Lucinid Bivalces, Lucinidae, are marine Bivalves which lack
syphons, but which have an elongate foot with a long channel that serves
the same purpose. All modern Lucinids host colonies of symbiotic
sulphur-oxidising Bacteria in their gills, which enable them to colonise
and thrive in sulphur-rich subtidal sediments. The first Lucinids
appeared in the Silurian, though they remained a minor constituent of
Bivalve faunas until the Late Cretaceous, when they underwent a major
radiation at roughly the same time as Mangroves and Seagrass meadows
appeared, which may have been when they acquired their symbiont
Bacteria. Lucinids also form a significant part of the communities that
live around hydrocarbon seeps (areas where hydrocarbons escape into the
sea from exposed deposits). During the last 20 years there has been a marked proliferation of generic categories within the Lucinidae building on and revising prior classifications. 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.
In a paper published in the journal ZooKeys on 12 December 2019, John Taylor and Emily Glover of the Department of Life Sciences at The Natural History Museum, present a review of the taxonomy of small Lucinid Bivalves, in which several new species are described and others redescribed and reclassified.
Taylor and Glover firstly erect a new genus, Rugalucina, to include species formerly included in the genus Pillucina, which they consider to be paraphyletic (i.e. the genus does not include all the decendents of the most recent common ancestor of the species within it). The name 'Rugalucina' derives from 'Ruga', meaning a crease or fold, plus 'Lucina' the first described species within the Lucinidae, and the genus which gives the group its name, in reference to the crinkled appearance of the shells.
The first species assigned to this new genus is Rugalucina angela. This species comprises populations from the Red Sea coasts of Egypt and Yemen, the Persian Gulf coasts of Kuwait, Qatar, and the United Arab Emirates, and the Indian Ocean coasts of Oman, Pakistan, India and Sri Lanka, which were previously assigned to Pillucina angela. These Bivalves reach a maximum size of about 15 mm in diameter and have white, yellow or orange shells with a waxy appearance and strong ornamentation.
Rugalucina angela. (A)–(C) Exterior of left valve and interiors of right and left valves. Gwadur, Pakistan, length 8.1mm. (D)–(F) Exterior of left valve and interiors of right and left valves, length 6.1 mm. (G)–(M) Ras al Khaimah, Arabian Gulf, (G) exterior SEM of right valve, length 5.0 mm, (H), (I) interior SEM of right and left valves, length 7.7 mm, (J) dorsal view, length 5.9 mm, (K) exterior of left valve, length 7.9 mm, (M) exterior of left valve, interior of right valve, length 7.6 mm. (N), (O) Exterior and interior of left valve, Gulf of Suez, length 13.7 mm. (P) Exterior of right valve and interior of right and left valves, Egypt, 7 km south of Hurgada, length 12.8 mm. (Q) Interior of left and right valves, Egypt, Port Safaga, length 9.4 mm. (R), (S) Detail of hinge teeth of (Q). (T) Exterior of left valve. Red Sea, Yemen, Orestes Point, north of Midi, length 9.8 mm. (U) Exterior of right and interior of left valves, Aden, length 10.6 mm. (V) Exterior and interior of right valve, Krusadai, India, length 8.4 mm. Taylot & Glover (2019).
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. Taylor and Glover 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 (pratical salinity units) but in shallow lagoons as high as 52–55 psu, and approximately 40–46 psu at Safaga in the northern Red Sea, and probably higher for the habitats occupied by Rugalucina angela.
The species Pillucina vietnamica is also re-assigned to Rugalucina, becoming Rugalucina vietnamica. This species is found in intertidal to shallow sub-tidal seagrass and muddy habitats on the coasts of Singapore, Peninsula Malaysia, Thailand, Cambodia, Vietnam and south China. The species was formerly thought to extend as far west as the Red Sea, but Taylor and Glover regard these western populations as probably belonging to Rugalucina angela. The species may also extend further to the east, possibly as far as Papua New Guinea.
Rugalucina vietnamica. (A)–(C) Exterior of right valve and interior of left and right valves, Hainan, China, length 5.5–8.9 mm. (D) Internal drawing of (C). (E), (F) Palau Semaku, Singapore, length 8.5 mm. (G)–(L) Kungkraben Bay, eastern Thailand, (G) exterior of right valve, length 7.0 mm. (H) exterior of left valve, length 7.0 mm (I), (J) interior of left and right valves, length 7.7 mm, (K) dorsal view, length 5.6 mm (L), (M) detail of hinge of (I) and (J). (N) Section through gill of Rugalucina vietnamica, Kungkraben Bay, Thailand showing filaments with ciliary (cz) and bacteriocyte zones (bz). Critical-point dried preparation. (O) Detail showing abundant Bacterial symbionts in bacteriocytes. Scale bars: 20 μm (N); 10 μm (O). Taylor & Glover (2019).
The third species assigned to the new genus is Rugalucina munda, formerly assumed to be the same species as the populations now assigned to Rugalucina vietnamica, which is restricted to intertidal and shallow subtidal muddy sands, nearshore seagrass beds and mangrove fringes in northern and north eastern Australia. This species has a subcirular, moderately inflated, white shell, with a yellowish interior and fine sculpture which includes low commarginal lamellae crossed by low, slightly fluted radial ribs
Rugalucina munda. (A)–(H) Moreton Bay, Queensland, Australia, (A)–(C) exterior and interior of left and right valves, length 11.3 mm, (D), (E) exterior of left valve and dorsal view, length 11.2 mm, (F)–(H) exterior of left valve and interior of left and right valves, length 10.4 mm. (I), (J) Rugalucina munda exterior of left valve and interior of right valve, Redland Bay, Moreton Bay, Queensland, length 6.8 mm. (K)–(N) Rugalucina munda Tin Can Bay, Queensland, (K) exterior left valve, length 6.9 mm, (L) interior of right valve, length 6.3 mm, (M) interior of left valve, length 5.5 mm, (N) exterior of left valve, length 6.7 mm. (O)–(S) Rugalucina munda Port Douglas, Queensland, (O) exterior of right valve, length 6.5 mm, (P) exterior of left valve, length 4.8 mm, (Q), (R) interior of right and left valves, L 7.2 mm S dorsal view, L 5.5 mm. Taylor & Glover (2019).
Two other species are assigned to Rugalucina on the basis of their morphology, although no genetic material is available for these species, so Taylow and Glover make these designations with caution.
The first of these is Rugalucina cypselis, formerly Divaricella cypselis, from the coasts of Pakistan and western India. This species is small, with a maximum shell length of about 5.2 mm. The shells are sub-circular and inflated, white or yellowish in colour, and have a sculpture of diverging, curved, radial ribs prominent to anterior and posterior are subdued or absent in middle parts of shell. These ribs are crossed by closely spaced, narrow, low, commarginal lamellae that are aligned obliquely to the ventral shell margin.
Rugalucina cypselis. (A)–(D) Karachi, length 5.2 mm. (E)–(L) Rugalucina cypselis, Karachi, Winckworth collection, (E) right valve, length 3.1 mm, ob – oblique commarginal lamellae, (F) left valve, length 3.0 mm, (G) right valve, length 3.2 mm, (H) dorsal view, length 1.9 mm, (I) interior left valve, length 3.2 mm. (J) interior right valve, length 3.2 mm, (K) interior left valve, length 3.0 mm, (L) protoconch. (M) Detail of ventral margin of (F) showing obliquely aligned commarginal lamellae. Scale bars are 100 μm in (L) and 1 mm in (M). Taylor & Glover (2019).
The second of these species without genetic material is Rugalucina cracentis, a new species created to include populations from the Red Sea coasts of Egypt and Yemen and Arabian Gulf coasts of Kuwait and Saudi Arabia, which were formerly thought to be the same species as the populations now assigned to Rugalucina vietnamica; the name 'cracentis' means 'graceful' in Latin. This species reaches about 9 mm in length, with an ovoid shell wider than it it long. The shells are white or yellowish in colour, and inflated with prominent umbones. They have a sculpture of diverging radial ribs, coarser and more widely spaced to anterior and posterior, and finer and more subdued in middle parts of shell. The ribs crossed by fine, low, closely spaced, commarginal lamellae.
Rugalucina cracentis. (A)–(E) Gulf of Suez, exterior and interior of right and left valves and dorsal view, length 9.2 mm. (F)–(H) Rugalucina cracentis exterior of right valve and interior of right and left valves Egypt, Dahab, Gulf of Aqaba, Red Sea, (H) length 6.7 mm. (I), (J) Rugalucina cracentis exterior and interior of right valve Dahab, Gulf of Aqaba, Red Sea, length 8.5 mm. (K), (L) Rugalucina cracentis exterior and interior of left valve Egypt, 4 km north of Port Safaga, length 8.3 mm. (M) Interior of right valve, length 8.8 mm. (N), (O) Exterior and interior left valve Egypt, 4 km north of Port Safaga, length 7.1 mm. (P), (Q) interiors of right and left valves, length 8.7 mm. (R), (S) hinge details of (P) and (Q). Taylor & Glover (2019).
Taylor and Glover erect a second new genus, Pusillolucina, also to include species formerly included in Pillucina. The name 'Pusillolucina' means 'very small Lucina'; the genus includes very small species. less than 3 mm in total length, which are sub-circular, and higher than they are long.
The first species included in this genus is Pusillolucina pusilla, formerly Pillucina pusilla, which is known only from Bohol Island in the Philippines. This species is very small, reaching a maximum length of about 1.8 mm, with a glossy shell with a sculpture of thin commarginal lamellae elevated to the anterior and posterior, with 17–25 low, rounded radial ribs, divaricate in anterior part of shell.
(A)–(D) Pusillolucina pusilla, (A) Philippines, Bohol Island, length 1.2 mm, (B) dorsal view, (C), (D) interior of left and right valves, length 1.05 mm, (E), (F) detail of hinge teeth in (C), (D). Taylor & Glover (2019).
The second species included in the genus is Pusillolucina denticula, formerly Pillucina denticula, which is known only from Durban Bay in South Africa. This species has a small shell with a maximum length of 3.5 mm. These shells are sub-circular to ovoid in outline, with a sculpture of fine, closely spaced, commarginal lamellae crossed by low, rounded radial ribs that are prominent and broader towards anterior and posterior and inconspicuous in central part of shell.
Pusillolucina denticula Durban Bay, South Africa; (G) exterior of right valve, length 2.9 mm, (H), (I) interior of left and right valves, length 3.5 mm (J), (K) detail of hinge teeth. Taylor & Glover (2019).
The third species assigned to this genus is Pusillolucina arabica, a new species based upon previously undescribed specimens dredged from Tarut Bay on the Arabian Gulf coast of Saudi Arabia. This species is small, with a maximum length of 2.4 mm, and an ovate shell with a sculpture of closely spaced, narrow, commarginal lamellae, sometimes elevated at the posterior and anterior dorsal margins, crossed at anterior and posterior by low radial ribs. The shells are white, and translucent when wet.
Pusillolucina arabica. Tarut Bay, Saudi Arabia, Arabian Gulf. (A)–(C) Exterior of right valve and interior of left and right valves, length 2.4 mm. (D), (E) interior of left and right valves, SEMs. (F)–(H) Exterior of right valve and interior of left and right valves, length 2.1 mm. (I), (J) Interior of left and right valves SEM. (K), (L) Exterior of left and right valves, length 2.0 mm. (M) Dorsal view, length 1.9 mm. (N) Protoconch of (D), scale bar 50 μm. (O), (P) Detail of multi-cuspate posterior lateral hinge teeth. Scale bar is 200 μm. Taylor & Glover (2019).
A second new species Pusillolucina africana, is included in this genus, described from previously undescribed specimens from Inhaca Island, Mozambique. This species reaches a maximum length of 2.4 mm, and has an ovate shell with a sculpture of closely spaced, narrow, commarginal lamellae, sometimes slightly elevated at the posterior and anterior dorsal margins, crossed at the anterior and posterior by low radial ribs.
Pusillolucina africana. Mozambique, Inhaca Island, Baia Campessuane, 3–4 m. (A) Length 2.3 mm. (B) Exterior left valve, length 2.1 mm. (C) Interior of left valve, length 2.1 mm. (D) Interior right valve, length 1.9 mm. (E) Exterior right valve, length 2.2 mm. (F) Exterior left valve, length 2.2 mm. (G) Exterior of left valve, length 2.2 mm. (H) Dorsal view, length 2.4 mm. (I) Interior of left valve, length 2.1 mm. (J) Interior of right valve, length 2.1 mm. (K) Interior of right valve, length 2.1 mm. (L)–(N) Detail of hinges of (I), (J), (K). (O) Protoconch. Scale bar is 50 μm. Taylor & Glover (2019).
A third new species is added to the genus, based upon previously undescribed species from Madagascar. and given the name Pusillolucina biritika, which means 'small' in Malagasy. This species reaches a maximum size of 1.5 mm, with sub-ovate shells, longer than high, with a sculpture of fine commarginal lamellae crossed at anterior and posterior by low, rounded, radial ribs. Some lamellae extended as scales along the posterior dorsal margin.
Pusillolucina biritika. (A), (B) South Madagascar east of Cap Antsirabe, 49–52 m, (C)–(E), (L), (M) South Madagascar off Baie Fort-Dauphin 54–56 m, (F)–(K) Northwest Madagascar, south of Cap St Sébastien, 42–44 m. (A) Length 1.1 mm. (B) Length 1.5 mm. (C) Exterior of left valve, length 1.3 mm. (D) Interior of right valve, length 1.3 mm. (E) Exterior of left valve, length 1.5 mm. (F) Exterior of right valve length 1.4 mm. (G) Exterior of right valve, length 1.1 mm. (H) Interior of left valve, length 1.4 mm. (I) Interior of right valve, length 1.3 mm. (J), (K) Detail of hinge teeth of (H) and (I). (L), (M) Exterior of left valve and interior of right valve, length 1.2 mm. Taylor & Glover (2019).
Finally Taylor and Glover create a new genus, Notocina, to incorporate the species Epicodakia falklandica (now Notocina falklandica), which is found around the Falkland, South Georgia and South Orkney islands. as well as on the continental shelf off Patagonia and Buenos Aires. This species reaches 2-3 mm in length and has an ovoid shell, slightly longer than high, with a sculpture of low rounded commarginal lamellae with narrow interspaces. Faint radial ribs are visible to the anterior and posterior. The is a microsculpture of dense fine punctae (2–3 μm in diameter).
Notocina falklandica. (A)–(D) Exterior and interior views of both valves, length 2.5 mm. (E)–(J) Falkland Islands, 208 m, (E) exterior of left valve, length 1.9 mm, (F), (G) interior of left and right valves, length 2.3 mm, (H) dorsal view, length 1.9 mm, (I) protoconch, arrow indicates boundary of P1 and P2, (J) detail of (E) showing punctate microsculpture. Scale bars are 100 μm in (I) and 20 μm in (J). Taylor & Glover (2019).
There is a general perception that bivalves having chemosymbiosis with thiotrophic or methanotrophic bacteria are large species 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 with shell lengths up to 170 mm and Codakia distinguenda at 160 mm, while the Eocene fossil, Superlucina megameris, attained a shell height of 310 mm. Nevertheless, most species of Lucinidae are much smaller, with recent studies revealing minute species as small as 1.5 mm.
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