Research Article |
Corresponding author: P. Graham Oliver ( graham.oliver@museumwales.ac.uk ) Academic editor: Richard Willan
© 2018 P. Graham Oliver, Anders Hallan, P.R. Jayachandran, Philomina Joseph, V.F. Sanu, S. Bijoy Nandan.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Oliver PG, Hallan A, Jayachandran PR, Joseph P, Sanu VF, Bijoy Nandan S (2018) Taxonomy of myid bivalves from fragmented brackish-water habitats in India, with a description of a new genus Indosphenia (Myidae, Myoidea, Myidae). ZooKeys 799: 21-46. https://doi.org/10.3897/zookeys.799.25843
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A group of small bivalves inhabiting Indian brackish-water estuaries and lagoons (known locally as backwaters), variously assigned to Corbula, Cuspidaria, and Sphenia, are reviewed and, based on shell characters, shown to be congeneric. Molecular (COI) and morphological data indicate that this group belongs to the family Myidae. Furthermore, the combined data suggest that these Indian myids are a sister taxon of the genus Sphenia. The Indian material studied herein exhibits a functional morphology typical of infaunal bivalves, whereas typical Sphenia are nestling and epibyssate. A new genus, Indosphenia, is thus erected for the Indian group and includes five species, one of which is named in this study.
Indosphenia kayalum Oliver, Hallan & Jayachandran, gen. et sp. n. is described from the Cochin Backwater on the western coast of India. Cuneocorbula cochinensis (Preston, 1916) is transferred to Indosphenia. Additionally, the west coast taxa I. abbreviata (Preston, 1907), I. abbreviata chilkaensis (Preston, 1911) and I. sowerbyi (EA Smith, 1893) are recognised herein. Corbula alcocki Preston, 1907, Corbula gracilis Preston, 1907, Corbula calcaria Preston, 1907 and Corbula pfefferi Preston, 1907 are placed in synonymy with I. abbreviata, and Cuspidaria annandalei Preston, 1915 is synonymised with I. abbreviata chilkaensis.
Anatomy, Bivalvia , brackish waters, COI, India, morphology, taxonomy
Extensive estuarine and lagoon systems are found around both the eastern and western coasts of India (Fig.
List of species considered in this paper recorded from the Indian subcontinent.
Original combination | Name currently used in MolluscaBase (2018) | Type locality |
---|---|---|
Sphenia perversa (Blanford, 1867) | Sphenia perversa (Blanford, 1867) | Pegu, Irawady Delta, Myanmar |
Sphenia sowerbyi EA Smith, 1893 | Sphenia sowerbyi EA Smith, 1893 | Pondicherry |
Corbula abbreviata Preston, 1907 | Potamocorbula abbreviata (Preston, 1907) | Port Canning |
Corbula alcocki Preston, 1907 | Potamocorbula alcocki (Preston, 1907) | Port Canning |
Corbula calcaria Preston, 1907 | Potamocorbula abbreviata (Preston, 1907) | Port Canning |
Corbula gracilis Preston, 1907 | Potamocorbula abbreviata (Preston, 1907) | Port Canning |
Corbula pfefferi Preston, 1907 | Potamocorbula abbreviata (Preston, 1907) | Port Canning |
Corbula chilkaensis Preston, 1911 | Potamocorbula chilkaensis (Preston, 1911) | Lake Chilka |
Cuspidaria annandalei Preston, 1915 | Cuspidaria annandalei Preston, 1915 | Lake Chilka |
Cuspidaria cochinensis Preston, 1916 | Cuneocorbula cochinensis (Preston, 1916) | Cochin Backwater |
Bearing this taxonomic background in mind, in 2016, one of the present authors (Philomina Joseph) collected a sample of fragile, thin-shelled bivalves living among filamentous green algae, on a muddy substrate, in the upper brackish regions of the Cochin Backwater. This bivalve could not be identified from the available literature including the latest book on Indian bivalves by
Two other species of Myidae have been recorded from Indian estuaries; Sphenia perversa was recorded from various localities on the coast of the Bay of Bengal by
Very little original material exists for those taxa described by
Specimens for molecular examination were preserved in 100% ethanol. The ethanol- preserved samples were re-hydrated in sterile distilled water for 10–12 hours at ambient room temperature prior to DNA extraction. Genomic DNA was extracted from macerated muscle tissue using the DNeasy Blood & Tissue Kit (Qiagen) following the spin column protocol. The polymerase chain reaction (PCR) mixture consisted of 25 μL Master Mix (Takara Clontech EmeraldAmp® GT PCR Master Mix), 1 μL forward primer, 1 μL reverse primer, 8 μL template DNA, and 15 μL distilled deionised water. The amplification primers were LCO-1490 F (5'- GGTCAACAAATCATAAAGATATTGG-3') and HCO-2198 R (5'-TAAACTTCAGGGTGACCAAAAAATCA-3'), used for amplifying mitochondrial cytochrome c oxidase subunit I (mtCOI) gene sequences (
Amplification was carried out in an Agilent thermal cycler (Sure cycler 8800). The amplification protocol followed a sequence of denaturation at 94 °C for 1 min., annealing at 37 °C for 2 mins and extension at 72 °C for 3 mins; 40 cycles were performed. Amplified products exhibiting distinct bands after agarose gel (1.2%) electrophoresis were purified and sent to SciGenom Labs (SciGenom Labs Pvt, Ltd. Ernakulam, India) for sequencing.
For the analysis, only the forward primer sequences were used. Sequences with a product length of 614–625 base pairs were obtained without any gaps or stop codons. The sequences thus obtained were assembled using BioEdit 7.0.9 (
BioEdit v7.0.9 (
MEGA7 was used also to conduct a Maximum Likelihood analysis with 1000 bootstrap repetitions. In all, six myid species plus three outgroup taxa (two corbulids plus the venerid Circe scripta [Linnaeus, 1758]) were chosen for the molecular analysis. GenBank accession vouchers for these outgroup taxa are shown in the phylogram. The phylogram was generated using MEGA7. Sequences have been submitted to GenBank and their identification codes are attached to the phylogram in Fig.
Phylogram showing Maximum Likelihood method based on the Hasegawa-Kishino-Yano model. The tree with the highest log likelihood (- 2576.44) is shown. The percentage of trees in which the associated taxa clustered together is shown next to the branches where such values exceed 95%. Initial tree(s) for the heuristic search were obtained automatically by applying Neighbor-Join and BioNJ algorithms to a matrix of pairwise distances estimated using the Maximum Composite Likelihood (MCL) approach, and then selecting the topology with superior log likelihood value. Grey area indicates Myidae.
Specimens were examined under a Leica MZ12 stereomicroscope and photographed with a Leica Z6 macroscope with Helicon Focus stacking software. Anatomical observations were made on formaldehyde-fixed specimens using the above microscopes; some specimens were stained in methylene blue to enhance contrasts, whereas some were fixed in Bouin’s solution. For scanning electron microscopy, specimens were dried and gold-coated prior to observation with a Jeol Neoscope. The shells were measured with an eyepiece graticule. The statistical analyses were applied from JMP™ statistical software.
Details of the specimens described in this study are given below for each taxon. Type material in the Zoological Survey of India was not studied directly because this institute will not lend material, and funds were not available to visit Kolkata. Syntypes of some species were available in British museums and original and subsequent illustrations were used. Attempts to borrow the material described as Sphenia sowerbyi by
Sphenia perversa 6 specimens, Kungkraben Bay, Thailand, Coll PG Oliver. 3 valves, Bombay, ex ME Deakin Coll., NHMUK1909.9.23.306-8.
Sphenia binghami 4 specimens, Pwlldu Bay, South Wales, coll. PG Oliver; many shells from Tenby and Weymouth.
Sphenia rueppelli 1 shell, Yemen, Red Sea,
Sphenia cf. rueppelli 12 shells, Karachi,
Mya truncata juvenile specimens from the British Isles,
Institutional abbreviations:
The evolutionary history was inferred by using the Maximum Likelihood method based on the Hasegawa-Kishino-Yano model (
Although based solely on a single gene and with a limited dataset, our phylogenetic analysis strongly suggests that the new species belongs in the family Myidae, thus corroborating the morphological evidence. We note that the bootstrap support throughout the phylogram is not strong overall (only two branches are statistically significant, see Fig.
The hinge morphology of the five species examined in this study (Sphenia sowerbyi, Corbula alcocki, Corbula gracilis, Cuspidaria annandalei and the Cochin Backwater sample) is consistent with that of the Myidae, notably with Mya arenaria, M. truncata, and Sphenia binghami.
Scanning electron micrographs of hinges of myids and Potamocorbula. Left hand series are right valves, left hand series are left valves with the chondrophore viewed from above. a Mya arenaria b Mya truncata c Sphenia binghami d Indosphenia kayalum e Potamocorbula amurensis. Scale bar: 500 µm. Abbreviations: as, anterior socket. at, anterior tooth. fl, posterior flange. lg, ligament plate. ps, posterior socket. pst, pseudotooth. r, ridge. rs, resilifer.
Of available myid genera, Sphenia is the most similar, and indeed EA
Given that we have both morphological and molecular distinctions between Sphenia sensu stricto and the endobyssate taxon, we describe the latter as a new genus, Indosphenia.
(here designated) Indosphenia kayalum sp. n.
Sphenia sowerbyi EA Smith, 1893; Corbula alcocki Preston, 1907; Corbula gracilis Preston, 1907; Cuspidaria annandalei Preston, 1915; Cuspidaria cochinensis Preston, 1916.
By inference from descriptions Corbula abbreviata Preston, 1907; Corbula calcaria Preston, 1907; Corbula pfefferi Preston, 1907; Corbula chilkaensis Preston, 1911.
Slightly inequivalve, left valve smaller than right valve, almost equilateral to posteriorly extended, rather inflated. Outline subovate, anterior end broadly rounded; posterior narrowed, sub-rostrate. Sculpture of commarginal lines and very thin, weak lamellae; rostrum with a defined keel, at least in early growth stages. Pallial sinus very shallow, adductor muscle scars subequal, posterior scar subcircular, anterior scar elongate. Right valve with sub-umbonal, depressed resilifer accommodating chondrophore from left valve. Anteriorly, small projecting pseudo-tooth appears as extension of anterior margin (Fig.
In larger specimens, anterior tooth on right valve can be eroded and scarcely visible, chondrophore can project further and flange can be reduced. Mantle edge fused except for pedal gape and short paired fused siphons. Mantle patterned with darkly pigmented radiating blotches. Gills with both demibranchs. Labial palps small. Byssus of very fine threads, but not observed in all species.
Indosphenia - combining the taxon provenance (India) with the related genus Sphenia. Gender feminine.
The molecular and morphological data strongly suggest that Indosphenia is a member of the Myidae.
The structure of the hinge of Indosphenia is generally identical to that of Sphenia sensu stricto as represented by its type species S. binghami, and its Indian Ocean counterpart S. perversa. However, this structure is also very similar to that seen in the juveniles of both Mya arenaria and Mya truncata. Consequently, hinge structure would not appear to be a useful character at the generic level. Instead, it suggests that Sphenia represents a neotenous retention of the juvenile byssate characters of Mya.
Compared to the deep burrowing habits of Mya, Sphenia exhibits a byssate nestling habit, which is manifested in their differing characters; typical Sphenia (Fig.
Indosphenia differs from Sphenia in being almost equilateral with a narrow, almost rostrate, posterior end and a very short pallial sinus. This reflects the infaunal lifestyle of I. kayalum. Some specimens of I. sowerbyi have the posterior end encrusted with epifauna while the anterior end is relatively clean, suggesting that this species lives in sediment with the anterior end at, or close to, the sediment surface.
Corbula
abbreviata
Preston, 1907: 215, fig. 1;
Corbula alcocki Preston, 1907: 215, fig. 2.
Corbula
calcarea
Preston, 1907: 216, fig. 3;
Corbula
gracilis
Preston, 1907: 216, fig. 4;
Corbula pfefferi Preston, 1907: 216, fig. 5.
Potamocorbula
abbreviata
(Preston):
Corbula alcocki, Syntype, 1 shell, Port Canning, purchased from Preston, HMUK1909.8.18.33.
Corbula gracilis, Syntype, 1 shell, Port Canning, purchased from Preston, HMUK1909.8.18.32.
The type material of C. abbreviata, C. calcarea, and C. pfefferi was not available for study.
Sphenia sp. 20 shells and single valves, “Port Canning” [can be interpreted as the Matla River at Port Canning], SE of Kolkata, 22°18.7'N, 88°40.6'E, ex Godwin-Austin Coll,
Sphenia sp. 5 shells and 1 single valve, Pt. Canning, Matla River, ex Blandford Coll, HMUK 20170343.
Corbula intumescens manuscript name of Stoliczka. 4 shells. Canning, ex. Dr F. Day, HMUK91.9.19.20-3.
Corbula tumescens manuscript name of Day 4 shells. Port Canning,
All species described by
Shell (Fig.
Shell variation in Indosphenia abbreviata. a–d typical form from Port Canning,
Prodissoconch (Fig.
Some shells in the unidentified samples from Port Canning are distorted with the rostrate posterior end upturned (Fig.
Scanning electron micrographs of the hinges of Indosphenia species. a chondrophore of I. abbreviata typical form from Port Canning b as a but from narrow form c as a but from distorted form d chondrophore of syntype of Cuspidaria annandalei e chondrophore of I. sowerbyi from Adyar f as e but from Cooum g right valve pseudo tooth in I. abbreviata h right valve pseudotooth in syntype of Cuspidaria annandalei i right valve pseudotooth in I. sowerbyi j–k chondrophores in small and large specimens of I. kayalum l–m right valve pseudo tooth small and large specimens of I. kayalum n–o chondrophore and right valve pseudo tooth of I. cochinensis. Scale bar: 5 mm.
The shells from Port Canning are highly variable, with many showing distortions of some degree, although the overall form is broadly rounded with a deep anterior end and a sub-rostrate posterior end. Some samples contain a higher proportion of shells that are less deep and more elongate, and these have the manuscript names of tumescens Stolizcka and intumescens Day.
While the extremes of this form look very different, there are intergrades, and in all of these the prodissoconch has the same pattern of radial striations.
While Port Canning is given as the locality, the precise habitat is not provided for these samples, although one does mention the Matla River.
Corbula
chilkaensis
Preston, 1911: 39, fig. 2;
Cuspidaria
annandalei
Preston, 1915: 308, figs 23, 23a;
Corbula chilkaensis. Type material was not available for study.
Cuspidaria annandalei. Syntypes, 3 shells, Lake Chilka [this can be interpreted as 19°42.9'N, 85°18.6'E]. Ex Preston,
Corbula chilkaensis. “Rambha, S. end of Lake Chilka” [this can be interpreted as off Rambha approximately 19°31.2'N, 85°6.3'E].
Cuspidaria annandalei. “Lake Chilka, 4–9 miles E. by S.1/2 S. of Patsahanipur, 4–5 ft” [this can be interpreted as off Patasanipur approximately 19°42.9'N, 85°18.6'E].
Examination of shells (Fig.
Sphenia sowerbyi EA Smith, 1893: 280, pl. 15, fig. 8.
Syntypes, 5 shells, Ariancoupar near Pondicherry [this can be interpreted as the Chunnambar River mouth, Ariyankuppam, Pudicherry, 11°52.8'N, 79°48.5'E].
4 shells,
50+ shells, Cooum River, Madras [this can be interpreted as the Cooum River mouth, Chennai, 13°4.3'N, 80°16.1'E]. Winckworth Collection, September 1931.
Ariancoupan, Pondicherry [this can be interpreted as the Chunnambar River Mouth, Ariyankuppam, Pudicherry, 11°52.8'N, 79°48.5'E].
The sample from Adyar most closely matches the form of the syntypes (Fig.
Variations of Indosphenia sowerbyi. a–c typical form from Adyar river d–e internal views of cleaned valves from Adyar river, pallial line false colour added d. f specimen with epifauna attached to posterior end, from Adyar river g–h specimen from Cooum river j internal showing blotched pattern on dry mantle tissue k posterior sculptured variety from Cooum river l–n Syntype of Sphenia sowerbyi from Ariancoupan, Pudicherry
Comparisons of shells from the Adyar and Cooum rivers show some significant difference in shape, despite the rivers being only 6 km apart. The shells from the Cooum River (Fig.
has a thicker shell than either I. abbreviata or I. kayalum and possesses a much more prominent sculpture. The beaks lie closer to the posterior end, which is the opposite of both I. abbreviata and I. kayalum. Furthermore, the posterior carina is not developed, whereas in I. abbreviata it is finely developed. Additionally, prodissoconch 2 shows no radial sculpture, as is present in I. abbreviata, making it more similar to I. kayalum in this character.
Holotype (Fig.
Ezhupunna region of Cochin Backwater, Vembanad Lake 9°50'43.9"N, 76°17'17.2"E.
Shell (Fig.
Anatomy (Fig.
a–d Gross anatomy of Indosphenia kayalum a left side after removal of the shell b left side after removal of shell and mantle c exterior view of the siphonal apertures d interior view of the siphonal apertures e Gross anatomy of Sphenia binghami from the left side after removal of the shell and anterior mantle. Abbreviations: aa, anterior adductor muscle. exh, exhalant siphon. ft, foot. id, inner demibranch. inh, inhalant siphon. irt, inner ring of tentacles. lp, labial palps. od, outer demibranch. ort, outer ring of tentacles. pa, posterior adductor muscle. per, periostracum. pg, pedal gape. ppr, posterior pedal retractor muscle. rod, reflected portion of outer demibranch. sph, siphons. sphm, siphonal muscles.
The specific name kayalum is from “kayal” the Malayalam (South Indian language) name for the backwaters of Kerala state in which this species lives. The name is intended as a noun in apposition.
The specimens were found attached to the filamentous alga Microspora sp. that was growing on wooden poles in the shallow (1 m) channel in the backwater. The bottom substrate consisted of silty-sand and the measured salinity was 5‰ (oligohaline). Occurring with I. kayalum were the bivalves Mytilopsis sallei and Arcuatula along with a large number of amphipods and polychaetes.
In overall shell shape, Indosphenia kayalum is most similar to the typical form of I. abbreviata, but it differs in being less tumid and with a less well-defined rostrate posterior end on which the carina is weak to obsolete. Prodissococh 2 in I. abbreviata has radial lines, whereas in I. kayalum P2 has only commarginal lines. In size, the prodissoconchs are similar, as is also the punctate sculpture of P1. Indosphenia sowerbyi is much larger, and more robust, such that the sculpture is of commarginal raised ridges, especially over the posterior area. In outline, I. sowerbyi is more tumid, less deep and the beaks are behind the midline. The sculpture of the larval shells is the same. Indosphenia cochinensis has a heavier shell that is rather narrow, it has very prominent ridges, and is distinctly rostrate posteriorly.
Cuspidaria cochinensis Preston, 1916: 39, figs 17, 17a.
Cuneocorbula cochinensis (Preston, 1916): Oliver et al. 2017: 1224–5, fig. 2.
Cuspidaria cochinensis (Fig.
The form of the hinge in Indosphenia is more similar to that of juvenile Mya and Sphenia supporting the placement of Indosphenia in the Myidae rather than the Corbulidae. The molecular phylogenetic analysis conducted herein, using COI places Indosphenia kayalum in the Myidae, although with no significant bootstrap support. However, in combination with the morphological data, we consider this molecular result as adding further confidence to this family placement.
The variability in shell form across the species herein assigned to Indosphenia can be seen both visually in the shells figured and in the graphical display of morphometric parameters (Fig.
Box plots of the ratios of four parameters for shell shape in five populations of Indosphenia. a length to height b posterior length to anterior length c length to tumidity d height to tumidity. Abbreviations: abb, typical I. abbreviata in lot
The environment may be more important than geographical separation in directing the form of the shell in Indosphenia, so although we have aggregated all of the Port Canning forms under I. abbreviata, it is possible that the two primary forms represent different species. The length/height and length/tumidity ratios (Fig.
The morphological complexity of Indosphenia suggests that the brackish waters of India may well represent fragmented habitats and as such would make excellent sites for the study of genetic isolation and divergence.
The senior author wishes to thank Andreia Salvador (