Family Microdesmidae, the Worm- and Dartfishes
by Robert Fenner and Anthony Calfo
Nemateleotris magnifica: known commonly as the Firefish, this fish is perhaps the most popular and recognized member of its family. They are easily cared for and make fine aquarium specimens with peaceful tankmates and a steady supply of zooplankton. Photo by Robert Fenner
Wormfishes are close kin to the large and familiar family we call gobies, as members of the same suborder - Gobioidei. They share many general features as aquarium candidates, from their small size to peaceful disposition, and the need for careful consideration with tankmates in kind. By and large, they are easily intimidated by active community fishes that will out-compete them for food, if not outright terrorize them. Active fishes like damsels, tangs and wrasses are unsuitable tankmates in all but the largest aquariums and may contribute to the slow attrition of shy Microdesmids. Yet given adequate cover for retreat, small frequent feedings, and opportunities to feed, this group is predominantly hardy and easily kept. A few species here are wholly prepared for the rigors of new aquarists and life in smaller aquariums, and several have been observed to spawn in captivity. The use of inline, mature, fishless refugium is strongly recommended to support continuous plankton production as a food source.
The Microdesmids are known by many common names in the trade such as Dartfish, Firefish, Ribbon, Bar & Gudgeon "gobies". The family is made up of less than a dozen genera and about sixty species. As aquarium candidates, they are ironically well understood yet commonly neglected. Too often, they find themselves in mixed community tanks with active or aggressive fishes in whose company they dwindle. This group at large has many common traits: elongate body forms (some anguilliform or "eel-like"), planktivorous feeding nature, and small scales. They are found distributed in tropical marine waters, with most favoring life above soft substrates. Most Microdesmids are limited in size to adult lengths of no more than a few inches (6-10 cm) total length.
The acquisition of a healthy specimen is one of the biggest obstacles to keeping Worm- and Dartfishes, because of their notorious sensitivities to transport. As small-scaled fishes, they suffer from rough handling and abrupt changes in water quality. Many are shy and skittish in brightly-lit systems. Once disturbed, these fishes dart remarkably quickly and hurl themselves into harm in the confines of an aquarium, if not literally launching themselves out of an open top or crevice. Many of these fishes have given themselves up as "carpet fodder" for lack of a secure cover on the aquarium. As you might guess, they are prone to the same high-speed, fitful displays at the slightest provocation by an aggressor. With no other practical means of defense, Microdesmids are remarkably fast swimmers!
When inspecting candidates for purchase, remember that many are naturally shy and inclined to hide. Assessment of gross health characteristics is the ticket here. Look for clean bodies, clear eyes, slow and deliberate gill movement, and intact fins/soft rays; many dartfishes have long filamentous pennants, which are easily chewed off by aggressive tank mates. Extended dorsal fins are flicked frequently while hovering and used, perhaps as a visual cue to kin, and are a normal behavior. Hollow or sunken bellies and weight loss are common afflictions with new imports. Boosting immunity with superb water quality and good diet is crucial to establishing healthy imports.
Infections are cause for great concern here, as these fishes do not tolerate popular therapies well. Metals and organic dyes can be harsh, and prophylactic freshwater dips may require the use of a tennis racquet to keep them underwater in the bath - yes… they can jump way out of water! It’s best instead to acquire apparently healthy individuals and quarantine them strictly in a quiet, dim tank for heavy feedings and undisturbed acclimation to captivity. Shy fishes such as these can also learn to be more at ease and take food readily from an aquarist in isolation before being forced to compete in the display.
Feeding is a straightforward endeavor with Microdesmids – they are strict diners on zooplankton. Most hover slightly above a soft sand or fine rubble seafloor and catch suspended planktonic microcrustaceans. Gut analysis has revealed that copepods make up a significant component of their diet. Note: understand the important difference between copepods and amphipods - they are not readily interchangeable prey for many fishes like Microdesmids, or Mandarin Dragonets, for example.
There is little doubt that fishless refugiums plumbed in-line for producing plankton are invaluable for keeping these and many such delicate fishes well-nourished. For zooplankton substitutes, "wet foods" have a decided advantage here among the suspension feeding planktivores. You can make a slurry of finely minced, thawed meats of marine origin (mysids, pacifica plankton, etc.) or you can purchase ready to serve (keep refrigerated), jar foods like Sweetwater brand plankton. Most Worm- and Dartfishes will also take dried prepared foods like fine pellets in time. Slowly sinking varieties are favored more than floating varieties. Argent-brand "Cyclop-eeze" is another excellent option (dry or frozen) for small reef planktivores. As with all fishes, you are warned to resist using frozen adult brine shrimp for anything more than an appetite-stimulant or minor component of the captive diet due to its hollow nutritive value. Freshly-hatched brine shrimp nauplii, however, can be very nutritious, just as cultured rotifers are. Any live foods can also be enriched further with HUFA- rich supplements or by gut-loading the prey (spirulina, yeast, etc.).
Worm- and Dartfishes are generally quite peaceful and tolerant of each other (the former moreso). Interspecific aggression with new imports is largely an artifact of captivity and the confines of small aquaria. Dive and field research observations cite most members of this family occurring naturally in pairs as adults and often in colonies as juveniles. With adequate space and feeding opportunities, two or more individuals of the same species can usually be kept in an aquarium.
Little is documented about their reproductive events in captivity [see Schiller (1990) for a report on Nemateleotris], although some at least are known to deposit their eggs on substrate and provide at least some level of parental guard. For these, courtship involves posturing and display in open water at dusk before the actual spawning event commences in a burrow or depression in the sand.
Keys to success with Microdesmids:
Best Bets of Worm and Dartfishes:
Burgess, Warren E. 1980. The genus Nemateleotris. TFH 6/80.
Carlson, Bruce A. 1982. Nemateleotris magnifica Fowler 1938. FAMA 1/82.
Debelius, Helmut. 1986. Gobies in the marine aquarium, pt 2.: Fire gobies. Today's Aquarium 3/86.
Pyle, Richard L. 1989. Helfrich's Dartfish, Nemateleotris helfrichi Randall & Allen.