I So BULLETIN OF THE BUREAU OP FISHERIES. 



ensis, Cancer borealis, Ovalipes ocellatus, Pagurus acadianus, Astarte castanea, Astarte 

 undata, Venericardia borealis, and Amaroucium stellatum. Now, a number of the fore- 

 going species, and in general a considerable proportion of those species which are 

 limited to the deeper waters, have already been mentioned among the northern forms 

 whose distribution is determined locally by temperature conditions. It must be 

 repeated, however (see p. 28), that the waters of the western end of Vineyard Sound 

 are little if any deeper on the average than those in the vicinity of Nobska and West 

 Chop. The preference of these species for deeper waters is shown by their scarcity in 

 the adlittoral zone. Certain of them, indeed, were dredged only by the Fish Hawk. 

 It is more than likely that the somewhat lower summer temperature of these bottom 

 waters, as compared with those skirting the shore, is the factor responsible for the 

 restriction of some species to the former. The temperature factor is not the one directly 

 concerned, however, in the case of all of the animals named. The distribution of 

 Ovalipes, for example, is probably wholly determined by the character of the bottom. 

 It is indeed known to occur on sand flats in shallow, warm water. The case of Amar- 

 oucium stellatum is interesting, since, although a deep-water species in the sense here 

 employed, it is for the most part restricted to the more easterly portions of the Sound, 

 where the bottoms are gravelly or stony. Thus its preference for deeper waters does 

 not appear to be related to the temperature factor, though this is not entirely certain, 

 since the deeper waters are everywhere somewhat cooler in summer than are the shoaler 

 ones. The marked restriction of this species to the former is in striking contrast to the 

 condition shown by the related Amaroucium pellucidum constellatum ( = A. constellatum 

 Verrill), which, although associated with A. stellatum at various points, is likewise 

 found in profusion in shallow waters and even upon piles. 



The vertical distribution of marine organisms is commonly designated by the term 

 "bathymetric," and it has been sometimes supposed that depth was one of the primary 

 factors determining distribution. There are, of course, at least four factors bound up 

 in this one, viz, pressure, temperature, light, and gas content. Now, it is not at all 

 certain to what degree, if any, pressure influences distribution. For the limited depths 

 within our region, we may certainly leave it out of account. 



Temperature is, as we have seen, definitely correlated with depth in the sea, just 

 as it is with altitude on land. But there is, in local waters, little difference between 

 surface and bottom temperature, except in those portions of the Sound and the Bay 

 which adjoin the open ocean. Some of the cold-water species inhabiting these last 

 are, as just stated, restricted to the greater depths. On the other hand, the restriction 

 of certain species (see above) to the shallow water immediately skirting the shore may 

 be due in some cases to the palpably higher temperature commonly met with at such 

 points during the summer. 



The relation of light to depth has been treated at some length in the botanical 

 section of this report (p. 447-449), to which the reader is referred. It is likely that 

 for relatively slight depths, such as those we are considering, the light factor has little 

 direct effect upon the bathymetric distribution of animals. Indirectly it may be of 

 influence in the case of certain forms which dwell upon algae, and it is possible that some 

 of the adlittoral species which have been discussed above are limited in this way. 



It may be repeated in conclusion, however, that, as regards the species taken 

 during our dredging operations, the great majority show little or no evidence of 

 bathymetric distribution. 



