VARIATION IN THE NUMBER OF FIN-RAYS IN THREE 



SPECIES OF FUNDULUS OF THE WOODS 



HOLE REGION. 



ROBERT PAYNE BIGELOW. 



While engaged in the study of the feeding habits of young fishes 

 in August, 1924, the author collected in Woods Hole Harbor a 

 number of young Fundulus having a total length of from 9 to 51 

 mm. All except the youngest of these fish showed the darkly 

 pigmented vertical bands, 9 to 1 1 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, 

 '07; 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 (14 and 20 mm.) had in 

 both D n, A 10. All the others had D 13 to 15, A 9 to n, of these 

 seven out of ten had more than 13 dorsal rays. 



Turning to Jordan and Evermann '96 for identification, the 

 fin-ray formulae for the species known to occur at Woods Hole 

 were found to be given as follows : F. heteroclitus , Dn,Aioorn; 

 F. majalis, D 12, A 10; F. diaphanus, D 13, A n. From the fin- 

 formulse 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. 



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