| Frogfish | |
|---|---|
| Striated frogfish, Antennarius striatus | |
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Actinopterygii |
| Order: | Lophiiformes |
| Family: | Antennariidae |
| Genera | |
|
Allenichthys |
|
Frogfishes, family Antennariidae, are a type of anglerfish in the order Lophiiformes. They are known as anglerfishes in Australia, where 'frogfish' refers to a different type of fish. Frogfishes are found in almost all tropical and subtropical oceans and seas around the world, the primary exception being the Mediterranean Sea.[1]
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Frogfish live in the tropical and subtropical regions of the Atlantic and Pacific, as well as in the Indian Ocean and the Red Sea. Their habitat lies for the most part between the 20-degree isotherms, in areas where the surface level water usually has a temperature of 20 C (68 F) or more. They extend beyond the 20-degree isotherms in the area of the Azores, Madeira, and the Canary Islands, along the Atlantic coast of the USA, on the south coast of Australia and the northern tip of New Zealand, coastal Japan, around Durban, South Africa, and at Baja California.[2] The most types are found in the Indo-Pacific region with the highest concentration around Indonesia. In the tiny Lembah Strait, north-east of Sulawesi, divers have found nine different species. Frogfish live generally on the ocean floor around coral or rock reefs, at most up to 100 meters (330 ft) deep.
There are a few exceptions to these general limits. The Brackishwater Frogfish is at home in ocean waters as well as brackish and fresh water around river mouths.[3] The Sargassumfish lives in clumps of drifting sargassum, which often floats into the deeper ocean and have been known to take the sargassumfish as far north as Norway.[4]
Frogfish have a stocky appearance, atypical of fish. Ranging from 2.5–38 cm (1-15 inches) long, their plump, high-backed, non-streamlined body is scaleless and bare, often covered with bumpy, bifurcated spinules. Their short bodies have between 18 and 23 vertebrae and their mouths are upward pointed with palatal teeth. They are often brightly colored, white, yellow, red, green, or black or spotted in several colors in order to blend in with their coral surroundings.[2] Coloration can also vary within one species, making it difficult to differentiate between them.
Rather than typical dorsal fins, the front-most of the three fins is called the illicium or "rod" and is topped with the esca or "lure". The illicium often has striped markings, while the esca takes a different form in each species. Because of the variety of colors even within a single species, the esca and illicium is a useful tool to differentiate among different varieties.[5] Some of them resemble fish, some shrimp, some polychaetes, some tubeworms, and some simply a formless lump; one genus, Echinophryne has no esca at all. Despite very specific mimicry in the esca, examinations of stomach contents do not reveal any specialized predation patterns, for example, only worm-eating fish consumed by frogfish with worm-mimicking esca. If lost, the esca can be regenerated. In many species the illicium and esca can be withdrawn into a depression between the second and third dorsal fins for protection when they are not needed.[6]
Frogfish have small, round gill openings behind their pectoral fins. With the exception of Butler's frogfish and the Rough Anglerfish, frogfish use a gas bladder to control their buoyancy.
The unusual appearance of the frogfish is designed to conceal it from predators and sometimes to mimic a potential meal to its prey. In ethology, the study of animal behavior, this is known as aggressive mimicry. Their unusual shape, color, and skin textures disguise frogfish. Some resemble stones or coral while others imitate sponges, or sea squirts with dark splotches instead of holes. In 2005, a species was discovered, the striated frogfish, that mimics a sea urchin while the sargassumfish is colored to blend in with the surrounding sargassum.[4] Some frogfish are covered with algae or hydrozoa. Their camouflage can be so perfect, that sea slugs have been known to crawl over the fish without recognizing them.
For the scaleless and unprotected frogfish, the camouflage is an important defense against predators. Some frogfish can also inflate themselves, like pufferfish, by sucking in water in a threat display.[7] In aquariums and in nature, frogfish have been observed, when flushed from their hiding spots and clearly visible, to be attacked by clownfish, damselfish, and wrasse, and in aquariums, to be killed.
Many frogfish can change their color.[2] The light colors are generally yellows or yellow-browns while the darker are green, black, or dark red. They usually appear with the lighter color, but the change can last anywhere from a few days to several weeks. It is unknown what triggers the change.[2]
Frogfish generally do not move very much, preferring to lie on the sea floor and wait for prey to approach. Once the prey is spotted, they can approach slowly using their pectoral and pelvic fins to walk along the floor.[6][8] They have two "gaits" that they can use. In the first they alternately move their pectoral fins forward, propelling themselves somewhat like a two-legged tetrapod, leaving the pelvic fins out. Alternately, they can move in something like a slow gallop, whereby they move their pectoral fins simultaneously forward and back, transferring their weight to the pelvic fins while moving the pectorals forward. With either gait, then can only cover short stretches.
In open water, frogfish can swim with strokes of the tail fin. They also have a kind of jet propulsion that is often used by younger frogfish. It is achieved by rhythmically forcing their breath-water out through their gill openings, which lie behind their pelvic fins.[8]
The sargassum frogfish has adapted fins which can grab strands of sargassum, enabling it to "climb" through the seaweed.[2]
Frogfish eat crustaceans, other fish, and even each other. When potential prey is first spotted, the frogfish follows it with its eyes. Then, when it approaches within roughly seven body-lengths, the frogfish begins to move its illicium in such a way that the esca mimics the motions of the animal it resembles. As the prey approaches, the frogfish will slowly move itself to prepare for its attack; sometimes this involves approaching the prey or "stalking" while sometimes it is simply adjusting its mouth angle. The catch itself is made by the sudden opening of the jaws, which enlarges the volume of the mouth cavity up to twelve-fold, pulling the prey into the mouth along with water.[6] The attack can be as fast as 6 milliseconds.[9] The water flows out through the gills, while the prey is swallowed and the esophagus closed with a special muscle to keep the victim from escaping. In addition to expanding their mouths, frogfish can also expand their stomachs to swallow animals up to twice their size.[6]
Slow-motion filming has shown that the frogfish sucks in its prey in just six milliseconds, so fast that other animals can't see it happen.[6] This is less time than it takes a muscle to contract so the source of the motion remains unknown. Frogfish have also been observed to eat lionfish; the latter's poison appears to have no effect.
The reproductive behavior of the normally solitary frogfish is still not fully researched. There are few observations in aquariums and even fewer from the wild. Most species are free-spawning, with females laying the eggs in the water and males coming in behind to fertilize them. Anywhere from eight hours to several days before the egg-laying, the abdomen of the female starts to swell as the eggs absorb water, sometimes as many as 180,000 eggs.[5] The male begins to approach the female around two days before the spawning. It is not known if the spawn is predetermined by some external factor, such as the phase of the moon, or if the male is attracted to a smell or signal released by the female. In all hitherto observed breeding pairs, one partner was noticeably, sometimes as much as ten times, larger than the other. When the gender could be determined, the larger partner was always the female.
During the free-spawning courtship ritual, the male swims beside and somewhat behind the female, nudges her with his mouth then remains near her cloaca. Just before the spawning, the female begins to swim above the ocean floor toward the surface. At the highest point of their swim they release the eggs and sperm before descending back. Sometimes the male pulls the eggs out of the female with his mouth. After mating the partners depart quickly as otherwise the smaller male would likely be eaten. A few species are subtrate-spawners, notably the genuses Lophiocharon, Phyllophryne, and Rhycherus which lay their eggs on a solid surface like a plant or rock. Some species guard their eggs, a duty assigned to the male in almost all species, while most others do not.[5][6] Several species practice brood carrying, for example the three-spot frogfish, whose eggs are attached the male, and those in the genus histiophryne, whose brood are carried in the pectoral fins.
The eggs are .5-1mm (.02-.04in) large and cohere in a gelatinous mass or long ribbon, which in Sargassumfish are up to a meter (3.3 ft) long and 16 cm (6 in) wide. These egg masses can include up to 180,000 eggs.[5][10] For most species, the eggs drift on the surface. After two to five days, the fish hatch and the newly hatched alevin are between .8 and 1.6 mm long (.03 and .07 in). For the first few days they live on the yolk sac while their digestive systems continue to develop. The young have long fin filaments and can resemble tiny, tentacled jellyfish. For one to two months they live planktonically. After this stage, at a length of between 15 and 28 mm (.6-1.1 in), they have the form of adult frogfish and begin their lives on the sea floor. Young frogfish often mimic the coloration of poisonous sea slugs or flatworms.
Hardly any fossil remains of frogfish have been found. In the north-Italian formation at Monte Bolca, which was formed from the sedimentation of the Tethys Ocean in the middle Eocene, a three centimeter (1.2 in) fossil was found and named Histionotophorus bassani and was initially described as a frogfish. However, today it is thought that the genus histionotophorus is the same as the still extant genus Brachionichthys or Handfish, a close relative of frogfish. In 2005, a fossilized frogfish from Miocene Algeria was described. Named Antennarius monodi, it is the first proven fossil from Antennariidae and is believed to be most closely related to the extant senegalese frogfish.[11]
Many public aquariums around the world keep frogfish. Frogfish are also available privately from appropriate specialty shops and websites. Though they are prized for their unique appearance, they are not easy to keep for a number of reasons. They often refuse food or become very fat when they do accept it. Furthermore, they must usually be kept alone as they will eat anything up to twice their size, including other frogfish, even potential mates. Also, as they can change their coloration, many lose the bright patterns they have initially, especially when kept in empty or more dull-colored tanks as opposed to the bright coral environments they are used to.
Though frogfish have been observed to spawn in captivity, captive breeding is difficult. It is currently not known how to sex most species without dissection so keeping an appropriate pair together raises problems. Furthermore even if they do spawn, raising fry through the planktonic phase is very difficult because the larval fish require extremely tiny live food and must be protected from other predatory fish.
Frogfish belong to the order lophiiformes commonly known as anglerfish, which is a member of the superorder Paracanthopterygii with four other orders. They belong to the infraclass Teleostei in the class Actinopterygii.
Within the family Antennariidae there are 45 species in twelve genera: