SEXUAL DIFFERENCES 33 



in mid-Atlantic belong to liuropean or to American species, but it seems probable 

 that the majority of them are of American origin. It is of interest to note that most 

 of those taken by Danish vessels were from the Sargasso Sea, which is practically 

 (lead water, uninfluenced by currents. This fact led Kyle (1913, p. 112) to suggest 

 that these post-larval forms " have been derived from adults living at or near the 

 spot where they were found," and that the " Sargasso Sea has a pelagic, flatfish 

 fauna of its own," a suggestion which seems very improbable. 



Regan (1916, p. 148) has written : " It may be inferred that the distribution of a 

 benthic species along a coast may be helped by a pelagic larval phase, but that unless 

 this be prolonged it will not serve to establish the species in places separated from its 

 original habitat by a wide sea." There is a species of Bothiis (B. mellissi) found only 

 at St. Helena and Ascension in the Atlantic Ocean, which is very closely related to 

 the European B. podas, which extends westwards to the Azores and southwards down 

 the coast of Africa to Angola. There can be little doubt that at some time examples 

 of B. podas were able to reach these islands by virtue of their prolonged pelagic life. 

 It also seems reasonable to infer that a species such as Syacium micyitritm, which is 

 found not only on the Atlantic coast of tropical America, but also on the coast of West 

 Africa, as well as pairs of closely related species occurring on the two sides of the 

 Atlantic {e.g. Citharichthys spilopterns and C. stanipflii), must have a larval life of 

 considerable duration. 



Most Flatfishes exhibit certain changes in form and proportions during the period 

 of growth after the metamorphosis and assumption of a demersal habit. As in most 

 Bony Fishes, the head and eye are proportionately larger in the young than in the 

 adult, the snout is generally shorter, and the interorbital space narrower. In adult 

 individuals the maxillary extends further back in relation to the eye than it does in 

 younger ones. The actual proportionate increase in the size of the mouth, however, 

 is very slight, the apparent difference being due mainly to the change in the position 

 of the eye. The depth of the body is nearly always le.ss in young examples.' In 

 species distinguished by having one or more rays of the dorsal and anal fins prolonged, 

 or in which some of the rays of the paired fins are elongate, the length of these rays 

 is proportionately less in the young and gradually increases with age. The develop- 

 ment of spines and other processes on the head, and of tubercles or granular plates on 

 the head and body, is also less marked in immature individuals. The number of 

 gill-rakers sometimes shows a slight increase with age. Finally, quite considerable 

 changes in the colours and markings may take place during the growth of certain 

 .species. 



VI. SEXUAL DIFFERENCES. 



The differences between the sexes in certain species is most marked, especially in 

 the family Bothidae, and may be concerned with the scales, rostral and orbital spines, 

 interorbital width, form of the fins, coloration, and (in one species) with the teeth. 



In the Pleuronectinae only two genera exhibit sexual dimorphism, namely, Liopsetta 

 and Pleuronectes, and in both this is connected with the character of the scales. In 

 all the species of Liopsetta the scales are much rougher in the male. In the Plaice 

 (Pleuronectes platessa) the .scales are usually mostly cycloid, but not infrequently some 

 of them are feebly ctenoid in the male, especially in the small Baltic race (see p. 354). 

 The closely related Pleuronectes pallasii shows no sexual differences. 



In those species in which cephalic spines or protuberances are developed, these 

 may be confined to the male, as in species of Perissias, Engvprosopon, Crossorhombus 

 and Bothus, or strong in the male and feebly developed in the female, as in Citha- 

 richthys cornutus, Lophonectes gallus and Ttsniopsetta ocellata. In the last-named 

 species the spines round the orbits are much stronger in the male than in the female, 

 and none of the rays of the dorsal and anal fins are prolonged in the latter ; in the 



^ E.xcept in very young Bothus. 



