's part of the appeal of keeping this rather unusual species.

Potential tankmates

The comet is generally peaceful to fish that are too large to be eaten, and its relatively sedentary nature means it doesn't tend to compete aggressively for swimming space. 

Small fish that could potentially be swallowed should obviously be avoided, as should particularly boisterous or aggressive species that may intimidate the comet or outcompete it for food.

Larger, relatively peaceful reef fishes are generally better choices, provided they don't harass the comet or compete too aggressively at feeding time. Suitable companions might include larger angelfish, tangs and peaceful wrasses. You'll need to research each species carefully to ensure it is compatible with both the comet and the size of your aquarium.

Diet and feeding

As a predatory reef fish, the comet requires a suitably meaty diet to thrive. The exact foods offered will depend largely on the size of the fish. Newly imported specimens are often relatively small and may initially accept foods such as enriched brine shrimp, Mysis shrimp and finely chopped krill. As they grow, more substantial offerings can be introduced, including pieces of lancefish, larger river shrimp and other chopped seafood.

Newly acquired specimens may not readily accept frozen foods, so be prepared to offer live alternatives while gradually attempting to wean them onto frozen foods. Gut-loaded live river shrimp can be particularly useful during this period, helping to provide a more nutritious meal while encouraging reluctant feeders to eat.

Prepared foods are also worth trying once the fish is feeding confidently, although acceptance can be variable. Some comets will eventually take suitably sized carnivore pellets, providing another useful option when establishing a varied long-term diet.

Given their nocturnal habits, offering food towards the end of the photoperiod or shortly after lights-out can also encourage more reluctant specimens to feed and ensure they get their fair share of the food. 

Is a marine betta right for you? 

The marine betta, or comet (Calloplesiops altivelis) is an unusual and attractive addition for a marine aquarium, but it's not necessarily a fish for every setup. Its secretive, nocturnal nature means it won't be the most visible inhabitant, while its predatory instincts limit the other species you can keep alongside it. 

However, for aquarists with a suitable aquarium, an appreciation of more unusual marine fish and the patience to wait for a glimpse of your prized fish, Calloplesiops altivelis can make a fascinating long-term addition. It might not be your average Betta, but that's precisely what makes it so interesting.

Comet fact file

Scientific name: Calloplesiops altivelis

Size: Up to 20 cm (7.9 in)

Origin: Indo-Pacific

Habitat: Reefs and drop-offs, hiding in crevices and caves during the day. 

Tank size: 100 x 55 x 45 cm (39 x 22 x 18 in)

Water requirements: Standard reef parameters, a of pH 8.1–8.4, 1.023–1.026 SG. 

Temperature: 24–27°C (75–81°F)

Temperament: Predatory and will eat smaller fish and crustaceans that will fit in its mouth. Otherwise, this species is fairly shy and peaceful and will not compete well with boisterous and aggressive tankmates. 

Feeding: A carnivore that requires plenty of meaty foods. May require live foods, such as river shrimp, to begin with but can be weaned onto frozen fare. Prepared carnivore diets may be taken once settled. 

Availability & cost: While not always in stock, stores should be able to source a comet fairly easily. Prices start from around £90.