This is a guest contribution from Joe Rowlett, one of the many talented fish geeks working the fish tanks at Old Town Aquarium in Chicago.
The taxonomy and classification of fish is not a subject that your average reefkeeper investigates of their own volition. There is a bewildering array of big words, and terminology to digest, and added to this is the lack of scientific consensus on the exact relationships within the fish tree of life.
Instead, it becomes incumbent upon the enterprising reefkeeper to consult the primary literature of scientific studies published in peer-reviewed journals, which are often inaccessible behind subscription paywalls. It is understandable why so few in the reefkeeping world, whether casual hobbyist or professional aquarist, have a real grasp of how the fish we keep are related to each other.
The fishes I see the most confusion with are the various small, benthic (i.e. bottom-dwelling) forms. There is a general trend amongst aquarists to lump all of these admittedly similar-looking fishes under the catchall terms goby or blenny. But doing so overlooks the separate evolutionary histories these groups have taken to convergently arrive at their similar forms.
The 14 Families of fish we’ll be discussing are indeed closely related—being members of the ‘Percomorphaceae’, or perch-like fishes. These can be thought of as being the more evolutionarily “advanced” fishes. But to put this in perspective, nearly every bony fish collected for saltwater aquariums (except for eels, squirrelfish, soldierfish, lizardfish, pineconefish, flashlightfish, & coral catfish) are percomorphs. Looking at the entirety of the fish tree of life shows that these 14 Families of goby-like bottom-dwellers evolved from at least 5 separate lineages of fish.
Gobiids & Gobionellids
The classification of the gobies (Family Gobiidae) has undergone significant change in the last decade. Traditionally this group was the largest family of fish, with 1,800+ described species, but recent advances have split the group up into two large families, Gobiidae (1200+ species) and Gobionellidae (600+ species), while reincorporating the former families Microdesmidae (wormfishes) and Ptereleotridae (dartfishes/firefishes).
The most observable character that unites this group is the fusion of the pelvic fins into a suctorial disk—a useful adaptation for a bottom-dwelling fish that clings to its substrate tightly. This feature is not universal, as it has been lost numerous times—Ptereleotris, Microdesmis, Amblyeleotris, Eviota, Asterropteryx. Also important for identification is the split dorsal fin, which differs from the confluent dorsal fin of the otherwise similar blennies and jawfishes. There are goby-like fishes that do have a split dorsal fin, but they will always lack the fused pelvic fins of a true goby.
There is unfortunately not any useful morphological character for delimiting the gobiids from the gobionellids. Even the subtleties that ichthyologists have teased out—head pore counts and pterygiophore insertion patterns—don’t stand up to rigorous examination. It is arguable what value there is in splitting these two groups apart to the Family level given their similarities, other than to indicate that they are each large and independent lineages of fishes. In fact, not all researchers have accepted the elevation of gobionellids to Family rank. The main difference that is relevant to the reefkeeping hobbyist is ecological: gobionellids tend to reside in estuaries, whereas gobiids are mostly saltwater.
Few gobionellids find their way into aquariums. The most commonly seen genus is likely Gnatholepis, which is typical of the family in leading an unspecialized life on sandy substrates. The Caribbean Oxyurichthys stigmalophius is a large and attractive species that makes rare appearances (from my personal experience with it, this is a timid, delicate species).
Closely related to the gobiid/gobionellids is the Family Eleotridae, known commonly as Gudgeons or Sleeper Gobies. Unlike a “true goby”, eleotrids always possess unfused, non-suctorial pelvic fins—these fish tend to swim near the bottom, but not directly on the substrate.
Most gudgeons are freshwater or brackish, and are commonly seen in the freshwater department of your friendly neighborhood fish store. The only reef-associated genus is Calumia, which consists of four small, colorful species that have sadly yet to make it into our nano-reefs. These fish have great potential as an aquarium inhabitant, if only someone would go about collecting them.
The fishes which I see most frequently confused with gobies are undoubtedly the jawfishes, Family Opisthognathidae. While they share a similarly elongate body and benthic habitat, this group is in fact far more closely related to the blenny families. Gobies (and the closely related Cardinalfishes) are thought to have diverged from the other fishes we’ll be discussing more than 120 million years ago, during the early Cretaceous. The non-gobyness of the jawfishes is driven home by the fact that the pelvic fins are not fused into a suctorial disk and the dorsal fin is not split.
It’s not surprising that hobbyists have such trouble identifying this group, as ichthyologists have had similar difficulties. It’s only with the advent of large molecular datasets that a consensus has been reached that this group is part of an assemblage of common aquarium fishes known as the Ovolentariae (O – vo – len – tar – ee – ee). The common character uniting these fishes (which includes the cichlids, dottybacks, damsels, grammas, assessors, jawfishes, and blennies) is the tendency for there to be a sticky filament on the eggs which allows them to be deposited and parentally guarded.
The Family Tripterygiidae consists of 175 species of small blenniod fishes which resemble the Trimma and Eviota gobies. The most salient character to identify this group is alluded to in the family name, which comes from the Greek for “three fins”. The “three fins” is in reference to the unusual configuration of the dorsal fin, which is broken into three separate fins. Another important identifying character for this Family and the following blenniod Families is the presence of feathery appendages on the head known as cirri (singular cirrus). This is a speciose group with many colorful members that would make for excellent nano-fish, but their tendency to reside in algae-dominated habitats has led them to be seldom collected for aquaria.
Labrisomid & Clinid Blennies
The Family Labrisomidae consists of 119 species of blennioid fishes found throughout the Western Atlantic and Eastern Pacific. Labrisomids have scaled bodies and more spines than soft rays in their dorsal fin. Labrisomids are uncommonly collected, with the Diamond Blenny Malacoctenus boehlkei and the Saddled Blenny M. triangulates being most common.
Their closest relations are a suptropical group of algae-dwelling blennioid fishes known as kelpfish, Family Clinidae. The only tropical clinid, the Indonesian Weedfish Springeratus xanthosoma, is rarely collected for aquaria.
One of the least blenny-like of the blennioid fishes are the Sand Stargazers of the Family Dactyloscopidae, whose 48 species are mostly found in Neotropical waters. The common name of Sand Stargazer alludes to the habit of burying in the sand with only the upwardly pointed head exposed—a behavior shared by the True Stargazers of the unrelated and non-blennioid Family Uranoscopidae. The convergence of form exhibited by these two Families is deceptive; the subtle differences in body and fin shape clearly show that these two fishes are only distant relatives, having diverged roughly 112 million years ago! Sand Stargazers are rarely available and are often mislabeled as uranoscopid stargazers.
Pikeblennies, Barnacle Blennies, Sailfin Blennies
The 97 species of the Family Chaenopsidae are found throughout the Eastern Pacific and Western Atlantic and exhibit a remarkable variety of forms. Identifying characters are a scaleless body and a dorsal fin that is enlarged anteriorly and which has more spines than soft rays. The Sailfin or Signal Blennies (Emblemaria) are common Floridian imports that make for lively nano-fish.
From the same part of the world comes the Pikeblenny (Chaenopsis alepidota), which looks very un-blennylike in lacking cirri.
Also lacking in cirri is the uncommonly seen Wrasse Blenny (Hemiemblemaria). This species mimics juvenile bluehead wrasses, and in doing so has lost its cirri to better its subterfuge.
Combtooth Blennies & Fangblennies
With 400+ species—many of which are commonly kept in aquaria—the Family Blenniidae is the largest of the blennioid families. These are scaleless fish which have more soft rays than spines in their dorsal fins. There are two groups in this family that differ quite markedly in behavior. The more predatory Subfamily Blenniinae contains common species like the Fangblennies (Meiacanthus), which are active swimmers that possess venomous canines.
The Combtooth Blennies of the Subfamily Salariinae contains mostly benthic, non-venomous, algae-eating fish. While these algae-eating blennies are enormously popular, it is often overlooked how aggressive they can be in small aquaria. I’ve seen them damage and kill numerous wrasses and anthias. They are also unlikely to directly help with the Bryopsis and Derbesia algae outbreaks that aquarists frequently purchase them for, as their preferred algal diet consists of shorter, tastier greens.
The 190 species of Dragonets, Family Callionymidae, are confusingly known as Scooter Blennies and Mandarin Gobies. Dragonets are quite goby-like in having a split dorsal fin, but the unfused pelvic fins immediately identify this as a non-goby. The split dorsal fin and lack of cirri indicate these fish are not blennies either. So what is this non-goby/non-blenny fish related to?
A surprising result from recent molecular studies has been the grouping of dragonets with the seahorses and pipefishes. This is an arrangement which intrinsically makes sense to aquarists, as both groups have small mouths and a very deliberate method of feeding. To think of a dragonet as a fat-bodied pipefish does not stretch the imagination too far, especially given the similarities required to care for them.
Parapercis, Family Pinguipedidae, are occasionally seen available in the aquarium trade—P. clathrata and P. schuainslandii being most common. These fish have a very goby-like demeanor, though oddly enough they have appeared with the common name of “Sandhopper Blenny”. This is a rather egregious misappropriation of the name “blenny”, as there is little about this fish that is blenny-like, nor is it very closely-related to them. These fishes are closest cousins to the True Stargazers. The alternate common name Sandperch is a better choice for these sand-dwelling, perch-like fish.
Osopsaron & Pteropsaron
These fish, while admittedly being a bit goby-like in appearances, are most assuredly not gobies. The confusion is luckily not of much importance within the reefkeeping world, as these deep-dwelling fish have yet to be collected for aquariums. More appropriate common names for these are the Signalfishes or Sand-divers.
The exact placement of these genera has bounced around recently, with the current consensus being that they are most closely related to the sand-divers of the genus Trichonotus, Family Trichonotidae, which may or may not be related to Parapercis.
The two species of Pholidichthys, Family Pholidichthyidae, are neither gobies nor blennies, but naturally go by the names of Engineer Gobies and Convict Blennies—a classic example of a fish’s common name giving no indication of its true relationship to its piscine brethren. Not surprisingly, this is another fish that has long been clouded with uncertainty as to its exact derivation. It is only in the last few years that its placement has been supported near the base of the Ovolentariae (the group of sticky-egg layers that includes blennies, dottybacks, etc.). It is the speciose freshwater Cichlidae that are its closest relations. Thus this non-goby non-blenny appears to be a near-cichlid instead.
… … …
Hopefully this taxonomic excursion will help in some small manner to clear up the confusion regarding these fishes. Haphazardly tossing around names like “goby” and “blenny” is in poor taste for the discerning reef aquarist. Cheers.