Written by staff reporter Monday, 17 August 2009 00:00
To find a frogfish on a dive is something of a triumph, as these ambush specialists are masters of disguise. Douglas David Seifert lifts the lid on the private life of one of the sea’s most secretive predators
Man’s earliest encounters with this cryptic fish have held sway over the imagination, and accounts – considered fishy tales at best or barefaced sailor’s lies at worst – were passed along until the most free-thinking and insightful naturalist in ancient Greece, Aristotle, first described ‘the fishing-frog’ in his Historia Animalium: ‘The fishing-frog has a set of filaments that project in front of its eyes; they are long and thin like hairs, and are round at the tips; they lie on either side, and are used as baits. Accordingly, when the animal stirs up a place full of sand and mud and conceals itself therein, it raises the filaments, and, when the little fish strike against them, it draws them in underneath into its mouth.’
No one can say where Aristotle encountered his ‘fishing-frog’. Although it is possible he came across a specimen while accompanying Alexander the Great into the Red Sea or the Indian Ocean, it seems more likely that he was describing a species of anglerfish, which are plentiful in the Mediterranean and are as highly prized at the fishmonger’s as the monkfish or the goosefish. The description of multiple hair-like lures denotes these anglerfish, which possess three such lures, whereas frogfish have but one and are more of a broad, flattened body type than the frogfish’s stumpy, squat form.
Frogfish, of the family Antennariidae, are members of the anglerfish clan (the order Lophiiformes, with more than 300 species in 65 genera and 18 families, including deep-sea anglerfish and the red-lip batfish). They are conspicuously absent from the Mediterranean, although they are otherwise found in tropical and temperate waters worldwide: some are endemic, some widely distributed; some are rare, yet five species can be found in all the oceans. With the exception of the sargassumfish (Histrio histrio), which lives exclusively in floating rafts of sargassum weed, all other frogfish are bottom-dwellers.
To avoid confusion, it is important to remember that although all frogfish are anglerfish, not all anglerfish are frogfish. The frogfish family is but one of 18 families of the anglerfish, and consists of 12 genera and between 40 and 50 species. Confusion further reigns within the diverse frogfish family due to widespread distribution and similarity of shapes and colour patterns between different species – and the extraordinary variety of shapes and colours and sizes within the same species – to the extent that even scientists have difficulties identifying frogfish at first glance.
When considering the frogfish, the most important feature is the one that makes it so fascinating; the evolutionary development that has made it the charismatic and successful predator of today: its natural, onboard fishing kit. The fishing rod is located at the tip of the frogfish’s snout and is, in fact, a modification of the first dorsal fin ray, called the illicium (from Latin, meaning ‘alluring’). Atop the illicium resides the esca (also from Latin, meaning ‘bait’). It is a fleshy appendage that is species-specific, meaning that each frogfish species has one that is unique in shape and size.
The esca dangles from the illicium and serves to mimic specialised prey items – such as fish, shrimp, worms or mysids – of the frogfish’s own intended and preferred prey. On close examination, the details of the esca’s mimicry of actual animals reveals aspects as enigmatic and astonishing as perfectly positioned eye spots or leg-like mobile appendages. Also, in some species, the esca also produces chemical stimuli that mirror the scent of the mimicked prey, further attracting and enticing the predators of such prey.
Not only does each species of frogfish have a mimic bait for its own preferred prey, but through manipulations of the illicium, they can make that bait wiggle, dance, vibrate, twitch or swim in the manner of the animal it is impersonating. This remarkable ability, coupled with the frogfish’s gift for camouflage, makes it an extremely efficient predator.
On rare occasion, a would-be meal gets the better of a frogfish and its lure and eats or damages its biological angling apparatus. In time, both parts can regenerate to an extent: if not completely replaced, at least enough grows back to provide a functional lure.
The frogfish is the ultimate lie-in-wait predator. Each species has its environmental niche, choosing the ideal background to blend with by means of coloration, often in combination with spots or stripes and a body texture comprising fleshy protuberances, warty nodules or filamentous tendrils. They can even change their coloration completely over a short period of time to match environmental conditions if it favours predatory success. Frogfish can vary in size from coin-sized juveniles to adults of 5–50cm, depending on the species.
Frogfish exhibit a stoic stealth that makes other ambush predators seem positively hyperactive; a frogfish can remain motionless for hours, if not days. Then, as potential prey wanders into the frogfish’s territory, even if that potential prey is the same size as the frogfish, the illicium and esca come to life, flicking rhythmically just in front of the frogfish’s mouth, enticing a greedy fish to its doom.
As the lured fish has its attention intently focused on the esca and its movements, the frogfish waits, preparing to spring like a mousetrap. Once the distance between the prey fish and the frogfish’s mouth has decreased to a sweet-spot proximity, one of the most astonishing feats of predation behaviour takes place. The frogfish throws its head up and rapidly expands its mouth outward in a lightning motion.
The amount of expansion in this spectacular gape is 12 times the volume of the closed mouth’s oral cavity. The energy created by this rapid expansion and equally rapid compression of the frogfish’s oral cavity acts as a vacuum as the frogfish simultaneously engulfs the prey fish and the surrounding water with a suction power that sets the record for gulping and swallowing in the animal kingdom.
Scientists Theodore W Pietsch and David B Grobecker conducted experiments to measure the predation behaviour and speed. The actual gape and suck happens so rapidly, it is almost impossible to perceive with the human eye. Only by using high-speed cameras to record and study the event could the scientists determine that the oral expansion and engulfing of the prey takes six milliseconds from start to finish (the only other marine creature with a similar feeding-suction power and speed is the seahorse, which coincidentally shares many of the frogfish’s habitats and is often frogfish prey).
Despite all these impressive techniques and adaptations, frogfish are, alas, not especially intelligent. Charming, yes; clever, no. In a study conducted by Jack Randall, ichthyologist par excellence, and others, ‘737 species of fish were measured for brain size relative to total weight. Fishes that sit well camouflaged on the bottom to be unnoticed by predators and their prey have the smallest brains. Frogfishes, scorpionfishes and soles are among the lowest in relative brain size.’ As Randall’s colleague, John Earle, summed it up: ‘How smart does a mousetrap have to be?’
Probably a bit more than they are. Frogfish frequently die from engulfing prey too large for them to digest. A frogfish may try to eat anything its own size and may survive, but eating animals bigger than itself almost always has fatal consequences for both outsize prey and over-ambitious predator. Interestingly, they are not above cannibalising their own species, alongside other frogfish family members. For this reason, they’re usually solitary.
When two or more frogfish are seen together on a reef, it is the prelude to reproductive behaviour. The males, generally smaller, are attracted to a female that is producing eggs internally. As the female swells with eggs, the male becomes extremely attentive, following her at close proximity, touching her with his fins, nuzzling her with his body. Other males may also be present, vying for the opportunity to fertilise her eggs when the moment is at hand.
When the time is right, she expels the eggs in a cataclysmic burst and the male immediately fertilises them. In some species, the eggs are laid in a spiral raft that will float away into the water column; in others, the eggs are laid on the male or on a substrate, and the male remains with them until they hatch.
Once the physical act of reproduction is concluded, the frogfish go their separate ways – given their cannibalistic bent, it’s healthier for the smaller partner this way... ■