Zoology Publications from Victoria University of Wellington—Nos. 54 to 57
Sexual Dimorphism in Heptranchias perlo
Sexual Dimorphism in Heptranchias perlo
In order to provide further information on the nature of sexual dimorphism in H. perlo we have listed in Table 1 various proportional dimensions which appear to be significant in this respect. Because of the small number of specimens involved, and the scarcity of males, we interpret the differences between the sexes with caution.
Expressed as a percentage of total length, the average predorsal length of females is slightly longer (0.8%) than that of males. Conversely, the average distance from snout tip to anal fin origin is slightly shorter (0.7% t.l.) in females than in males. The greater proximity of the dorsal and anal fins in the females compared to the males seems due, therefore, to equal differences in the positions of both of these fins between the sexes rather than to a more marked difference in the position of either one of the fins. In females the average prepelvic length is slightly longer (0.9% t.l.) than in males. This situation coupled with the shorter snout to anal fin length in females provides a more notable difference between the sexes in the distance between the pelvic fin origin and the anal fin origin. In females this distance averages 13.7% t.l., whereas in males it is 15.2% t.l.
Thorson, Watson and Cowan (1966, p. 395) discussed sexual dimorphism in the Bull shark, Carcharhinus leucas, from Lake Nicaragua and showed that in that species also the distance between the pelvic and anal fins was shorter in females than males. They concluded that "externally, the pelvic fins and cloaca are displaced posteriorly by about 2.5% of the body length in females, while other features are placed approximately as in the males". Our conclusions for H. perlo agree, in general, with those of Thorson et al., but the differences we have observed are much less trenchant.
Hubbs, Iwai and Matsubara (1967) have shown that in the dwarf pelagic shark Euprotomicrus bispinatus the length of the abdomen increases with size of fish, and that this increase is most marked in females. They consider this "an adaptation to fecundity in a dwarfed shark", but suggest, after noting the findings of Thorson et al., that a relatively larger abdomen in females "may well be a common characteristic of sharks".
The function of such sexual dimorphism is not well established, but it possibly hinges on two quite different requirements. For females a greater prepelvic length would permit a longer body cavity and hence be advantageous for a live-bearing species. For males a shorter prepelvic length, which in H. perlo is combined with a more posteriorly sited anal fin, provides for a longer pelvic-anal fin interspace and hence allows space for the large claspers.page 6
Table 1. Selected dimensions of Australian, New Zealand and Atlantic specimens of Heptranchias perlo. Dimensions in columns A-B are those which according to Whitley (1931) should separate H. dakini from H. perlo. Dimensions in columns B-F illustrate sexual dimorphism in H. perlo.page 7 page 8
Table 2. Heptranchias perlo: proportional dimensions as percentage of total lengthpage 9 page 10
We have not searched the literature on other notidanoid sharks to see if they show sexual dimorphism comparable with that in H. perlo, but we do note that illustrations of the male holotype and a female of Hexanchus vitulus, a new species of Sixgill shark described by Springer and Waller (1969), show the male with the anal fin origin towards the posterior end of the dorsal base, whereas in the female it is below the middle of the base. Through the kindness of Mr. Stewart Springer, co-author of Hexanchus vitulus, we have been given measurements of the type specimens. These establish that in the male holotype the proportion (%) of the dorsal fin base anterior to the anal fin origin is 81%, whereas in the female paratype it is 67% and in another smaller female it is 74%. The situation in Hexanchus vitulus, is, therefore, akin to that in H. perlo, and it seems likely that such sexual dimorphism is common to all notidanoids.