DEEP-SEA PROTOBRANCHIA (BIVALVIA) 



Table 2 Yoldiella jeffreysi; ratios of Posterior Length (PL), Height (H), Width (W), to Total Length (TL) and other parameters. 



65 



Basin 



Max. recorded 



PL/TL 



H/TL 



W/TL 



ProdissoconchN 



length (mm) 



(mean) 



(mean) 



(mean) 



length ((Jim) 



3.70 



48/60 



69/75 



33/53 



187-198 (54) 





(54) 



(72) 



(45) 





3.49 



52/58 



68/75 



32/48 



185-198(17) 





(56) 



(70) 



(41) 





2.84 



51/56 



70/76 



38/46 



190-200 (52) 





(54) 



(72) 



(42) 





2.54 



47/55 



69/77 



35/52 



182-189 (38) 





(51) 



(73) 



(40) 





2.96 



48/58 



69/76 



34/43 



200-210 (35) 





(54) 



(73) 



(40) 





2.95 



44/60 



69/75 



36/49 



182 (25) 





(53) 



(72) 



(41) 





W. European 

 Incal DS07 

 Canary 

 Sta. 6704 

 Cape Verde 

 852 1 6 



N. America 

 Sta. 126 

 Guyana 

 Sta. 291 

 Argentine 

 Sta. 259 



Fig. 101 Yoldiella jeffreysi. Lateral view of a shell from the left 

 side and an inner view of a right valve to show detail of the 

 hinge-plate. Specimens from Sta. 316, West European Basin. 

 (Scale =1.0 mm). 



The populations in different basins also differ somewhat in 

 overall shape (Fig. 105). In the Atlantic, more southerly 

 populations are somewhat less inflated and large specimens 

 have an extended posterior margin and thus a more inequilat- 

 eral shape. Overall, populations in the western Atlantic have 

 a similar shape to the majority of specimens from the 

 northern part of the West European Basin and they are 

 inflated to a similar degree. The more southern populations 

 are more extended posteriorly. Specimens from the Argen- 

 tine Basin are slightly more inflated but less so than those 

 from the northern West European Basin. 



It is a general feature of all populations that the width/ 

 ; length and post-umbonal length/total length increase with 

 I increasing length. In contrast, there is little change in the 

 , height/length ratio (Fig. 105). 



The inter- and intra- variability in the shape of populations 

 of deep sea protobranchs has been noted in many taxa and 

 most recently for the family Malletiidae (Sanders & Allen 

 1985). Yoldiellid species are no exception and Y. jeffreysi is 



an extreme example (see Table 2). So much so that we have 

 made a particular study of this species and we propose to 

 present our results in more detail in a following publication. 



The most closely related (but distinct) species to Y. jeffreysi 

 is Yoldiella lata. Jeffreys (1876) failed to distinguish between 

 the two species in his samples from depths where their 

 distributions overlap (Y. lata is confined to slope depths (see 

 p. 32). In general Y. jeffreysi is more inflated than Y. lata, has 

 more hinge teeth and the posterior adductor muscle (usually 

 visible through the shell in live specimens) is smaller and 

 more elongate. 



Internal morphology (Fig. 106-107). The exhalent and 

 inhalent siphons are combined the latter being open ventrally 

 (Fig. 106). There is a moderately well-developed feeding 

 aperture immediately ventral to the inhalent siphon. A large 

 siphonal tentacle, more frequently on the left side originates 

 close to the base of the siphon. Antero-ventrally there is a 

 well-developed mantle sense organ. The adductor muscles 

 are unequal in size. The anterior is between two and three 

 times larger than the posterior. The posterior is oval in shape 

 while anterior is 'bean'-shaped; the 'quick' and 'catch' parts 

 are clearly distinct. 



The gills are relatively well-developed with 12-19 alternat- 

 ing gill plates, the number depending on the size of the 

 animal. The most posterior plate lies close to the junction 

 between the inhalent and exhalent siphons and to which the 

 gill axes join. The labial palps are relatively small with long 

 and slender palp proboscides. In their contracted state they 

 extend across one quarter to one third of the body. The palps 

 have been 11-14 palp ridges, again the number depending on 

 the size of the animal. The foot is large and in some 

 specimens it is preserved in a very long, anteriorly extended 

 fashion. It has a deep papillate sole. A large byssal gland is 

 present and in many specimens in the region of its aperture at 

 the posterior margin of the foot there is a considerable 

 amount of mucous material present. 



The species has a very large stomach, the dorsal wall of 

 which lies close to the hinge plate, the stomach lies off centre 

 slightly to the left. A large style sac penetrates deep into the 

 foot. The gastric shield extends close to the opening of the 

 oesophagus, the latter slightly to the right on the antero- 

 dorsal wall. Right and left digestive diverticula are anterior 

 within the body and material similar to that found in the 

 stomach was observed in sections of both right and left 

 diverticula. The hind gut forms a clockwise loop to the right 



