| Zusammenfassung: | The deep Red Sea represents a unique thermal environment with constantly high temperatures of about 21.5°C throughout the water column. In this warm ocean, secondary deep-sea fishes have colonized a wider vertical range than in adjacent areas of the Indo-Pacific. At great depths food availability is rather low, requiring distinctive morphological, physiological, or behavioural adaptations. In a quantitative morphological comparison, 2 populations of the ophidiid species Neobythites stefanovi from the deep Red Sea and the adjacent Gulf of Aden were examined with respect to possible differences in 17 morphometric and 8 meristic characters. Both by univariate methods and by canonical variate analysis (CVA), considerable variations in 3 morphometric and 3 meristic characters were found. Accordingly, the Red Sea population shows a larger mouth and the ocellus on the dorsal fin has a vertically larger and more anteriorly positioned dark spot than that in the Gulf of Aden population. In accordance with earlier data on morphological variability between populations of the bythitid fish Oligopus robustus from the Red Sea and the Gulf of Aden, the larger mouth indicates an adaptation of foraging style to the low energetic returns in the deep Red Sea. The ocellus of N. stefanovi resembles that of other closely related Neobythites species in form, position, and size and most probably serves an antipredator function. The increased vertical size of the ocellus spot in the deeper-living, Red Sea population may intensify the signalling effect. A finding that deserves further investigation in other ocellus-bearing species is the population difference in spot position. The classification results of the CVA based on the morphometric and meristic data sets both confirm the existence of 2 morphologically divergent populations, but do not support earlier assumptions of a high intraspecific variability among deep-sea fishes from different parts of the Gulf of Aden.
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