DEEP-SEA PROTOBRANCHIA (BIVALVIA) 



89 



is further extended to three loops to the right and two to the 

 left with one loop on each side short and anterior to the 

 stomach. The latter is a configuration unique among proto- 

 branchs. In these relatively large rounded species the liga- 

 ment tends to be relatively large and the hinge plates robust 

 and elongate. Most of these species extend from the abyssal 

 rise across the abyssal plain. 



Comparison of this latter group with Y. similis is of 

 interest. While Y. similis species has a double hind gut loop 

 its anatomy is similar to Y. artipica. In particular it has small 

 palps with a few broad ridges. 



One species of Yoldiella remains to be mentioned. Yoldi- 

 ella fabula differs markedly in its shell characters from all 

 other species of Yoldiella (and Portlandia) yet in its internal 

 morphology it is close to the basic stem group of the genus. 

 Thus, while it is elongate with the umbo set far posterior to 

 the mid-vertical line, it has relatively short separate hinge 

 plates with a small amphidetic ligament. Very large subequal 

 adductors are present and the hind gut is a simple, single loop 

 to the right. Although its shell outline is unique, these 

 characters fit best with the yoldiellids and we are presently 

 persuaded to keep it within this subfamily. 



In the foregoing discussion, stress has been placed on 

 changes in the shape of the shell and the disposition of the 

 gut. It is clear that for the most part species that occur at great 

 depths have longer guts than those in shallow water and that 

 this increased length has been accommodated within a small 

 body space. This in turn relates to the digestive requirement 

 to deal with sparse complex organics in deep-sea sediments. 

 Indeed, in general the body space of deep-sea bivalves as a 

 percentage of shell volume is significantly smaller than that of 

 shallow-water congeners. Similarly, in regard to shell shape, 

 species in shallower depths appear to be of an elongate-ovate 

 shape, whereas those from the deep have higher shells which 

 are either more rounded or have the greatest length measure- 

 ment dorsal to the mid-horizontal plane. This we believe is 

 related to the softness of the abyssal sediments and the ease 

 of movement within them. 



There are other evolutionary trends that may or may not be 

 depth related. For instance, the size of the adductors clearly 

 relates inversely to the strength and length of the hinge plates 

 and the size of the ligament. In contrast, the size of the palps 

 appears to increase with increasing depth range, while gills 

 tend to reduce in size. We believe this is for a different reason 

 from that of the change in shape of gut and shell, and relates 

 in part to a difference of energy demand at high pressures and 

 in part to the lack of importance of the protobranch gill in the 

 feeding process. Because of this latter there is a reduction in 

 the size of the gill, however, the loss in ciliated tissue is 

 compensated by an increase in palp area which is required in 

 order to maintain ciliary flow within the mantle cavity. At the 

 same time it provides ciliary activity where is is most needed 

 in the processing of fine abyssal sediments. 



As might be expected the siphons also show modifications. 

 Reduction in the gill area results in lower inhalent siphonal 

 flow rates. In contrast, larger palp surfaces result in higher 

 inflow via the feeding aperture. The processing of large 

 quantities of fine sediment must produce increased numbers 

 of faecal pellets. The predicted result from these changes is 

 : realized in the increased importance and size of the feeding 

 | aperture, the reduction and in some cases elimination of the 

 'inhalent siphon or the loss of division between exhalent and 

 inhalent siphonal lumena such that the combined siphon is 

 largely used for the passage of faecal material to the outside. 



Finally, we speculate that not only that the small size of the 

 body in comparison with the shell volume in the deep-water 

 species is related to reduced food resources (as in reduced 

 numbers of ova) but it is also related to the reduction of 

 overall metabolic energy requirements at high pressures. 



Like all protobranchs, the yoldiellids have large eggs and 

 larval development is almost certainly short-lasting, non- 

 feeding and takes place close to the sea floor. Although there 

 are subtle basinal differences in shell form, some of which 

 may be sufficiently distinct to establish subspecies (e.g. Y. 

 obesa and Y. inconspicua) there is little doubt that either 

 widespread gene flow occurs and/or that genetic change is 

 slow. Clearly, the yoldiellid form is one that is extremely 

 successful. The subfamily contains by far the most species of 

 all the protobranch families and subfamilies. 



Much of the scientific discussion on the distribution of the 

 Yoldiellinae will be incorporated into a following and final 

 round-up paper on the diversity and zoogeography of the 

 deep-sea protobranchs of the Atlantic. Nevertheless, because 

 it is such a large group, the distribution of the 29 species of 

 the genus Yoldiella described here reflect many of the general 

 features of protobranch distribution. Thus, of the 29 species 

 only a limited number can be regarded as being widespread 

 (Table 3). Only six species are present in five or more of the 

 Atlantic abyssal basins and of these, five are abyssal and one 

 is lower slope/abyssal rise in its depth distribution. There are 

 16 endemic species, of these five species and one subspecies 

 are restricted to the Argentine Basin. A further four endemic 

 species are found in the Cape or Angola Basins. Thus, it is 

 clear that most endemic species are in the South Atlantic. 

 Furthermore, the South Atlantic has been much less sampled 

 than the North and one would suspect that more rare 

 endemic species will be reported in the future. This is clearly 

 of considerable importance in speculations on the origin of 

 the protobranch fauna of the Atlantic. Although endemic 

 species are not restricted to upper slope depths, most of the 

 species at upper slope depths are endemic and those few that 

 are not, are restricted to the North European and North 

 American Basins. Thus, in general, the deeper the species 

 occurs, the more widespread is likely to be its distribution. 

 This may simply reflect the fact that the abyssal plains contain 

 enormous areas of sediment of similar characteristics and that 

 distribution simply reflects the commonality of the environ- 

 ment. 



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