DEEP-SEA PROTOBRANCHIA (BIVALVIA) 



131 



The hind gut in Neilonella, like that in Tindaria, has a wide lumen 

 and single pronounced typhlosole. Although the course that the hind 

 gut takes in neilonellids makes a single loop on the right side of the 

 body, it does not penetrate mantle space as it does in tindariids 

 (Sanders and Allen, 1977). In this respect the hind gut of neilonellids 

 probably represents the more primitive condition. We have argued 

 elsewhere (Allen, 1992) that elongation and the complexity of form 

 of hind gut configuration are related to food procurement at great 

 depths, and this applies to the tindariids (Sanders and Allen, 1977). 

 The neilonellids are for the most part upper slope species and the 

 hind gut would be expected to be less specialized and less elongate. 



In Nuculana posterior elongation becomes more extreme and the 

 ventral margins of the combined siphons are fused such that the 

 exhalent and inhalent lumena are separate and entire. The shell 

 remains robust, but is more slender The ligament is restricted to a 

 small internal structure separating elongated anterior and posterior 

 series of hinge teeth. We believe that elongation is correlated to the 

 almost vertical orientation of the animal in the sediment but which 

 retains contact with the surface via the extended posterior body and 

 siphons. The genus Nuculana is found mainly in shelf and upper 

 slope sediments and as such the available food resources are rela- 

 tively abundant. The hind gut is not greatly extended and remains as 

 a single loop to the right side of the body. 



In Propeleda the evolutionary trend of posterior body elongation 

 seen in Nuculana becomes is more extreme, particularly posterior to 

 the posterior adductor muscle. The posterior adductor muscle is 

 more elongate and dorso-ventrally narrow, and the gill, gill axes, 

 siphons and the palp proboscides are exceptionally long and slender. 

 The shell of Propeleda, particularly in abyssal species, is much more 

 fragile and is further specialized in that it possesses an internal 

 posterior longitudinal ridge. The function of this ridge is not entirely 

 clear and has await examination of the living animal but, possibly, it 

 is involved in the separation of excretion, feeding and respiratory 

 functions in the extremely elongate posterior mantle cavity. It may 

 also help to strengthen the otherwise very fragile shell and assist in 

 predation damage limition. In Propeleda post-adductor elongation 

 involves body tissues that can be relatively easily regenerated, much 

 in the same way as has been reported in deposit feeding tellinids 

 (Edwards, Steele and Trevallion, 1970). Specimens showing shell 

 repair posterior to the posterior adductor are present in our samples. 



The evolution of the Ledellinae and an assessment of their func- 

 tional morphology was discussed earlier (Allen and Hannah, 1989). 

 In respect of the species of Ledella and Tindariopsis described here, 

 little needs to be added to that account other than to note, again, that 

 the hind gut in these abyssal protobranchs is extraordinarily length- 

 ened and takes the most complex courses within the visceral mass. 



The other item of note is the description of yet another ledellid in 

 which the shell, after reaching a certain length, changes its direction 

 of growth. In Ledella aberrata as in L. ultima the result of this 

 change is to produce a broad shell margin and lateral expansion of 

 the shell cavity. This adaptation has been construed as possibly 

 providing more space for the gonads that begin to develop at about 

 the time the change in direction occurs. 



Acknowledgements. It is fitting that in this paper we thank our numer- 

 ous friends and colleagues, particularly French colleagues of the Biogas and 

 Walda cruises, who provided much material and gave much advice and 

 encouragement. We would like to mention three colleagues by name. George 

 Hampson of the Woods Hole Oceanographic Institution and Fiona Hannah 

 (Lonsdale) of the University Marine Biological Station, Millport, who have 

 given us tremendous close support over the years and without their help it is 

 likely that we would not have succeeded in this task within our lifetimes, and 

 Kathy Way of the Natural History Museum, London, who has been so quick 



in responding to our requests for help particularly in our quest for early 

 literature and specimens for comparison. Much of the work was supported by 

 grants from the Natural Environment Research Council. 



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