127 



thermoclines, which often foUow a period of intense upwelHng. Dissolved 

 oxygen in the water column below the thermocline is rapidly depleted by 

 the oxidation of the organic debris (resulting from the death of the bloom) 

 as it falls to the bottom. Even so, much of the organic matter does reach 

 the bottom unchanged, thereby providing food for the detritus feeders 

 and scavengers living there. 



Virtually nothing is known concerning the larval development and 

 reproduction of the mollusks living in the upper bathyal regions. A pre- 

 liminary examination of the prodissoconchs of some of the lamellibranchs 

 by W. K. OcKELMANN and the author indicated that at least some of them 

 have a lecithitrophic development and probably no pelagic veliger or 

 feeding stage. Ockelmann (1958 pp. 243-45) has shown that the number 

 of non-pelagic larval species of lamellibranchs increase rapidly from Boreal 

 to Arctic waters, while Thorson (1944) indicated that none of the species 

 of prosobranch gastropods investigated by him in the Arctic had a pelagic 

 development. According to a preliminary analysis of the Gulf of California 

 data, a similar phenomenon occurs in the Gulf where the number of species 

 of mollusks having a pelagic larval development decreases with increasing 

 depth. Similar observations were also reported by Thorson (1950, pp. 

 26-28). Since bottom water temperatures approach Arctic values as one 

 descends into abyssal depths, and food becomes less plentiful, it would 

 seem that temperature and availability of food may influence the distri- 

 bution of mollusk species with non-pelagic larval development. 



X. Middle Continental Slope, 731 to 1,799 Meters: 



The mid-slope assemblage is a typically deep-sea one, composed entirely 

 of lower bathyal and upper abyssal fauna. Three of the eight species of 

 mollusks taken in this environment were listed as abyssal by Clarke 

 (1962) and all of the genera are primarily abyssal in distribution. Bottom 

 water temperatures (3 to 6°C.) are partly in the abyssal range, according 

 to the description of the abyssal environment given by Madsen (1961). 

 There seems to be no real dividing line between abyssal and bathyal, 

 based on depth or temperature alone. Whether a fauna in any region is 

 bathyal or abyssal depends upon many circumstances. For instance, depth 

 is not a valid limiting factor, when one considers the fauna of the deep 

 isothermal basins, such as the Red Sea, Ballenas Channel in the Gulf of 

 California, and the Sulu Basin. These areas have what have been called 

 abyssal depths, but are characterized by high outer shelf temperatures at 

 the bottom. Likewise, the Arctic and Antarctic regions have abyssal 



