Stephanoberyciforms are a morphologically diverse lineage, ranging from the large-headed whalefishes to the small-headed megalomycterids and mirapinnids. In stephano-beryciforms generally, the head is somewhat large, with numerous bony ridges in the stephanoberycoids (giving them a highly armored look), usually with small eyes, the dorsal and anal fins end in opposition posteriorly on the body, the pectoral fins are of moderate size, and the caudal fin is truncated and not very large. All families have a single dorsal fin, either with very few spines (stephanoberycoids) or lacking them completely. Pelvic fins can be well developed (as in melamphaids), reduced (as in stephanoberycids), absent (as in cetomimids), and even winglike and aberrant (as in the hairyfish, Mirapinna esau). The caudal fin may have procur-rent spines (as in stephanoberycoids), and is uniquely subdivided in Mirapinna. The jaw bones are rather weak and easily bent.
Many stephanoberyciform species appear velvety due to small protrusions from the epidermis (as in Mirapinna), or even from numerous spines on the scales (as in stephanobery-cids) that are usually deciduous. Many species, such as the ce-tomimids, lack scales, others, such as the melamphaids, have large cycloid scales (their scales are rarely seen as they are easily lost on capture). The teeth are very small; numerous pores are usually visible on the head and lateral line, and some species may have luminous tissues and highly distensible stomachs, and are soft and flabby (as in whalefishes). One family, the Megalomycteridae, has extremely well-developed olfactory organs and nostrils. Species of the mirapinnid genera Eutaeniophorus and Parataeniophorus, and those in the family Megalomycteridae are morphologically quite distinct from other stephanoberyciforms.
The fishes of this order are usually small, rarely surpassing 9.8 in (25 cm) in length, and usually below 4.7 in (12 cm); only a few specimens above 13.8 in (35 mm) are known. Coloration is drab, brown, grayish black, or reddish (as in whale-fishes). Many stephanoberyciform species are known from very few specimens, and sometimes these represent only a single sex or a juvenile stage; no adult mirapinnid has been collected to date.
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