But They Don't Look Like a Rat with a Fuzzy Tail: The Family Holocentridae

Members of the fish family Holocentridae will never take home an award for being the most sought after marine fish. They cannot compare with the remarkable colors of fairy wrasses, the classic beauty of marine angelfish, or the puppy-like nature of puffers. Perhaps it is just this - the lack of beauty or charismatic personality, which makes the Holocentridae good aquarium candidates. While hobbyists can travel to countless aquariums stretching across the world, throughout their travels they will likely become bored seeing Yellow Tangs, Coral Beauty Angelfish, or (insert any other common fish). But, in all these loacations Holocentridae species might be absent from all those aquariums. With a holiday toast to being unique and swimming against the flow, I present the unheralded Squirrelfish.

A magnificient photo of a captive Neoniphon sammara.
Photo courtesy of Greg Rothschild.

The marine fish family Holocentridae is divided into two subfamilies: Myripristinae and Holocentrinae. Frizzell and Lamber (1961) and Hecht (1982) concluded differently regarding this classification, however. They used a combination of recent and fossilized otoliths to determine that these two subfamilies should actually be regarded as two separate families. However, to this day most other authorities still regard the two as subfamilies. These two subfamilies are known commonly as the Soldierfishes (Myripristinae) and the Squirrelfishes (Holocentrinae). Although the two subfamilies appear to be nearly identical to the casual observer, there are significant external differences, most notably the preopercle, a bone in the cheek region. Holocentrids have a sharply angled preopercle, which has a long spine at the corner. In many species this spine is venomous (Randall, 1998). The Myripristids lack this spine in most cases, but some species may have one short, broad, non-venomous spine. Of course, there is an exception to this rule; the monotypic Corniger spinosus has 1 - 2 sharp spines. In addition to the spine, Holocentrids have a narrow mucous channel, compared to the wide mucus channel of Myripristids (Randall, 1998).

Holocentrinae has three genera, Holocentrus, Neoniphon and Sargocentron. Thirty-three species are now classified as Sargocentron (see below), though it took several revisions to arrive at this current classification. Fowler (1904a) initially launched the subgenus of Sargocentron and placed it within the Holocentrinae. After additional investigation, Fowler (1944) forty years later, raised Sargocentron to genus level. Later, however, Woods (1955) disagreeing with Fowler's interpretation, ignored this revision and referred to Sargocentron as a subgenus of Holocentrus. Adding to the confusion, when Shimizu and Yamakawa (1979) concluded their review of Holocentrinae, Sargocentron was not included at any level. However, Matsuura and Shimizu (1982) concluded Adioryx and Sargocentron were congeneric. Adioryx was originally described by Woods (1965) with the western Atlantic Adioryx poco. Sargocentron, having been originally described 61 years prior to Adioryx, takes precedence and therefore Adioryx is a junior synonym of Sargocentron.

Flammeo is often used instead of Neoniphon because Woods and Sonoda (1973) used this name in preference to Neoniphon, because Neoniphon was "based on a species whose status is uncertain." However, the original description of Neoniphon by Castelnau (1875) is accurate, and thus Flammeo is a synonym of Neoniphon (Randall and Heemstra, 1985).

In addition to the above mentioned species, a complex of five species, Sargocentron inaequalis, S. iota, S. lepros, S. macrosquamis, and S. shimizui, is recognized. All are smaller and possess a distinguishing retrorse spine on the surface of the nasal bone (except S. shimizui, which lacks this spine).

Squirrelfish of the genus Sargocentron can be found throughout the tropical and subtropical waters of the world. Five species (Sargocentron bullisi, S. coruscum, S. furcatum, S. lacteoguttatum, and S. vexillarium) originate from Atlantic waters in the Caribbean; a single species (Sargocentron suborbitalis) ranging from the Gulf of California to Ecuador, comes from the Eastern Pacific, and the remaining 27 species are found in the Indo-Pacific range. Members of the genus Holocentrus are located entirely in the Caribbean, while one Neoniphon (N. scythrops) is located in the Caribbean and the remaining four are from the Indo-Pacific. The majority of Squirrelfish are widespread geographically, including S. punctatissimum and S. tiereoides, which can be found from the east coast of Africa to Easter Island in the far southeast of the South Pacific. One species, Sargocentron shimizui, has the smallest distribution, only being described from Sulawesi.

Holocentrus adscensionis is rather common in the Caribbean, from Florida
south to Brazil. They are regularly available within the trade, and generally can
make a quick transition into the home aquarium. Photos courtesy of Greg Taylor.

The depth range of this genus ranges from one or two meters, (S. hormion, S. caudimaculatum, S. cornutum) to over 180 meters (juvenile S. punctatissimum). However, the majority of adult Squirrelfish are found shallower than 50 meters. Squirrelfish inhabit both the shallow inshore reefs and the deep offshore reefs and generally maintain their presence underneath overhangs. They are almost always found in small groups, usually consisting of eight to ten individuals.

Typically, upon first seeing a Squirrelfish, the unusually large eyeball is quickly noted. Squirrelfish have this large eye because they are primarily nocturnal and the large eye assists with gathering all available light, usually moonlight. Specimens can sometimes be seen during daylight hours, although they will appear reclusive and will rarely venture out from underneath their overhangs.

The cave or overhang is most often used both as a shelter and a hiding place. Squirrelfish have few natural defenses other than staying out of sight from larger piscivores. Once in view of a predator, squirrelfish will attempt to use their bony, spiny nature to deter the attack. All of the dorsal and anal rays are flexed, displaying their spines, and attempting to appear larger than they truly are. As a last ditch effort, venom from the spine on their operculum may be injected into fish that attempt to swallow them.

The diet of squirrelfish is comprised mostly of crabs and shrimps, though occasionally small fish may be taken. One study (Hiatt and Strasburg, 1960) investigated the stomach contents of 13 Sargocentron diadema, revealing a diet of gastropods, polychaetes, crustaceans (in particular xanthid crabs), small clams, and a single solenogastre. In a separate study (Harmelin-Vivien, 1979) 36 of the 76 specimens collected during the day had an empty stomach. In sharp contrast, however, only one specimen of the 46 collected in the evening had an empty stomach.

Squirrelfish have the ability to produce a multitude of sounds. By contracting muscles, causing vibrations of their swim bladders, they can create audible sounds that range from quick cracks and pops to a constant rumble at frequencies from 75 - 85 Hz (Carlson and Bass, 2000). Interestingly enough, the swim bladder is not only responsible for vocalization, but it also assists in their hearing sounds produced by other squirrelfish. Because there is a tight association between the rostral end of the swim bladder and the auditory bulla, the swim bladder functions as a pressure transducer, resulting in enhanced hearing sensitivity (Carlson and Bass, 2000). Carlson and Bass hypothesized that squirrelfish are able to communicate both within and outside of their own species.

Oddly enough, knowledge of the mating and spawning habits of Squirrelfish is woefully inadequate for a group found in such large numbers in shallow water. This is presumably due to their nocturnal nature. It is likely that mating and spawning occurs entirely after nightfall and includes all members of the small group. Of the minimal information available, it appears females are routinely larger than males, though the difference is often 10mm or less. No other external differences are readily observable between the two sexes. Eggs are pelagic, though no one is sure exactly how long they are free-living. What is known, however, is that juvenile squirrelfish settle out of the plankton at a size that is relatively large for a reef fish, sometimes measuring over one inch of total length.

Squirrelfish are hardy aquarium fish and usually make a seamless transition to captive life provided a few basic needs are satisfied. The first obstacle that must be overcome is providing them with a suitable habitat. Squirrelfish will require a fairly decent sized cave or overhang. The larger your captive group of squirrelfish, the more room that will be required underneath the ledge. Squirrelfish will spend the vast majority of their daytime hours below their overhang or within their cave, patiently waiting for nightfall. With a little forethought, the hobbyist can set up their display to both meet this need of the fish, and their own desire for fish watching.

Squirrelfish are relatively disease resistant in the home aquarium. Potential problems that may arise are likely due to the opercular spine. Much like the similar spine of marine angelfish or the spine near the base of the tail of the surgeonfish, the spine of squirrelfish has a tendency to become caught in nets or embedded in the hands of their caretakers. When caught in nets, the spine may break off or become damaged. If this occurs, the wound may become a potential hazard to a bacterial infection, most notably Aeromonas or Vibrio. The future is not promising for these fish, and it is best to avoid the purchase of squirrelfish with damaged operculum spines. However, if this spine becomes embedded in your hand, special care does need to be taken. As noted above, some squirrelfish harbor a venom within this spine. The reaction for humans is reported to be less severe than similar stings from lionfish or other scorpaenids. However, seeking out immediate medical attention is still in the aquarist's best interest, especially if there is a history of known allergies to insect stings.

The aquarium size for squirrelfish is not a demanding requirement. Squirrelfish remain fairly inactive throughout much of their life. They are nearly entirely inactive during the day, and at night only become active enough to find food. The main consideration is the size of the fish - many squirrelfish can reach 12" or larger in an aquarium, though most remain six to seven inches. In order to house a small harem or group of squirrelfish, each at 12", a rather large aquarium will be required. If space is a at a premium, you may prefer to house only one, or possibly two or three, squirrelfish or opt for smaller members of the family. In this instance, a traditional 75 gallon aquarium would be more than suitable. However, what is more often the deciding factor for aquarium size is the type of tankmates that are desired. Squirrelfish, although not threatening to larger fish, still prefer a peaceful aquarium. If you prefer to house your squirrel with active swimmers, be sure to increase the aquarium size accordingly.

Although fairly non-aggressive, squirrelfish have a propensity to swallow most anything that will fit into their mouths - fish are no exception. Therefore, any tankmates must first be considered as potential prey. Any fish that has the probability to fit into the mouth of squirrelfish is best left to another aquarium unless the intention is to offer it as food. For that matter, the aquarist would be wise to not only include fish in this category but also any mobile invertebrate. Shrimps, crabs, snails, bristleworms, and starfish all fit into the profile as potential prey. To keep this simple, if it walks, crawls, slithers, or swims, it might be squirrelfish food. It should be obvious at this point that squirrelfish are not good inhabitants for the traditional reef aquarium that is typically teaming with small invertebrates. However, if your goal is to remove invertebrates, such as bristleworms, from your aquarium, then a squirrelfish may be a good option.

Compatibility chart for Holocentrids:

Fish	Will Co-Exist	May Co-Exist	Will Not Co-Exist	Notes
Angels, Dwarf			X	Adult squirrelfish will consume dwarf angels.
Angels, Large	X			Should be good tank mates.
Anthias			X	Will be consumed by squirrelfish.
Assessors			X	Will be consumed by squirrelfish.
Basses			X	Will be consumed by squirrelfish.
Batfish		X		Adult squirrelfish can consume juvenile batfish.
Blennies			X	Will be consumed by squirrelfish.
Boxfishes		X		Adult squirrelfish can consume juvenile Boxfishes.
Butterflies		X		Adult squirrelfish can consume juvenile butterflies.
Cardinals			X	Will be consumed by squirrelfish.
Catfish	X			Should be good tank mates.
Comet			X	Will be consumed by squirrelfish.
Cowfish		X		Adult squirrelfish can consume juvenile cowfish.
Damsels			X	Will be consumed by squirrelfish.
Dottybacks			X	Will be consumed by squirrelfish.
Dragonets		X		The noxious mucous may not be enough to ward off a hungry squirrelfish.
Drums	X			Should be good tank mates.
Eels		X		Some eels require a tank unto themselves.
Filefish	X			Should be good tank mates.
Frogfish	X			Should be good tank mates.
Goatfish	X			Should be good tank mates.
Gobies			X	Will be consumed by squirrelfish.
Grammas			X	Will be consumed by squirrelfish.
Groupers	X			Should be good tank mates.
Hamlets	X			Should be good tank mates.
Hawkfish		X		Adult squirrelfish can consume juvenile hawkfish.
Jawfish			X	Will be consumed by squirrelfish.
Lionfish	X			Should be good tank mates.
Parrotfish	X			Should be good tank mates.
Pineapple Fish	X			Should be good tank mates.
Pipefish			X	Will be consumed by squirrelfish.
Puffers	X			Should be good tank mates.
Rabbitfish	X			Most should mix well, but some Rabbitfish can become aggressive over time.
Sand Perches		X		Adult squirrelfish can consume juvenile sand perches.
Scorpionfish	X			Should be good tank mates.
Seahorses			X	Will be consumed by squirrelfish.
Snappers	X			Should be good tank mates.
Soapfishes	X			Should be good tank mates.
Soldierfish		X		Some species may not co-exist.
Spinecheeks		X		Adult squirrelfish can consume juvenile spinecheeks.
Squirrelfish		X		Some species may not co-exist.
Surgeonfish	X			Should be good tank mates.
Sweetlips	X			Should be good tank mates.
Tilefish			X	Will be consumed by squirrelfish.
Toadfish	X			Should be good tank mates.
Triggerfish	X			Should be good tank mates.
Waspfish	X			Should be good tank mates.
Wrasses		X		Adult squirrelfish can consume juvenile wrasses.

Note: While many of the fish listed are good tank mates for Holocentrids, you should research each fish individually before adding it to your aquarium. Some of the mentioned fish are better left in the ocean or for advanced aquarists. In addition, for any species listed as "will co-exist," it is assumed that the fish is large enough to not be swallowed by the Holocentrid.

It should also be obvious at this point that squirrelfish are not particularly picky about their prey. This translates into a fish that is easily fed in the home aquarium. However, they must be fed a lot of food. The traditional aquarium favorite of flake or pellet foods will not sustain a squirrelfish for long. Instead, opt for pieces of thawed shrimp or fish, or any food geared towards the diet of large fish such as triggers or puffers. Most squirrelfish will adapt to consuming food during daylight periods, however a training period may be required. If the fish is initially difficult to feed, the aquarist would be best served by offering live shrimp after nightfall to entice feeding. Once feeding has commenced on a regular basis, attempt to wean it off the live food, and finally transitioning it over to a daylight feeding period.

As noted above, squirrelfish have the ability to communicate with each other. Be advised… the aqurist should become accustomed to hearing a multitude of sounds coming from the aquarium. Undoubtedly, it is their frequent chirping sounds that have garnered them their common name… not their fluffy tail.

Possibly the most regularly offered squirrelfish for the American trade is Holocentrus adscensionis, or the Big-Eyed Squirrelfish. They are a typical squirrelfish in all aspects except size - these get very large. They can reach up to 24" of length. Instead, you may wish to opt for the Longspine Squirrelfish, Holocentrus rufus, which is nearly identical to H. adscensionis except it reaches only 12" in captivity. In addition, the yellow of the dorsal fin is absent, but instead the yellow highlight is located on the anal, caudal, and pectoral fins.

The dorsal fin of Holocentrus rufus is seen here on the left, with the dordal fin of Holocentrus adscensionis
seen on the right. Note the color difference. Photos courtesy of Greg Taylor.

Neoniphon sammara is a wide-ranging squirrelfish owning the common name of the Samara Soldierfish, despite it actually being a squirrelfish. This species is relatively more outgoing than many of the members of the family. Typically, it can be seen hovering just outside its preferred overhang in bright daylight. Preferred territories usually include overhangs created by thickets of Acropora species. Over 66% of the diet for this species is crabs, with another 15% attributed to small fish and 10% to shrimp (Randall, 1972). Neoniphon argenteus is nearly identical. In fact, Beaufort (1929) incorrectly placed N. argenteus in synonymy with N. sammara because the two species are so closely related.

Sargocentron diadema, often called the Crown Squirrelfish, is the most often offered Indo-Pacific species. It is generally found in lagoons or bays, not outer reefs, unlike most of the family. Of all the Sargocentron species, this is the most likely candidate for venturing out into bright daylight. Adults may reach nine inches in length.

Squirrelfish definitely fit the bill as being anything other than a normal, run-of-the-mill fish. With requirements that are not shining examples of the perfect reef-aquarium candidate, caution needs to be exercised before purchasing one. Once the proper setup is established, however, the addition of a squirrelfish will be a unique addition to your aquarium.