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Anglerfishes differ radically from all other fishes. With the first dorsal-fin spine mounted on the snout and modified to serve as a luring apparatus, and gill openings narrowly constricted to form tubelike structures that open posteriorly behind the base of the pectoral fin, they can hardly be confused with anything else. The deep-sea suborder Ceratioidei is by far the largest, most highly derived, and certainly the least known of the five primary lineages of the order, which has come to be called the Lophiiformes. When compared with its less-derived lophiiform relatives, it too is easily distinguished. In fact, a single character complex is sufficient to diagnose the suborder: anglerfishes that display an extreme sexual dimorphism in which the males are dwarfed, reaching only a fraction of the size of the females. Although enough to encompass what is meant by “Ceratioidei ,” this single concise statement only begins to convey the enormous morphological uniqueness and diversity found within the group. The present chapter is intended to provide an overview of this diversity of form, to describe in some detail the various characters of biosystematic significance, and to demonstrate how they vary, not only among the various taxonomic subunits, but between genders as well. While the families and genera of the Ceratioidei are well defined by combinations of the characters described below, the problems of differentiation are nearly all confined to the species level. The soft bodies and weakly ossified skeletons of ceratioids make nearly all morphometric characters traditionally utilized in teleost taxonomy useless in distinguishing the many morphologically similar species. This is especially true for those belonging to the species-rich genera, such as Himantolophus , Oneirodes, Gigantactis, and Linophryne, which together account for 70% of all recognized ceratioids. Values for body width and depth; head length, width, and depth; length of the upper and lower jaws, pectoral lobe, and bases of the unpaired fins; and length and depth of the caudal peduncle are often difficult to establish with accuracy, and so plastic and highly variable intraspecifically depending on the mode of preservation , that they are of very limited taxonomic importance. With regard to female ceratioids, most frustrating has been the inability to discover characters of taxonomic value at the specific level other than those of the escal morphology. While the importance of this character complex cannot be overstated, the illicial apparatus of ceratioids is often damaged or lost upon capture, particularly in larger specimens, making positive identification difficult if not impossible. For example, nearly half the known specimens of Ceratias greater than 75 mm have lost the esca (Pietsch, 1986b:479), making assignment to one of the three recognized species of this genus next to impossible . The percentage is significantly higher in certain other genera like Oneirodes, in which a large number of damaged females in collections around the world remain unidentified (Pietsch, 1974a:107). Ceratioid families are distinguished from one another primarily by osteological features, many of which are shared by members of both sexes: primarily the (1) presence or absence of certain cranial elements; (2) branchiostegal-ray counts; (3) fin-ray counts; and (4) number of pectoral radials. In contrast, differentiation of ceratioid genera is based primarily on characters present only in the females, the more important of which include the following: (1) shape of the skull and other bones of the head, including the development of head spines; (2) structure of the jaws, including dentition; (3) structure of the luring apparatus, including the basic pattern of escal appendages and filaments; (4) pigmentation of the skin; and (5) development of skin spines and spinules. Some of the distinguishing osteological characters of the females are shared with the males, for example, the shape of the opercular bones, which within some families (e.g., the Oneirodidae) show distinct and consistent intergeneric differences . On the other hand, certain structures restricted to the males, such as the denticular teeth, show distinct intergeneric differences that agree in full with distinctions based on characters of the females (Bertelsen, 1984:325). In addition to fin- and branchiostegal -ray counts, larvae are differentiated primarily on the basis of body shape, degree of skin inflation, size of the pectoral fins, and the pattern of subdermal pigmentation. TWO What Makes an Anglerfish? At present we do not know certainly for any one species its vertical or horizontal distribution, the conditions most favorable for its existence, or the nature of its food; but we may be certain that these are different for the different species and that the...

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