Antennarians – Frogfish
Reef Hobbyist Online Magazine
By: Anthony Calfo
image from flickr.com
Frogfishes and Anglers are found circumtropically, but are commonly imported from the tropical Atlantic, Hawaii and the Indo-Pacific for the aquarium trade. Most are collected small and have modest adult sizes ranging from 3-6" (7.5-15 cm), although a few popular species can approach or exceed 12" (30 cm). Live fishes and shrimp are the standard fare offered and accepted. Better candidates will accept frozen meaty foods in time (teasingly offered with forceps or a skewer). Few common names are needed for this group when the earliest ones are so befitting; Antennarian’s are called Anglerfish for their specialized "fishing apparatus" used for luring unsuspecting fishes, and Frogfishes for their large mouths and squat amphibian-like appearance. In popular literature, the Antennarians imported for the aquarium trade are perhaps best called "frogfishes". The name "anglerfishes" is generally ascribed to the deepwater denizens of the family with truly exaggerated lures. With little consequence or meaning, I'll favor the term "frogfishes" here and rest in the knowledge that its always best to cite species by scientific name whenever possible instead of or in addition to common names.
The first step to finding a healthy frogfish is just like preparing for the acquisition of any other aquarium specimen: know and understand their natural habitat and behavior first. Most frogfishes are benthic creatures that spend their entire lives on hard substrates (excluding the adult surface-dwelling Sargassum Histrio). By any measure, they are remarkably inactive creatures. As such, we must pay close attention to other aspects of behavior and carriage when evaluating new imports for purchase. Healthy lophiiform fishes are "bright" in appearance and behavior. Their eyes should be clear and aware, shifting to follow you and other stimuli that come near them. Respiration should be slow and deliberate. You’ll notice that their gill slits are very discreet in an effort to conceal their presence as living aspects and ambush predators on the reef. Closing one gill while pumping the other, in contrast, is a possible sign of gill parasites.
Unless recently fed, these fishes should respond positively to the introduction of food or prey by stalking or "angling" for them. If irritated by imposed stimuli, you can expect them to "walk" away… crawling across the seafloor with modified pelvic fins that resemble feet in form and function. Although it is their nature and habit to be inconspicuous… indeed cryptic… they will break formation and evade your exploratory net handle, gentle stick, or hand in the tank at some point. Complete inactivity may indicate a stressed or sick animal.
Newly arrived specimens should be permitted to settle in for some days (a week or more ideally) before taking them home for quarantine. It may be necessary or appropriate to offer your merchant a deposit to hold such fishes during that time. Patience in moving recently airshipped or otherwise transported specimens will go a long way towards lower rates of morbidity and mortality. Care should be taken to transfer all lophiiform animals in slow, deliberate motions, and not exposing them to the air where they may gulp in the atmosphere, or suffer under the unnatural weight of their gelatinous, scaleless bodies above water. Rest assured that they are slow and predictable enough to easily catch by coaxing them to walk along ("pushing" them) into a submerged plastic bag or specimen container. Wrangling and moving frogfishes with nets can cause serious and unnecessary damage to delicate tissues.
The specific coloration of individuals in this most cryptic family of reef creatures means little to nothing for identifying their health or even species. Their visage is ever-changing and adaptable, particularly with regard for coloration. The substrate upon which they rest or migrate to naturally is highly influential on the colors they show. Some species are especially variable and will match the shades of numerous benthic reef growths in colors of red, yellow, black, orange and much more. They may even appear to mimic textures like pores and exhalent openings in sponges. And we can find examples of species with physical extensions that resemble seaweed! All of this magnificent evolution testifies to their lifestyle as an ambush predator.
You will also notice that frogfishes have strategically upturned mouths – a clear indication of their lifestyle. They sit on the seafloor camouflaged, braced or wedged with their frog-like feet… and wait. When potential prey draws close enough to be of interest, the famous fleshy lure-tip (the esca) is utilized to draw them nearer. Where body coloration alone is useless to differentiate between like resembling species of frogfish, the esca can often be distinguishing. Some species have esca that have evolved to be wormlike, while others may remarkably resemble a small fish. If bitten off, then fleshy esca can indeed be regenerated after some months. The lure-pole of an anglerfish is, in fact, a modified dorsal fin spine (called the illicium). Much like a fishing pole, some lophiiform fishes angle with the extended tip of this spine like a fisherman angles for sport. Unlike a fisherman, however (well… a sober fisherman, assuming such a creature exists), anglerfish keep their lure very near to the mouth. When prey finally makes it to within striking distance, they draw them in with a sudden and convulsive suction produced by a rapid expansion of their mouth. Anglerfishes can expand their mouths into remarkable caverns by a ten-fold increase in size. The action takes mere milliseconds and is one of the fastest recorded movements in the animal kingdom (lagging just behind the lighting speed of the opening of a beer can by the aforementioned thirsty fisherman). This strategy can be described as aggressive mimicry (the use of a lure by frogfishes, that is… not the beer drinking by a fisherman).
Habitat is one of the first things a keeper needs to address with new fishes. As mentioned before, most frogfishes inhabit rocky environments, but many do indeed occur on soft sandy substrates and flats. There is of course the pelagic Sargassum species too (provide natural Sargassum seaweed or a like substitute both floating and anchored for display). From all niches, though, the lophiiforms tend to favor slower water flow and quiet environments. They can regularly be spotted in the wild near piers and seagrass beds. Where modern reef aquariums need at least 10-20X water flow per hour in the aquarium, keepers of these fishes need not exceed the range. Larger specimens producing larger amounts (volume and particle size) of waste will benefit from high water flow to process solid matter more efficiently. Naturally, heavy bio-filtration will be needed as with any predaceous fishes for the copious amounts of nitrogenous matter generated by large, heavy or messy feedings. Employ over-sized trickle/wet-dry filters and/or fluidized bed filters here.
The decor of the aquarium is a subject or many possibilities and great fun with frogfishes. Their proclivity to change color dramatically and rapidly is legendary. Their talents at mimicry challenge a keeper to provide ever-more interesting colors and textures to the artifacts in the aquarium with hope of seeing even more impressive displays of camouflage. Colored sponges in the wild are some of the very best backdrops for which to view frogfishes against. For their challenging nature in aquariums, however, live sponges are not recommended for most aquarists; they can be quite noxious and even toxic! Artificial ornaments are taken as perches just as readily as natural substrates in the aquarium. Some aquarists like to have great fun with swapping out various colored rocks and ornaments to the see the evolving show of color that unfurls with each change of venue as the fish moves around the tank. Provide some significant measure of hard substrate in the aquarium for essentially all in this group. Live seagrass displays (Thalassia or Syringodium) make outstanding habitats for most all species in this group overall.
Foods/Feeding/Nutrition and Compatibility
Interestingly perhaps, I lump "compatibility" in with "feeding" coverage here. This is done for some very practical if not obvious reasons with this predatory family. The rules that apply to both are very simple: if it can be eaten whole, it will… and if it can’t be eaten whole, it might be attempted anyway! Ahhh… right. These are simple rules to follow. And forgiving the slight exaggeration, you will do well to heed this warning, and your fishes will live longer for it. Antennarians will eat fish and most any motile invertebrate. Shrimps and small bony fishes are favored prey. Even members of their own species are fair game! The stories and legends of what some frogfish have eaten is amazing: with seabirds, lionfish and inflated spiny puffers, and more. And while tankmates clearly too large to swallow whole generally will be ignored, it’s best instead to keep only one frogfish per tank and perhaps in isolation as a species-tank. As mentioned previously, numerous active community fishes if not predated may in turn nip or harass lophiiforms. We should not avoid the toothy predators alone here, but even the smaller or less predatory fishes that naturally graze upon the reef are a calculated risk; they can mistake the lophiiform animal for part of the living substrate and rasp dangerous wounds into its flesh as if it were encrusted benthic fauna! Without traditional scales, venom, spines or other exaggerated means of defense, the soft-bodied Antennarians are fairly vulnerable at large. One means of defense employed is to quickly inflate their stomachs with water to foil attempts by some predators to inhale them. Unfortunately, the strategy is useless against predators with large enough mouths or those with larger teeth like puffers and triggers. There is also the concern of territoriality between frogfishes. Although some will tolerate each other, often times they demonstrate intolerance except briefly during breeding season. You can expect the same intolerance of other fishes in aquaria in most cases.
Keeping frogfishes in species tanks also makes feeding time much easier with these very slow predators. As one can imagine, active feeding fishes will simply steal small live feeder shrimp or fishes away before the lumbering frogfishes can get to them. Some individuals after acclimation will take prepared foods while others are rather slow to wean off of preferences for live prey. Most however, can be trained in time to take dead meaty foods from a feeding stick. And much like other commonly "stick-fed" predators (eels, octopuses, mantis stomatopods, etc.), lophiiforms will demonstrate individual preferences for how they "like" to be fed. Some will respond to a moderate to vigorous tease of killed prey dancing by the movement of your hand on the stick, while others show irritation to excess stimulation and may even require repetitive drops of food chunks through the water column sans stick. Experimentation with the delivery of dead or prepared foods is required on a specimen-by-specimen basis.
Try to always offer saltwater aquarium inhabitants foods of marine origin. Few freshwater or terrestrial foodstuffs are adequately nutritious to constitute any significant part of a marine animal’s diet. Even prepared meats of marine origin are deficient in some ways if cleaned, gutted or otherwise rendered incomplete. Whole prey are best (head, guts, legs, fins, wristwatches, whatever… intact). Shrimp, krill, and silversides are common fare for frogfishes. HUFA rich supplements are recommended here too to soak thawing foods in. For live foods, be sure to maintain a proper holding tank of prey as if in quarantine, and never feed live prey that has been only freshly acquired for fear of transmitting an infectious disease. Keep feeders for a minimum of 2 weeks to reduce the chance of a pathogenic transmission. It would be better still to maintain your own breeding colony of live-bearing mollies, for example, that are brackish or saltwater tolerant and can be gut-loaded with nutritious foods before being offered as prey. Avoid freshwater goldfishes as a staple food item (nutritive concerns) or un-quarantined saltwater baitfishes (disease risk), but rely on killed prey (frozen meats) to exclusion instead if you must. Small live ghost/grass shrimp (Palaemonetes) are also quite good food items. And true to form with all of this hype about frogfishes being voracious predators, they can be observed angling with their lure day or night! Some have suggested that nocturnal prey can sense the feel or vibration of the lure at night, but at least one species of frogfish has been documented to contain bioluminescent bacteria in its lure - lending the frogfish full-time feeding opportunities.
Whether you opt for live or thawed frozen foods for your frogfishes, be very mindful of prey size: items that are too large may still be accepted but nonetheless are dangerous. Proffer no items larger than 20-30% of the animals total body size; smaller is always better. Oversized prey can harm or kill greedy lophiiforms by taking too long to digest. It will hinder respiration and, less commonly, may build up gasses in the digestive system from decay that causes the animal to struggle with buoyancy issues. Much like anemones, which also get commonly overfed with food chunks that are too large, the animal will often regurgitate the meal later – perhaps after the lights go out and causing attrition if repeated habitually, despite the keepers best intentions. Most marine fishes fare better with small frequent feedings. Frogfishes will tolerate only a few hearty feedings weekly with extra offerings to the smaller and younger specimens.
Spawning and reproductive activity in frogfishes is fairly well documented and frequently observed. In fact, they appear to be one of the earliest species observed to reproduce in modern aquaria. Frogfishes are even commented upon in classical history by none other than Aristotle, 344 B.C.! Like so many other marine fishes, it is the rearing of their larvae that has been so elusive to aquarists. Oftentimes, these events in aquaria occur soon after the import of a gravid female. Unfortunately, fertilization of the eggs is external, and fruitless without a male of course! Dimorphism is not apparent in most species – the girls and boys generally look alike... to us humans at least. Reproductive females become quite swollen and egg-laden just prior to copulation (mere hours/days prior). Some frogfishes move to deeper waters to spawn, but overall they are regarded as relatively shallow water denizens of the reef. At least Histrio, the Sargassumfish, has been observed to spawn year around with no apparent season or reproductive cycle. Courtship occurs by day, although the spawning event itself may occur day or night. Males chase the gravid and clumsy females by "nudging" them along and above the seafloor just prior to a brisk dash by the pair to the surface for egg release and subsequent fertilization. Spawns are comprised of gelatinous rafts or ribbons that usually float. Both Antennarius and Histrio have been observed to spawn in aquariums. We have good reason to ultimately be hopeful of rearing frogfishes successfully in aquaria as the eggs are large and the planktonic stage for larvae in some can be fairly short (21 days, Thresher 1984), although the range in this family extends as far as 2 months or more. Although broadcast spawns are the rule in this group, some demersal strategies have been observed with the parental care of egg clusters upon the flanks of the adult frogfish. Its best to separate frogfishes in aquaria after a spawning event as males can become aggressive or belligerent. Frogfishes by nature are solitary animals.
Disease and Health Issues
On a scale of susceptibility to disease, frogfishes rank on the weak side; they are rather prone to external parasites, even beyond the common Cryptocaryon and Amyloodinium infestation. For the treatment of external parasites, use copper strictly and with close supervision on these scaleless fishes. Maintain therapeutic levels diligently with small daily doses, and be sure to monitor its use with a copper test kit. Formalin based medications have also been shown to be helpful here. With good water quality and regular feeding, bacterial infections are uncommon in this group. Yet, new imports may show such symptoms or receptive wounds from the stress of capture and repetitive, abrasive contact with the walls of the shipping vessel. Treatment in quarantine with broad-spectrum antibiotics is effective on such fishes. And lastly, issues of gas accumulation that lead to swimming difficulties are observed in frogfishes. Although not immediately perilous, they are a source of great duress for the clumsy swimming or struggling victim (difficulty feeding, avoiding features of set-up and hardware like intakes and overflows, etc.). The two most common causes are gulping air from being inappropriately removed from water, or from being fed food that’s too large. The former cause can often be remedied with a (latex) gloved hand in the water grasping the fish and gently massaging it or harnessing a wriggling attempt at escape; orient the mouth of the afflicted upwards with the intent that stimulation will burp then air free. Overall, issues of disease and health among frogfishes and walking batfishes are relatively uncomplicated and can easily be tempered by good selection, proper quarantine and dutiful husbandry of specimens in species tanks.
This group at large can make very interesting and worthwhile aquarium guests. Their needs are somewhat specific if not challenging (hand or live feeding, large bio-filters, exclusion from community tanks), but their merits abound. They are fascinating physically and behaviorally, and occur in seemingly countless and changeable colors and textures. The potential for captive reproduction in Antennarians is very plausible for aquarists too. Seek specimens from the nearest points of collection (Florida and Hawaii for American aquarists) to reduce the stress of import on captive specimens.