Unloved, paraphyletic or misplaced: new genera and species of small to minute lucinid bivalves and their relationships (Bivalvia, Lucinidae)

Abstract Species identified as Pillucina are paraphyletic in molecular analyses and a new generic name, Rugalucina, is introduced for a complex of three similar species Rugalucina angela from the northern Indian Ocean and Red Sea, R. vietnamica from South East Asia, and R. munda from northern and north eastern Australia. Lucina concinna from the Red Sea, previously synonymised with P. vietnamica/angela is recognised as a Rugalucina-like species but with a very short anterior adductor scar. Divaricella cypselis from Karachi is similarly now recognised as a distinct species, probably related to Rugalucina but with oblique commarginal sculpture and a short adductor scar. A group of minute Indo-West Pacific lucinids with highly unusual multi-cuspate lateral teeth and previously classified as Pillucina are separated under a new genus Pusillolucinagen. nov., with the description of three new species P. arabica, P. africana, and P. biritika from the Arabian Gulf, Mozambique, and Madagascar. Finally, a new genus, Notocina, is introduced for the small southern Atlantic species, Epicodakia falklandica, shown in molecular analyses to be misplaced at subfamily level and now classified in Lucininae and not Codakiinae with Epicodakia.


Introduction
Within the chemosymbiotic bivalve family Lucinidae, genera in the Loripes group have been identified as monophyletic (Taylor et al. 2011 and characterised by a distinctive internally inset ligament. In the eastern Atlantic and Mediterranean Loripes orbiculatus is recognised as the most abundant lucinid in shallow water seagrass habitats and its important role in ecosystem function has been much studied (Johnson et al. 2002;Heide et al. 2012;Rossi et al. 2013;Geest et al. 2014). Other genera within the Loripes group include Lucinella and Keletistes in the Atlantic and from the Indo-West Pacific Pillucina, Wallucina, and Chavania. Of these, Pillucina is the most diverse with eleven currently recognised species (Glover and Taylor 2001; distributed from the Red Sea to the Hawaiian Islands. Some species have been recorded with high population densities in seagrass and peri-mangrove habitats (Nakaoka et al. 2002;Uede andTakahashi 2008, Meyer et al. 2008;Rattanachot and Prathep 2015). However, recent and ongoing molecular analyses indicate that Pillucina is paraphyletic with P. vietnamica Zorina, 1978 andP. pusilla Glover & aligning distantly from five other Pillucina species (Taylor et al. 2011. Moreover, it is also now apparent that the reportedly widespread Pillucina vietnamica is a complex of three similar species with distinct distributional ranges. In view of the paraphyly and the separate status of the Pillucina vietnamica species group we propose a new genus and a reappraisal of the constituent species including two names resurrected from synonymy and recognition of an early available name from Australia.
The other lucinid separated by paraphyly and not closely related to the other Pillucina species is the minute Pillucina pusilla from the Philippines that possesses distinctive multi-cuspate lateral teeth. This extremely unusual character was first recognised in Pillucina denticula Glover & Taylor, 2001 from near Durban, South Africa. Recently, we have identified herein other species of minute lucinids with similar dentition from the Arabian Gulf, Mozambique and northern and southern Madagascar. Species with this dentition form a morphologically distinct clade that we recognise with a new generic name.
Additionally, amongst these small lucinids it is apparent that the southern Atlantic species Epicodakia falklandica Dell, 1964 is misclassified because molecular results (Taylor et al. 2011 place it in the large subfamily Lucininae rather than Codakiinae with other Epicodakia species. We review the morphology and relationships of this species and place it in a new genus.
During the last 20 years there has been a marked proliferation of generic categories within Lucinidae building on and revising the prior classifications of Chavan (1969) and Bretsky (1976). This is a reflection of increased taxonomic activity following the discovery of chemosymbiosis but also the clear indication from molecular results and more detailed morphological studies that a number of existing genera are paraphyletic or polyphyletic, with some species misclassified even at subfamily level. At species level increased sampling effort, with closer attention to smaller sieve size fractions, has revealed an unexpected range of minute lucinids and this, coupled with greater attention to museum collections and type material, has highlighted previously neglected species.  Figure 1. Single gene tree for the Loripes group of genera and species based on cytochrome b gene sequences, using Bayesian inference as implemented by MrBayes. Support values are posterior probabilities. Methods as in Taylor et al. (2011Taylor et al. ( , 2016 Melvill, 1918 andLucina concinna H. Adams, 1871. Comparison with other genera. Rugalucina is part of the broader Loripes group within the Lucininae, all having an obliquely inset internal ligament. Other genera within the group are Pillucina, Pusillolucina Wallucina, Lucinella, Chavania, and Keletistes. Of these only Pillucina, Pusillolucina, and Rugalucina have prominent radial sculpture. In shell characters Rugalucina differs from Pillucina in the more strongly divergent radial sculpture, the longer anterior adductor muscle scar and the more coarsely crenulate inner shell margin.
In molecular analyses members of the Loripes group form a monophyletic subclade of Lucininae. Seven species of putative Pillucina have been included in analyses , P. pisidium, P. symbolica, P. australis, P. profusa, P. pacifica, 'P.' pusilla, and 'P.' vietnamica, the latter from Thailand, Abu Dhabi, and Queensland. Although the type species P. hawaiiensis has not yet been included, P. pacifica is similar in morphol-ogy and could be regarded as a proxy. The Pillucina species are not monophyletic in published trees or in the mitochondrial cytochrome b gene tree (Fig. 1), with those identified as P. vietnamica and P. pusilla separated from the other species and now classified herein into two new genera with distinct morphological characters. Note: we use the cyt b gene in the analysis of the Loripes group shown in Fig. 1 as this best reflects species level relationships within clades of the subfamily Lucininae as shown by previous analyses using three genes (Taylor et al. 2011. Previously, we regarded Pillucina vietnamica as a wide-ranging species in the northern Indian Ocean through south east Asia to southern China (Glover and Taylor 2001), the name replacing the earlier but preoccupied Lucina fischeriana Issel, 1869, with P. angela as an possible phenotype from the northern Arabian Sea. Although more comprehensive sampling across the whole range is desirable, evidence from analysis of the cytochrome b gene (Fig. 1) shows that specimens from Tin Can Bay and Moreton Bay, Qld, Australia differ from samples from Kungkraben Bay, Thailand and Abu Dhabi supporting a species level separation. Our evidence suggests that Pillucina 'vietnamica' from Arabia, Thailand and Queensland are distinct. After further study of type material, we use the name R. angela for the northern Indian Ocean species, P. vietnamica for the southeast Asian species, and P. munda for the northern Australian species.
Lucina concinna H. Adams, 1871 previously placed in the synonymy of P. vietnamica by Glover and Taylor (2001) is now recognised as having distinct morphological characters and renamed because the original species name is preoccupied. We also resurrect Divaricella cypselis Melvill, 1918 from the synonymy of R. angela and recognise it as another morphologically distinct species, from the Arabian Sea and southern India.  Description. Small (L to 15 mm), subcircular, inflated. Colour white, yellow or orange. Waxy appearance. Sculpture of strong diverging radial ribs, broader and more widely spaced to the anterior and posterior. Ribs crossed by fine, closely spaced, commarginal lamellae which curve over ribs producing a roughly scabrous appearance. Central parts of shell generally without ribs. Lunule large, broadly lanceolate, smooth. Ligament internal, obliquely inset. Hinge: RV with single large cardinal tooth and short anterior and posterior lateral teeth, LV with two cardinal teeth, lateral teeth consisting of small sockets. Anterior adductor scar narrow, elongate, detached from pallial line for ca. half of length. Pallial line irregularly lobate, or slightly divided. Inner shell margin coarsely crenulate to anterior and posterior.
Rugalucina angela (as Pillucina vietnamica) is recorded as an invasive species off Israel in the eastern Mediterranean (Steger et al. 2018).

Remarks.
Rugalucina angela shows variation in shell morphology between various localities around the Arabian Peninsula and Red Sea. For example, shells from the Arabian Gulf are usually smaller, while those from the northern Red Sea as on Gulf of Suez shores are generally larger, and the marginal crenulations stronger. We regard these differences as ecophenotypic probably associated with the extreme environmental conditions such as the very high salinities experienced in the southern Arabian Gulf, usually more than 40 psu but in shallow lagoons as high as 52-55 psu (Price 1982) and approximately 40-46 psu at Safaga in the northern Red Sea (Zuschin and Oliver 2003) and probably higher for the habitats occupied by Rugalucina.
Rugalucina angela is closely similar in shell morphology to R. vietnamica and R. munda. The differences are subtle; externally they share diverging radial ribs that are stronger to anterior and posterior, and the fine commarginal lamellae. Rugalucina angela has a shorter anterior dorsal area, larger hinge plate and teeth, and a slightly more divergent anterior adductor scar. Rugalucina vietnamica is higher, with a longer anterior dorsal area. Rugalucina munda is similar but the radial sculpture is much less pronounced with finer margin denticulations and subdued commarginal sculpture.
Type locality. Intertidal, south coast of Hainan, China. Description. Small (L to 10 mm), sub-circular, longer than high, posteriorly slightly truncate, moderately inflated. Shell white, slightly translucent and waxy in appearance. Sculpture of many, fine, low, commarginal lamellae and low radial ribs which are broader and more prominent towards the anterior and posterior. Radial ribs are conspicuously fluted where commarginal lamellae cross giving a crinkled appearance. Lunule elongate, lanceolate and impressed, slightly asymmetrical. Ligament internal, short, situated on a broadly triangular resilifer. Hinge: right valve with single, cardinal tooth, anterior and posterior lateral teeth small, posterior tooth elongate. Left valve with two narrow cardinal teeth, a small anterior lateral socket and posterior lateral narrow socket. Anterior adductor muscle scar medium-long, detached for ca. 50 % of length. Posterior scar ovate. Pallial line entire, sometimes partially discontinuous or irregularly lobate. Shell margin crenulate, with crenulations coarser towards anterior and posterior.
Remarks. Although we formerly recorded Rugalucina vietnamica with a broad longitudinal range from the northern Red Sea to southern China (Glover and Taylor 2001) we now regard it as having a narrower range along the continental margin of south eastern Asia with few records from islands. Despite intensive sampling by the MNHN Paris Expeditions around Panglao, Philippines (2004, 2005 and Papua New Guinea (2012Guinea ( , 2014 no R. vietnamica were recorded (Glover and Taylor 2016 and unpublished data). Description. Small, L to 11 mm, sub-circular, posteriorly slightly truncate in juveniles, shallow posterior sulcus, moderately inflated. Shell colour white, internally often yellowish. Sculpture of fine, low commarginal lamellae crossed by low radial ribs which are broader and more prominent towards the anterior and posterior. Radial ribs are slightly fluted where crossed by commarginal lamellae. Lunule elongate, lanceolate, slightly asymmetrical. Ligament internal, short, situated on a broadly triangular resilifer. Hinge: right valve with single, narrow, cardinal tooth, anterior and posterior lateral teeth small; left valve with two narrow cardinal teeth, small anterior lateral and posterior lateral teeth. Anterior adductor muscle scar medium length, detached for ca. 50 % of length. Posterior scar ovate. Pallial line entire, irregularly lobate. Shell margin finely crenulate, with crenulations coarser towards anterior and posterior.
Distribution. Northern and north eastern Australia.
Habitat. Intertidal and shallow subtidal muddy sand, nearshore seagrass and mangrove fringe. Remarks. Rugalucina munda is closely similar in shell morphology to R. vietnamica and R. angela and we previously confounded the species (Glover and Taylor 2001). In shell morphology the differences are subtle; R. munda is more ovoid, the radial ribs are finer, and the inner shell margin is less coarsely crenulated compared to R. angela and R. vietnamica. Furthermore, Glover and Taylor (2001: fig. 16) recorded a distributional gap between Australian and Asian records of 'P. vietnamica'. Features of the anatomy and ctenidial bacteria of R. munda from Port Douglas, Qld were described by Glover and Taylor (2001, as Pillucina vietnamica). Hedley (1913: 267) recommended that the name Lucina munda be rejected as unrecognisable because at that time type material had not been identified and the original description ambiguous. Later, because the incorrect type material had been isolated in the NHMUK collection the name Lucina munda had been regarded (e.g., Lamprell and Whitehead 1992) as a synonym of Ctena bella (Conrad, 1837). More recent recognition of the original type material (JDT & EAG) showed that Lucina (Codakia) munda (syntypes figured here for the first time from the Cuming collection and collected in Moreton Bay by Mr Strange) is a species similar to Rugalucina vietnamica and R. angela. In his description Adams (1856) highlights the dichotomously radiating ribs and the radially grooved inner shell margin and the yellowish shell interior. Rugalucina munda is thus the earliest name for the Australian species.

Rugalucina sensu lato
Here we include two species from the northern Indian Ocean and Red Sea that are similar externally to Rugalucina vietnamica but differ from it and each other in shell sculpture, hinge, and muscle scar characters. No molecular material is available for firmer placement of either species.

Distribution.
Known from the northern Arabian Sea and southern India. Probably more abundant in the northern Indian Ocean but unrecognised.
Remarks. Dekker and Goud (1995: 6) excluded D. cypselis from Divaricella more correctly suggesting placement in Pillucina. Whereas previously we included Divaricella cypselis in the synonymy of Pillucina angela because of the similarity of external sculpture (Glover and Taylor 2001) we now recognise it as a morphologically distinct species. The holotype and other specimens from the Karachi area all have a very short anterior adductor muscle scar (Figs 3, 7I-K) that is barely detached from the pallial line compared with the longer and ventrally detached scar of Rugalucina angela, R. vietnamica and R. munda. It also has a distinctive sculpture of oblique commarginal lamellae set at an angle to the ventral shell margin that is not seen in other Rugalucina. Similar radial ribs and dentate inner shell margin are present in Pusillolucina species, but they have multi-cuspate posterior lateral teeth and lack the oblique commarginal sculpture.
The obliquely inset internal ligament indicates placement in the Loripes group of genera and while the external sculpture resembles Rugalucina in radial ribbing the obliquely aligned commarginal lamellae are similar to that seen in Lucinella divaricata (Linnaeus, 1758) from the eastern Atlantic but that species lacks any radial ribbing. Etymology. cracentis Latin, genitive singular of cracens meaning neat, graceful. Adjective.
Diagnosis. Ovoid shape, slightly higher than long, diverging radial ribs, ligament short, largely internal, hinge with ventral flexure, right valve with single large cardinal tooth, anterior adductor muscle scar short.
Description. Small, L to 9 mm. ovoid, slightly higher than long (H/L 1.01), inflated, umbones prominent, rounded. Colour white or yellowish. Sculpture of diverging radial ribs, coarser and more widely spaced to anterior and posterior, ribs finer and more subdued in middle parts of shell. Ribs crossed by fine, low, closely spaced, commarginal lamellae. Lunule short, heart shaped. Ligament short, obliquely inset. Hinge line with ventral flexure (Fig. 7R, S), right valve with single, relatively large cardinal tooth and small Remarks. Dekker and Orlin (2000) used Pillucina cypselis as a synonym of the preoccupied Lucina concinna but, as shown above, the species differ in external morphology. Although Glover and Taylor (2001) included this species in the synonymy of Pillucina vietnamica (NW Indian Ocean forms now Rugalucina angela) we consider it distinct. Externally, it is similar to Rugalucina angela but differs in the ovoid shape, the finer radial ribs, the much shorter anterior adductor scar, the ventral flexure of the hinge line and large cardinal tooth in the right valve.
The ovoid shape and flexured hinge line with the large cardinal tooth in the right valve are features of Pillucina s. s. but Pillucina species usually have less prominent radial ribbing Taylor 2001, 2016 Diagnosis. Very small, L to 3 mm, sub-circular, higher than long. Umbones prominent. Sculpture of thin commarginal lamellae, elevated to anterior and posterior, crossed by radial ribs that are more prominent to anterior and posterior. Lunule broadly lanceolate, concave. Ligament short, largely internal. Hinge: RV with single cardinal tooth, large anterior lateral tooth and a complex multi-cuspate posterior lateral tooth consisting of up to ten cusps; LV with two cardinal teeth, the anterior larger, posterior lateral tooth formed of sockets for projecting cusps of the right valve. Anterior adductor scar very short, barely detached from pallial line, pallial line irregularly discontinuous. Inner shell margin crenulate.
Etymology. Derived from Latin pusilla meaning very small and Lucina. Feminine Comparison with other genera. Pusillolucina is similar in many external characters including shape and sculpture to some Pillucina (type species Pillucina spaldingi Pilsbry, 1921 = Pillucina hawaiiensis (Smith, 1885)). By comparison no Pillucina species possess the unusual multi-cuspate posterior dentition. Moreover, Pusillolucina pusilla is distinct in molecular analyses ( Fig. 1 and Taylor et al. 2016) from most Pillucina including five common shallow water Indo-West Pacific species but aligns with 'Pillucina vietnamica'. This latter is now placed in a separate genus, Rugalucina (see below), whose species are larger and, significantly, even in minute juveniles, lack the multi-cuspate posterior lateral teeth of P. pusilla and its congeners.
Distribution. Indo-West Pacific, low intertidal to 70 m.

Pusillolucina pusilla Glover & Taylor, 2016 Figs 8 A-F
Pillucina pusilla Glover and Taylor  Description. Very small, glossy, L to 1.8 mm, sub-circular, higher than long, inflated. Umbones prominent. Sulci and dorsal areas poorly defined. Sculpture of thin commarginal lamellae elevated to anterior and posterior, with 17-25 low, rounded radial ribs, divaricate in anterior part of shell. Juvenile shell with elevated commarginal lamellae but no radial ribs. Microsculpture of fine growth increments only. Protoconch P1 +P 2 = 134 µm, P2 with numerous growth increments. Lunule broadly lanceolate, depressed, asymmetric, larger part in left valve. Ligament short, internal, oriented Etymology. arabica from Latin arabicus. Used as an adjective. Diagnosis. Pusillolucina with posterior lateral teeth divided into four or five cusps and sockets.
Description. Shell very small, L to 2.4 mm, ovate, umbones prominent, sculpture of closely spaced, narrow, commarginal lamellae, sometimes elevated at posterior and  anterior dorsal margins, crossed at anterior and posterior by low radial ribs, juvenile shells with commarginal lamellae only. Colour: white, translucent when wet. Protoconch: P1 84 µm, P1 + P2 = 155 µm, P2 with numerous growth increments. Lunule broadly lanceolate, smooth. Ligament internal, short, set alongside cardinal teeth. Hinge: right valve with single cardinal tooth, anterior lateral tooth located above anterior adductor muscle. Posterior lateral tooth long, divided into four or five cusps, left valve with two cardinal teeth, the anterior larger, anterior lateral tooth small, posterior lateral tooth divided into four or five sockets for cusps of right valve. Anterior adductor muscle scar short, barely detached from pallial line, posterior scar ovoid. Inner shell margin crenulate, more strongly to anterior and posterior.
Remarks. Pusillolucina arabica differs from the Philippine P. pusilla by the more ovate outline, and the less prominent commarginal and radial sculpture. By comparison, P. africana has finer more closely spaced commarginal sculpture and more cusps (7-8) on the posterior lateral tooth. From South Africa P. denticula has much finer commarginal sculpture and up to 10 smaller cusps on the posterior lateral tooth. From Madagascar P. biritika sp. nov. has only three cusps on the posterior lateral tooth.
Despite its small size, P. arabica has the characteristic, thick inner ctenidial demibranchs with a well-developed bacteriocyte zone packed with symbiotic bacteria ca 3-4 µm (Fig. 10N, O) similar to other lucinids. Etymology. Named for Africa, used as an adjective. Diagnosis. Pusillolucina with posterior lateral teeth divided into seven or eight cusps and sockets.

Pusillolucina africana
Description. Shell very small, L to 2.4 mm, ovate, umbones prominent, sculpture of closely spaced, narrow, commarginal lamellae, sometimes slightly elevated at posterior and anterior dorsal margins, crossed at anterior and posterior by low radial ribs, juvenile shells with commarginal lamellae only. Protoconch: P1 ca 75 µm, P1 + P2 = 140 µm, P2 with numerous growth increments. Lunule long, broadly lanceolate, smooth. Ligament internal, short, set on triangular resilifer alongside cardinal teeth. Hinge: right valve with single cardinal tooth, anterior lateral tooth located above anterior adductor muscle. Posterior lateral tooth long, divided into seven or eight cusps, left valve with two cardinal teeth, the anterior larger, anterior lateral tooth small, posterior lateral tooth with seven or eight sockets for cusps of right valve. Anterior adductor muscle scar short, barely detached from pallial line, posterior scar ovoid. Pallial line continuous. Inner shell margin crenulate, more strongly to anterior.
Remarks. For comparison with other species see P. arabica above. Etymology. Biritika, meaning extremely small in Malagasy. Used as a noun in apposition.
Diagnosis. Pusillolucina with posterior lateral teeth divided into three cusps and sockets.
Description. Very small, L 1.5 mm, sub-ovate, longer than high, umbones prominent. Sculpture of fine commarginal lamellae crossed at anterior and posterior by low, rounded, radial ribs, juvenile shells with commarginal lamellae only. Some lamellae extended as scales along posterior dorsal margin. Lunule long, lanceolate. Ligament short internal on short resilifer. Hinge: RV with single cardinal tooth and single anterior lateral tooth, posterior lateral tooth short, divided into three cusps, LV with two cardinal teeth the posterior-most thin, posterior lateral tooth with three sockets for cusps of the right valve. Anterior adductor muscle scar short, barely detached from pallial line, posterior scar ovate. Pallial line entire. Shell margin coarsely crenulate, more strongly to anterior and posterior.
Diagnosis. Small (L to 3 mm), sub-ovoid, slightly longer than high. Umbones prominent. Posterior dorsal margin straight. Sculpture of low, rounded, commarginal lamellae with poorly defined fine radial ribs to anterior and posterior. Microsculpture densely punctate. Ligament short, protrudes above dorsal margin. Lunule broadly lanceolate. Hinge line narrow, left valve with two small cardinal teeth and anterior and posterior lateral teeth. Right valve with single, slightly bifid cardinal tooth and anterior and posterior lateral teeth. Anterior adductor muscle scar short, slightly detached from pallial line, inner shell margin finely denticulate. Etymology. notos in Greek meaning south, -cina as an abbreviation of Lucina. In reference to the southern Atlantic distribution. Female gender.
Remarks. Dell (1964) placed N. falklandica in Epicodakia (type species E. consettiana Iredale, 1930) because of some similarity of hinge teeth and sculpture. He remarked that only two species of Epicodakia were known (E. consettiana Iredale, 1930 and E. neozelanica Powell, 1937) but since then a number of other species from the Indo-West Pacific, Western Atlantic and Eastern Pacific have been classified in the genus (Glover and Taylor 2007;Taylor et al. 2016;MolluscaBase). Molecular analyses place Epicodakia in the subfamily Codakiinae, close to Ctena species    from the Philippines and we are aware of further undescribed species. There is no close similarity of Troendleina to E. falklandica in shell characters: species of the former are larger with lengths of 30-40 mm, a sculpture of growth increments with fine radial threads, small cardinal teeth, obscure or absent lateral teeth, and a finely dentate inner shell margin. Notocina falklandica has some similarity to Parvilucina species in size and shell characters but is not closely aligned in molecular trees. Another unusual small species Guyanella clenchi from the southern Caribbean ) is similar to Notocina falklandica in size but differs in having a distinctive internal ligament and lacks any radial sculpture. Neither Troendleina, Parvilucina nor Guyanella species possess the punctate microsculpture that occurs in N. falklandica and sporadically amongst other Lucinidae including species of Myrteinae, some Ctena, Epicodakia and Codakia species (Codakiinae) and Funafutia levukana (Lucininae) . (Dell, 1964) Fig. 13 Epicodakia falklandica Dell, 1964: 206, fig. 4 (17, 18, 19 Description. Shells small, L 2-3 mm, ovoid, slightly longer than high. Umbones prominent. Posterior dorsal margin straight. Sculpture of low rounded commarginal lamellae with narrow interspaces. Faint radial ribs visible to anterior and posterior. Microsculpture of dense fine punctae (2-3 µm in diameter). Protoconch: P1 = 188 µm, P1 + P2 = 242 µm, P2 with fine growth increments (South Georgia shell). Ligament short, protruding above posterior dorsal margin (Fig. 13J), set in shallow nymph. Escutcheon well defined, smooth. Lunule slightly impressed, broadly lanceolate. Hinge line narrow, left valve with two small cardinal teeth and anterior and posterior lateral teeth. Right valve with single, slightly bifid cardinal tooth and anterior and posterior lateral teeth. Anterior adductor muscle scar short, barely detached from pallial line, posterior adductor scar ovoid. Pallial line entire. Inner shell margin finely denticulate.

Discussion
There is a general perception that bivalves having chemosymbiosis with thiotrophic or methanotrophic bacteria are large, as exemplified by Vesicomyidae (Calyptogena magnifica Boss & Turner, 1980, 263 mm), Modiolinae (Bathymodiolus boomerang Cosel & Olu, 1998, 360 mm), Solemyidae (Acharax bartschi (Dall, 1908), 210 mm) that live at hydrothermal vents and hydrocarbon seeps. Among the several bivalve families with chemosymbiotic life habits Lucinidae is by far the most diverse with more than 400 species occupying a wide range of habitats from the intertidal to bathyal depths and spanning a wide size range from 1.5 to 170 mm. The largest living species are Meganodontia acetabulum Bouchet & Cosel, 2004 with shell lengths up to 170 mm and Codakia distinguenda (Tryon, 1872) at 160 mm, while the Eocene fossil, Superlucina megameris (Dall, 1901), attained a shell height of 310 mm (Taylor and Glover 2009). Nevertheless, most species of Lucinidae are much smaller, with recent studies revealing minute species as small as 1.5 mm Taylor 2016, Taylor et al. 2016). Additionally, with availability of molecular phylogenies, paraphyly is increasingly being recognised in genera and families previously classified by morphological characters. Such paraphyly has been found in the small lucinids previously assigned to Pillucina; one group consists of several species similar to the genotype of Pillucina and the other includes species some previously neglected but now assigned to the new genera, Rugalucina and Pusillolucina, that are also distinctive in morphology.
One of the foci of this paper has been the recognition of some very small lucinid species within the new genus Pusillolucina with adult shell lengths of 1-3 mm. All the species in this genus have a very unusual multi-cuspate lateral dentition not seen in any other lucinids. The recognition of these minute lucinids extends the morphological and functional range of Lucinidae. Even the smallest have chemosymbiosis with thickened ctenidial demibranchs occupied by symbiotic bacteria. Other minute lucinids include Guyanella clenchi (Altena, 1968) 1-2 mm and Parvilucina latens Taylor & Glover, 2016, 2-3 mm, both documented from Guadeloupe but likely occur more widely in the western Atlantic ). The latter species was first detected in molecular analyses having initially been confounded with Parvilucina pectinella (C.B. Adams, 1852). Additionally, most species of Liralucina, have shell lengths less than 5 mm (Glover and Taylor 2007), they are little recorded but common in some coral reef associated habitats (Glover and Taylor 2007).
Another small species that has been phylogenetically misplaced is Notocina falklandica widely recorded from outer shelf and bathyal depths around the Falkland Islands, Argentina and Uruguay. Since the original description by Dell (1964) its placement in Epicodakia (Codakiinae) had been problematic but molecular data now shows that it should be classified in the subfamily Lucininae in a monospecific genus close to Troendleina from the Indo-West Pacific with no comparable taxa known from the southern Atlantic.
Two species described here, Rugalucina angela and 'R.' cracentis, are abundant in the northern Red Sea where a surprising number of other lucinids have been recorded. From intensive sampling around Safaga Bay (Zuschin and Oliver 2003;2005) lucinids were amongst the most abundant bivalves with 15 species comprising nearly 40 % of the shells recovered, most frequently Cardiolucina semperiana (Issel, 1869), Wallucina erythraea (Issel, 1869) and 'Pillucina fischeriana'. With taxonomic refinements 19 species are now recognised from the Safaga area (Taylor and Glover 2005, herein). The northern Red Sea is a highly oligotrophic environment and, with a nutritional strategy of chemosymbiosis utilising sulphides from the benthic substrate, lucinids can survive in nutrient poor habitats where food availability for suspension feeding bivalves is limited.