DEEP-SEA PROTOBRANCHIA (BIVALVIA) 



83 



and West Atlantic, Y. frigida and Y. robusta that have 

 characters which we believe a 'stem' group might possess. 

 These include, in addition to the single hind gut loop, a 

 symmetrical ovate shell outine, relatively short fragile sepa- 

 rate hinge plates and a moderately large central amphidetic 

 ligament (Fig. 148). In addition, they have large siphons with 

 combined lumena which are ventrally unfused, moderately 

 large, oval, subequal, adductor muscles, a few large fringing 

 papillae to the foot, a relatively elongate gill with a moderate 

 number of plates and broad palp ridges which are relatively 

 few in number. 



From this basic form, we derive a number of evolutionary 

 pathways. The most simple derivation appears to be that 

 shells become somewhat higher in proportion to length and 

 the antero-dorsal and postero-dorsal margins become more 

 horizontal such that the anterior and posterior limits of the 

 shell are dorsal to the horizontal midline (Figs. 136-147). 

 (For details of the comparative overlay technique, see Fig. 

 136). In other respects, a characters are similar to those given 

 above. These latter species include Y. inconspicua, Y. extensa 

 and Y. argentinensis, again from the North and West Atlantic 

 but further downslope than Y. frigida and Y. robusta. In 

 addition, Y. curta, a common and widespread species from 

 the base of the continental slope would also appear to belong 

 here, however, unlike the species mentioned so far, the 

 hindgut of Y. curta passes anterior to the mouth to form a 

 single loop on the left side of the body. This disposition of the 

 hind gut is a simple derivation from the primitive condition 

 and can be explained simply in terms of elongation and 

 accommodation of the hindgut. In all other respects Y. curta 

 is similar to the species of the Y. frigida group. 



In juxtaposition to this possible stem group is a group of 

 species centred upon Y. lucida (shelf/upper slope), Y. obesa 

 (mid/lower slope) and Y. similiris (upper/mid slope). 

 Although having similar ovate shape to Y. frigida and Y. 

 robusta, these species have much longer but still separated 

 hinge plates, smaller adductor muscles of which the posterior 

 is significantly smaller than the elongate anterior muscle. The 

 hind gut loops are somewhat longer and more broadly looped 

 and the lumena have a wider diameter. Most other characters 

 are in common with the 'stem group'. These include an 

 internal amphedetic ligament ventral to the hinge plate which 

 is somewhat smaller than that of the stem group, a relatively 

 elongate gill, small palps with broad ridges which are few in 

 number and papillae of the foot which are relatively large. 

 The differences between Y. lucida, Y. obesa and Y. similiris 

 and the species of the 'stem group' relate to the strength of 

 the hinge plate. As the length and robustness of the hinge 

 ^increases there is less requirement for large adductors and a 

 large ligament to ensure the integrity of the two valves. In 

 addition, as the maximum depth limit of the species 

 increases, the hind gut tends to enlarge either in length or 

 diameter or both. 



As in the case of the 'stem group', we believe evolution 

 from the 'lucida' group also involves an increase in the height 

 of the shell and the antero- and postero-dorsal shell margins 

 becoming more horizontal such that the posterior limit of the 

 hell margin becomes characteristically sharply rounded. The 

 posterior adductor muscle is reduced in size, the hind gut 

 penetrates deep into the foot and the palps are small with 

 j/ery few ridges. The siphonal lumena are separate. Species 

 vith these characterstics include Y. bilanta, Y. hanna and Y. 

 apensis, all of which are restricted to slope depths. 



Y. artipica is intermediate in its characters to the two above 



groups, however the hind gut is more sinuous in its course to 

 the right. This represents the initial stage in a trend that leads 

 to the coiling of the hind gut on the right side of the body. 

 Note the Y. artipica has a deeper distribution than those 

 species described above and is found on the abyssal rise. 

 Similarly, Y. sinuosa, Y, blanda and Y. biscayensis which 

 have similar characteristics to the 'bilanta'' group, all have 

 sinuous hind guts. There are other internal differences shown 

 by these three species. Thus, while the adductor muscles are 

 dissimilar in size, the posterior muscle is not greatly reduced 

 in size, the palp ridges, although wide, are more numerous 

 and thus the palp is large and the siphon is particularly large 

 with combined lumena. As in the case of Y. artipica these are 

 species from the abyssal rise. 



The logical sequence to the trend is seen in Y. lata in which 

 the hind gut has a larger diameter, has lengthened such that 

 the 'reverse-S' course has progressed to a double-loop. Also 

 to be noted are that the adductors are moderately large and 

 are equal in size, and the hinge plates, although elongate, are 

 narrow and much less robust. Similarly the gill is relatively 

 small and the palp has numerous narrow ridges. 



As a continuation of this trend, a specialized group of 

 species comprising Y. ella, Y. enata and Y. jeffreysi is arrived 

 at. A sequence of events can be envisaged. Starting from the 

 condition in Y. lucida the hind gut lengthens, remains to the 

 right of the body and becomes increasingly coiled. (Up to 

 four times). The shell becomes characteristically rounded, 

 high with large umbos. The hinge plates become very strong 

 and the ligament small and rounded. The adductor muscles 

 are small, and may be subequal in size. The generating 

 outline curve of the shell is rotated somewhat to the right so 

 that the umbo is distinctly anterior and the maximum ventral 

 limit is posterior to the vertical midline. We can envisage a 

 sequence of increasing hind gut complexity from Y. lucida 

 through Y. blanda, Y. lata, Y. jeffreysi to Y. enata, however, 

 we have some reservations as to whether Y. ella is the 

 terminal species of this line. While Y. ella has the most coiled 

 hind gut of all the yoldiellids in our collections, the more 

 evenly rounded shape, the extremely small subequal adduc- 

 tor muscles and the massive hinge plates might indicate a 

 separate derivation or at least a marked terminal divergence 

 in the series. These species have a depth distribution from the 

 abyssal rise to the greatest abyssal depth. There is little doubt 

 in our opinion that the differences in morphology exhibited in 

 both the latter group are related to the requirements of life at 

 abyssal depths. 



We also derive an even more complex but distinct group 

 comprising Y. americana, Y. subcircularis , Y. biguttata and V. 

 ovata from the Y. lucida stock. These four species also have 

 characteristically rounded, high shells which have large 

 umbos. The evolutionary sequence probably included an 

 intermediate stage with a form similar to that of Y. perplexa 

 in which shell characters are similar but in which there is a 

 relatively short single hind gut loop to the right, robust 

 subequal adductor muscles, small palps and gills, large pedal 

 papillae and long hinge plates. Y. americana differs relatively 

 little from this, except that the hind gut is considerably 

 lengthened and which takes a course to the left and right of 

 the stomach and the palp is a deep semicircular shape with a 

 large number of ridges (see p. 54). A similar morphology to 

 that of Y. americana is found in Y. subcircularis except that 

 the hindgut loops are doubled on either side of the body. 

 Similarly, Y. biguttata and Y. ovata also have two hind gut 

 loops to the right and the left and in Y. insculpta, the hind gut 



