The antennarid frogfishes are bizarre and comical. We take a close look at these great marine fish.
Few fish are stranger in appearance than the marine frogfishes, or anglerfishes as they’re also known. They have prehensile fins used like hands and legs to crawl along the substrate and grip rocks, tufts of 'hairs' on their skin and strange shapes and colours that enable them to blend with their environment.
Then, as their name suggests, they also have lures on their heads which they use like miniature fishing rods to catch unwary prey.
And in what you might think is something of an evolutionary oversight for a fish, they’re also pretty rubbish at swimming — though at least one has evolved jet propulsion to get around!
While too predatory to be kept with many species, they are great additions to a species tank and can be kept alongside placid fish too large to be considered food. They generally also leave sessile invertebrates, such as corals, well alone.
Although they look different, these small fishes — which range from 5-30cm/2-12” — are related to the anglerfish or monkfish you can buy in fancy restaurants and supermarkets.
They are all members of an order called the Lophiiformes, which includes goosefishes (or lophiids), walking batfishes (or ogcocephalids), handfishes (or brachionichthyids) and anglerfishes (or antennarids). There are more than 80 species in the order spanning some 15 genera and found in cold deepwater trenches and warm, shallow reefs.
The antennariid family includes more than 40 species in three subfamilies: antennariinae, lophichthyinae and tetrabrachiinae. The latter two contain a single species each: the flattened frogfish (Lophichthys boschmai) and the Four-armed frogfish (Tetrabrachium ocellatum).
The other 12 genera are all members of the antennariine group which includes Allenichthys, Antennarius, Antennatus, Echinophryne, Histiophryne, Histrio, Kuiterichthys, Lophiocharon, Nudiantennarius, Phyllophryne, Rhycherus and Tathicarpus. Of these only Antennarius and Histrio are commonly seen in the shops.
Although distributed throughout the marine tropics, around Mexico, Red Sea and Persian Gulf, most species are found around the Indo-Australian Archipelago. In fact the group is found in all tropical marine waters — except the Mediterranean.
Antennarius, which includes around 24 described species, is found globally and the group most often encountered in shops.
According to frogfish expert Dr Theodore Pietsch, Antennarius have been recorded from the Western Atlantic, near New York, the southernmost coast of Brazil, the coast of Senegal in the Eastern Atlantic and the Indo Pacific from the bottom of South Africa right over to Japan and New Zealand. They’ve also been recorded on the other side of the USA off California, the Galapagos Islands and off Chile.
Most antennariid frogfishes are from the shallows and many have evolved to blend with the bright colours of coral reefs. Some do this so well you’d have a tough job to spot them — as would their prey! Many are orange, red or yellow to blend with bright sponges, so although they may stand out like a sore thumb in the shop, they’re masters of disguise on the reef.
These fish have modified scales bearing structures called dermal spinules, as well as skin flaps and various fin filaments which make them look hairy and give them a comical bearded appearance. In most frogfishes these are relatively subtle, but in some species are so developed that they blend with coral polyps, and prey (and humans) find it difficult to make out the shape of the fish beneath.
Like all members of the Lophiiformes group, antennariine frogfish species have a modified dorsal fin spine near the front of the head that they use like a tiny fishing rod. This 'rod', formed from the spinous section of the dorsal ray, is called the illicium and has a lure on the tip called an esca.
The frogfish is an aggressive mimic. It pretends to look like a rock, bit of coral or a sponge, wiggles its lure to attract prey and swallows it whole. Very little energy is required to move the lure, so the anglerfish can sit motionless for long periods.
By finding a suitable place to sit on the reef and keeping very still, it just settles and waits for a passing fish, shrimp or crab. When it spots a snack, it wiggles the illicium and the attached esca to attract the prey which moves in for a closer look — only to be swallowed by that capacious, toothy gob.
One of the most fascinating things about frogfishes is their unusual mode of locomotion. In fact, frogfishes actually have three different ways of moving around. They can swim using beats of their tail fin — a technique called subcarangiform swimming. They can use enlarged muscles in their massive mouth and gill cavities to pump water rapidly out through small holes in their gills (called opercular openings) to allow them to swim via jet propulsion — and they can also 'walk'.
Frogfishes have muscular stumpy pectoral fins and similarly developed pelvic fins which they use in a similar manner to tetrapod limbs. They can use these modified fins to grip rocks and to walk around on the bottom. Indeed, they’re rather poor swimmers so this is their preferred mode of transport when they can be bothered to move.
How do I keep them?
Antennariid frogfishes range from around 6-25cm/2-10” and sometimes a fraction more. It’s therefore important to try to determine which species you’ve spotted for sale to ensure you’re providing it with the appropriate tank before any purchase.
They vary so much in colour pattern and shape that they can be tricky to identify and the shops or wholesalers rarely do this for you, so take a good fish guide along to check identities before you buy. There are currently dozens of described species found in six distinct species groups.
Most frogfishes are identified by colour pattern, shape and position and structure of the illicium and esca. Get a good look at this structure to identify any frogfish with any accuracy, as the esca varies between species. The illicium usually sits in a recessed channel in the head.
In some Antennarius the esca can resemble a fish and may even have stripes and a spot to mimic the position of an eye!
What size of aquarium would I need for these fish?
These are not a great choice for the typical reef tank, as small inhabitants such as clownfish, wrasse, shrimps and damsels will soon fall victim to their lure.
However, although not a reef-safe species, these fish behave most naturally in tanks with live rock and happily co-exist with corals and other sessile invertebrates, making them ideal for the fish-only with live rock or FOWLR set-up.
For smaller varieties you’ll usually be able to keep two in a tank of 60cm/24” or so. However, this set-up will need a decent protein skimmer as these can be big polluters. Larger species need proportionally larger aquariums, but need not be huge as these fish wait for prey to swim past rather than actively chasing them.
Should they be housed with tank mates?
Ideally, frogfishes ought to be the tank’s only piscine inhabitants. However, in a bigger tank you can get away with keeping larger more placid fishes too. Just ensure they’re too large to be lunch!
Frogfish can be mixed with their own species, but males can be aggressive with each other. Pairs usually co-exist happily.
Most Antennarius species have such big mouths they’re easily capable of gulping down fish and invertebrates larger than themselves. Once they’ve sucked in their prey, their needle-like teeth ensure it rarely gets away. The other factor that allows them to make the most of a mouthful is their stretchy stomach — seeming to expand beyond belief.
While they don’t do a lot, that’s part of their appeal. They prefer live foods, such as shrimp, but often adapt to defrosted frozen whitebait dangled on tongs after a short time. There’s usually no need to offer live fish as food.
Keeping them well fed is certainly important, as some fishkeepers who didn’t do so have had one of their frogfish swallow its partner!
This item first appeared in the Christmas 2009 issue of Practical Fishkeeping magazine. It may not be reproduced without written permission.