VARIATION IN THE NUMBER OF FIN-RAYS IN THREE SPECIES OF FUNDULUS OF THE WOODS HOLE REGION

Abstract

While engaged in the study of the feeding habits of young fishes in August, I924, the author collected in Woods Hole Harbor a number of young Fundulus having a total length of from 9 to 5! mm. All except the youngest of these fish showed the darkly pigmented vertical bands, 9 to II in number, characteristic of the young of all three of the species of Fundulus known to occur in the vicinity of Woods Hole. (Jordan & Evermann, †? 96; Newman, †? o7; Sumner et al., †? 13.) The color markings give no clue to specific identity at this stage, and the anatomical proportions, changing as they do with growth, are equally useless for identification. On the other hand, certain supporting structures, such as vertebrae and fin-rays, are developed at an early stage and remain constant in number throughout life. Therefore it was hoped that the number of fin rays would furnish a means of assigning these young fish to their proper species. The number of fin-rays in the dorsal and anal fins of the smallest fishes in the collection (total length 9 and 12 mm.) were found to be D 9, A 7 and 9. The two next in size (†4and 20 mm.) had in both D II, A io. All the others had D 13 to 15, A 9 to II, of these seven out of ten had more than 13 dorsal rays. Turning to Jordan and Evermann †? 96 for identification, the fin-ray formuhe for the species known to occur at Woods Hole were found to be given as follows: F. heteroclitus, D I I, A 10 or II; F. tflajalis, D 12, A 10; F. diaphanus, D 13, A II. From the fin formulae alone it might have been supposed that the specimens in question were F. diaphanus. But this was improbable because they were taken in the pure sea water of Woods Hole Harbor, whereas F. diaphanus is supposed to inhabit only brackish or fresh water. On the other hand, the number of rays in the dorsal fins was much greater than the numbers given for the two purely marine species.