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The dartfishes of the family Ptereleotridae have been taxonomically mobile in recent years and some taxonomists now include them in the wormfish family Microdesmidae. I place them here alongside the family Eleotridae because they are also gobioids and share the clearly-divided pelvic fins of the eleotrids. Larval ptereleotrids most closely resemble the "long" larvae of my Group 4 gobies. There are only two dartfishes in the region, a pair of sibling species that vary only slightly in color: their larvae are likely identical.
The sleepers of the family Eleotridae (some use Eleotrididae) are similar to gobies but have divided and well-separated pelvic fins (photograph of the separated pelvic fins in
Eleotrids are typically found in tropical freshwater habitats, but they do penetrate brackish and mangrove environments. Some species get large as adults and can even become gamefishes in the major river systems of central America. Their larvae, however, are small to medium-sized and exhibit similar body shapes and marking patterns to the larvae of their goby relatives.
Larval eleotrids share many basic characters of larval gobies. While most of the true gobies have fused pelvic fins, several genera have divided pelvic fins like the eleotrids (although they do not have the fins completely separated at the base as do the sleepers). pelvic-fin morphology is not always easily apparent on small larvae, but fortunately there are only a few species of eleotrids in the Caribbean and larval eleotrids do have a somewhat different appearance from the usual goby gestalt.
Most eleotrid larvae share a distinctive suite of characters. They have long ventral midline streaks of melanophores that extend onto the abdomen. Most also have linear internal melanophores extending up from the anal-fin base along myomere edges. Pre-transitional stages usually have odd-shaped narrowed eyeballs, some with unusual pigmented membranes over the iris that can expand to essentially cover the shiny surface completely. Also unusual is the high number of procurrent caudal-fin rays, up to 14, in several eleotrid species; true gobies almost always have 10 or fewer, often many fewer. The goby exceptions are the larvae of the river gobies of Sicydium and Awaous. Interestingly, the common factor is freshwater habitat; the high number of procurrent caudal-fin rays is likely an adaptation for living in fast-flowing streams. Eleotrid larvae also exhibit some of the more dramatic eye-shape changes during development and at transition found in larval fishes.
Eleotrid larvae tend to share basic melanophore patterns and general morphology, and fin-ray count differences are slight. There is also a marked degree of variation within species, making species identifications more difficult. Some characters common in one larval type will occur occasionally (or later in transition) in another larval type; for example, the characteristic melanophore patterns along the jaws of Eleotris amblyopsis larvae match closely those found on the late transitional larvae of Erotelis smaragdus. Typically, a suite of characters in combination serve to distinguish the larval types and unite transitional series.
The literature reports of fin-ray counts of sleepers can differ by two fin rays or more, and sometimes disagree on whether there are equal numbers of dorsal and anal-fin rays or more or less. The literature on larval sleepers is also not helpful, since features common to the entire family are typically cited as unique to one species or other and line drawings omit other diagnostic features (likely a result of inadequate sample sizes of highly-variable larvae).
Complex neuromast patterns develop on the head, body and caudal fins of late transitional eleotrid larvae. These patterns are more developed on juveniles and adults and are commonly used for taxonomic studies. Unfortunately, the neuromasts can be hard to highlight on most transitional larvae (photograph below, from the top, larval Erotelis smaragdus, Gobiomorus dormitor, and Eleotris amblyopsis).