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The external morphology of the light organs of ceratioids— those aspects that provide the essential characters that delineate the numerous species of the suborder—have been described in Chapter Two, What Makes an Anglerfish? The internal anatomy, functional morphology, and luminescence of these structures are dealt with here, along with a discussion of the biological significance of bioluminescence in these fishes (Fig. 240). Specialized structures adapted to produce bioluminescence in ceratioids are restricted to females, the most conspicuous of these being the bait, or esca, located at the tip of the illicium. Other structures of females known or thought to luminesce include the esca-like bulb on the tip of the second dorsal-fin spine of diceratiids and ceratiids, the caruncles of ceratiids , and the jaw teeth, fin rays, dermal spines, and skin patches of various taxa. Beebe (1932b:101, fig. 29) thought that the somewhat swollen tips of the six innermost caudal-fin rays of a larval male of Melanocetus murrayi were “slightly but distinctly luminous in the fresh fish.” Young and Roper (1977) described bioluminescent countershading in an attached pair of Cryptopsaras couesii, in which the skin of both female and male luminesced (see Luminescent Countershading, below). But apart from these two observations, neither light organs nor luminescence have ever been seen in ceratioid males, or larvae of either sex. That the esca of deep-sea anglerfishes might be bioluminescent was first mentioned by Rudolph von Willemoes-Suhm (1847–1876), in a letter to his professor, Carl Theodor Ernst von Siebold (1804–1885), written while aboard the HMS Challenger in the Pacific in 1875 (see Büchner, 1973). While describing luminescence in various deep-sea fishes, Willemoes-Suhm (1876:lxxxi) wrote that “they are probably all phosphorescent, definitely so in Sternoptyx, which once, when the trawl came up at night, hung in the net like a shining star. The source of the light is probably the strange side organs [Seitenorganen, i.e., lateral photophores], which may be found in a similar fashion at the tip of the head barbel [illicium] of certain deep-sea lophioids.” Lütken (1878a:313), in describing the silvery distal escal appendages of Himantolophus, agreed: “This specialization of the tips makes one think of a possible function, e.g., phosphorescence [fosforescens]. I mention this so that it can be studied further if a new opportunity arrives, but I lately recall that R. v. Willemoes-Suhm, in his ‘letters from the ChallengerExpedition ’ . . . suggested the presence of a ‘light-source’ at the tip of the illicium in deep-sea lophioids.” The earliest recorded direct observation of bioluminescence in ceratioids is apparently that of William Beebe (1926b:80), in his original description of Diabolidium arcturi, now recognized as Linophryne arcturi (Fig. 241): “The white base of the candlelike organ [distal appendage of the esca] and all the longer teeth showed distinct luminescence in the dark room during the first three minutes after capture.” Somewhat later, this time in L. arborifera, Beebe (1932a) again described glowing teeth— which he thought might be covered with a luminous mucus— as well as light emanating from the esca and hyoid barbel. A number of additional sightings were recorded by Beebe (1926b, 1926c, 1932a, 1934a; Beebe and Crane, 1947), but none are as well known as his eye-witness account and subsequent formal description of an anglerfish seen from the window of the bathysphere , as detailed in Half Mile Down (Beebe, 1934a:211; see also Beebe, 1934b, 1934c; Ellis, 2005:41; Fig. 242): One minute later, at 2470 feet, all my temporarily relaxed attention was aroused and focused on another splendid piece of luck. A tie rope had to be cut and in this brief interval of suspension , extended by my hurried order, a new anglerfish came out of all the ocean and hesitated long enough close to my window for me to make out its dominant characters. I am calling it the Three-starred Anglerfish, Bathyceratias trilychnus. It was close in many respects to the well-known genera Ceratias and Cryptopsaras, but the flattened angle of the mouth and the short, even teeth were quite different. It was six inches long, typically oval in outline, black, and with small eye. The fin rays were usual except that it had three tall tentacles or illicia, each tipped with a strong, pale yellow light organ . The light was clearly reflected on the upper side of the fish. In front of the dorsal fin were two pear-shaped organs S I X Bioluminescence and...

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