12 



J.A. ALLEN, H.L. SANDERS AND F. HANNAH 



Synopsis. Four species of the genus Portlandia and twenty-nine species and five subspecies of the genus Yoldiella 

 from the deep Atlantic are described - many for the first time. The subfamily Yoldiellinae includes more species 

 than any other higher taxon of deep-sea protobranchs. The differences in morphology are for the most part subtle 

 and there are many closely related species. These close relationships have been analysed, the analysis taking into 

 account shell shape, hinge morphology, musculature and the extent and course taken by the hindgut. Taken 

 together with geographical and depth distribution a pattern of evolution is derived. This supports the view that the 

 derivation of the deep-water species of Yoldiella in the Atlantic has been derived mainly via downslope migration 

 and speciation rather than by deep water migration from the Southern Ocean. 



INTRODUCTION 



This is the eighth paper in a series on the biology and ecology 

 of the deep-sea protobranch bivalves of the Atlantic (Allen & 

 Sanders, 1973, 1982; Allen & Hannah (1989); Rhind & Allen 

 (1992); Sanders & Allen 1975, 1977, 1985. Our interest is 

 widespread and includes, ecology and adaptations to life at 

 great depths, morphology, reproduction, distribution and 

 evolution. Here we turn to the subfamily Yoldiellinae and 

 what has become our major and most difficult task of all our 

 protobranch studies to date. Nowhere have the problems of 

 elucidating evolutionary trends and specific and subspecific 

 divisions within the Protobranchia been so acute as in this 

 large group. Of necessity descriptive and taxonomic studies 

 have played a major role in all our studies. This is because so 

 many species from the deep oceans are new. Now that we 

 have studied more than three quarters of the protobranch 

 material in our collections, it has become obvious that major 

 questions on the evolution of the subclass - particularly those 

 taxa in the deep ocean - remain to be answered. We have 

 described (Sanders & Allen, 1985) intra- and inter- 

 population variations in various species and the difficulties in 

 separating even higher taxa with satisfactory, clear cut, 

 definitions (Allen & Hannah, 1986). These difficulties can be 

 no better illustrated than in our studies on the Yoldiellinae. A 

 considerable effort has been put into the analysis of the most 

 subtle differences in shape and form of the many species of 

 the subfamily. As a consequence we have decided to record 

 our observations in two parts. In the first, here, we describe 

 36 species in a way that has become standard for this series of 

 papers, describing those population variations that are perti- 

 nent to description and taxonomy. In the second, we will 

 report in detail on diversity and the quantitative aspects of 

 the ecology of sibling species which are distributed widely in 

 many Basins of the Atlantic. 



Species of the subfamily Yoldiellinae, are among the most 

 common protobranchs of the deep sea and many are recorded 

 in the literature. The difficulties we have experienced in 

 accurately distinguishing the species is not new and confusion 

 is apparent in both past and recent literature and in museum 

 identifications of this group. 



Descriptions of genera and higher taxa are based on the 

 recent studies of Shileiko (1985) and Allen & Hannah (1986), 

 but complemented from the results of this study. Holotypes 

 have been lodged in either the Natural History Museum, 

 London, or the Museum National d'histoire Naturelle, Paris. 

 The paratypes, together with the remainder of the specimens 

 collected, for the time being are in the care of JAA, but at the 

 conclusion of the studies will be lodged in appropriate 

 Museums. 



Measurements of height, length, width and postumbonal 



length have been taken and in the case of larger samples 

 ratios have been plotted. For species of which we have few 

 specimens the measurements have been tabulated. While 

 these record the variation in the major axes, they do not 

 measure subtle variation in shell outline and curvature. Much 

 time has been spent on computerized analysis of shell shape 

 and on this work we hope to report later but, to date, this has 

 not improved on visual recognition from comparative accu- 

 rate drawings. We prefer drawings to photographs for their 

 clarity. 



In recognizing subspecies we comply with the ICZN. 

 Subspecies occur at different depth ranges and/or different 

 basins. In a few cases we recognize 'forms', infrasubspecific 

 units which, in compliance with ICZN, cannot clearly be 

 distinguished in their distribution patterns but which may 

 indicate a species in the process of subspeciation. 



ABBREVIATIONS TO TEXT FIGURES 



AA anterior adductor muscle 



ME mantle edge 



AN anus 



MT major typhlosole 



AS anterior sense organ 



NV nerve 



BG 'byssal' gland 



OE oesophagus 



CG cerebral ganglion 



PA posterior adductor muscle 



CS combined siphon 



PG pedal ganglion 



DD digestive duct 



PL palp 



EG digestive diverticula 



PM pallial muscles 



DH dorsal hood 



PP palp proboscis 



ES exhalent siphon 



PR pedal retractor muscle 



FA feeding aperture 



PSA posterior sorting area 



FM pedal muscles 



RM longitudinal muscle 



FT foot 



SC statocyst 



GC gland cells 



SE siphonal embayment 



GD duct of gland 



SF sole of foot 



GI gill 



SS style sac 



GS gastric shield 



ST stomach 



HG hind gut 



SY fold of sensory organ 



HT heart 



TE tentacle 



IF inner muscular fold 



TM transverse muscle 



IS inhalent siphon 



TS tooth of gastric shield 



LI ligament 



VG visceral ganglion 



Family Nuculanidae Adams & Adams 1858 



Shell elongate, usually moderately compressed, may be ros- 

 trate, shell gape if present, restricted to short posterior 

 margin where siphons protrude, concentric sculpture usually 

 present which may be strongly incised, middle and inner shell 

 layers non-nacrous; teeth chevron-shaped; ligament internal 

 or external with resilium; combined siphons present, usually 

 a simple siphonal tentacle attached to the left or right side of 

 the siphonal embayment. 



