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Of the five major taxa of lophiiform fishes, the deep-sea Ceratioidei is the most phylogenetically derived (Bertelsen, 1984; Pietsch, 1984). Ceratioidei are distributed throughout the world’s oceans below a depth of 300 m. With 160 species, it constitutes by far the most species-rich vertebrate taxon within the bathypelagic zone and below—more than twice as many families and genera and more than three times the number of species as the Cetomimoidei, the next most species-rich deep-sea vertebrate taxon (see Paxton, 1998; Herring, 2002). At the same time, new species are being added to the suborder at a steady if not increasing rate.
Members of the group differ remarkably from their less-derived, bottom-living relatives by having an extreme sexual dimorphism (shared by all contained taxa) and a unique mode of reproduction in which the males are dwarfed—those of some linophrynids, adults at 6-10 mm standard length, competing for the title of world’s smallest vertebrates (see Winterbottom and Emery, 1981; Roberts, 1986; Weitzman and Vari, 1988; Kottelat and Vidthayanon, 1993; Watson and Walker, 2004)—and attach themselves (either temporarily or permanently) to the bodies of relatively gigantic females. In Ceratias holboelli, where the most extreme examples are found, females may be more than 60 times the length and about a half a million times as heavy as the males (Bertelsen, 1951; Pietsch, 1976, 1986). The males lack a luring apparatus and those of most species are equipped with large well-developed eyes (Munk, 1964, 1966) and relatively huge nostrils (Marshall, 1967a, b), the latter apparently used for homing in on a female-emitted, species-specific pheromone (Bertelsen, 1951; Pietsch, 1976; Munk, 1992). Normal jaw teeth are lost during metamorphosis, but are replaced by a set of pincer-like denticles at the anterior tips of the jaws for grasping and holding fast to a prospective mate. In some taxa male attachment becomes permanent and parasitic through fusion of male and female tissue (see Sexual Parasitism in Ceratioid Anglerfishes).
Ceratioid anglerfishes differ further from their shallow-water relatives in having a bacterial light-organ that serves as bait to attract prey, a structure technically called the “esca”—exceptions among members of the suborder include the monotypic family Neoceratiidae (Bertelsen, 1951), the three species of the gigantactinid genus Rhynchactis (Bertelsen et al., 1981; Bertelsen and Pietsch, 1998), and the five members of the family Caulophrynidae (Pietsch, 1979). Parr (1927) was the first to recognize the diagnostic value of the external morphology of escae in ceratioids, pointing out the need for a closer examination of individual variation in the structure of this organ. Since that time, differences in the number, shape, and size of escal appendages and filaments, as well as variation in external escal pigment patterns, have been, for the most part, the sole basis on which new species have been described (e.g., see Pietsch, 1974; Bertelsen et al., 1981; Bertelsen and Krefft, 1988).
The internal structure of ceratioid escae is infinitely more complex, involving a confusing array of bacteria-filled vesicles, light-absorbing pigment layers, reflecting tissues, tubular light-guiding structures, nerves, blood vessels, and smooth muscle fibers (Munk and Bertelsen, 1980; Munk, 1988, 1998, 1999; Herring and Munk, 1994; Munk and Herring, 1996; Munk et al., 1998). There is some evidence also that ceratioid escae contain pheromone-producing secretory glands that function to attract a conspecific male (Munk, 1992), but the true nature and adaptive significance of these structures and most of the other internal parts of escae are unknown.
In addition to the esca, all 23 currently recognized species of the ceratioid genus Linophryne (family Linophrynidae) bear an elaborate bioluminescent hyoid barbel, the light of which does not originate from symbiotic luminescent bacteria but rather from a complex array of intrinsic, intracellular, paracrystalline photogenic granules; the bacteria-filled esca is ectodermal in origin, whereas the barbel light organ appears to be derived from the mesoderm (Hansen and Herring, 1977). This remarkable dual system, involving two entirely separate mechanisms of light production, is unique among animals.
In summary, ceratioid anglerfishes are among the most intriguing of all animals, possessing a host of spectacular morphological, behavioral, and physiological innovations found nowhere else. The suborder is taxonomically diverse: with 160 currently recognized species (and many more certain to be discovered in the future), it forms a major contribution to the biodiversity of the deep-sea. It is exceedingly widespread geographically, occurring in deep waters of all major oceans and seas of the world, from high Arctic latitudes to the Southern Ocean; while some species appear to be almost cosmopolitan in distribution, many others have surprisingly small, restricted, vertical and horizontal ranges. Their relative abundance, high species diversity, and trophic position as the top primary carnivores in meso- and bathypelagic communities make them important ecologically. Their unique mode of reproduction has significant biomedical implications to the fields of endocrinology and immunology. Yet, despite these many aspects of biological interest and importance, as well as a large amount of revisionary work published in the 1970s and early 1980s, including repeated attempts to resolve phylogenetic relationships, ceratioid anglerfishes have remained poorly known.