Research Article |
Corresponding author: Appukuttannair Biju Kumar ( bijupuzhayoram@gmail.com ) Academic editor: Richard Willan
© 2018 Anitha R. Arathi, P. Graham Oliver, Raveendhiran Ravinesh, Appukuttannair Biju Kumar.
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:
Arathi AR, Oliver PG, Ravinesh R, Kumar AB (2018) The Ashtamudi Lake short-neck clam: re-assigned to the genus Marcia H. Adams & A. Adams, 1857 (Bivalvia, Veneridae). ZooKeys 799: 1-20. https://doi.org/10.3897/zookeys.799.25829
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The economically valuable bivalve mollusc, known as the short-neck clam, is the major fishery resource of the brackishwater Ashtamudi Lake in Kerala, India. This fishery carries a Marine Stewardship Council certification for sustainability wherein it and all hitherto published reports identify the short-neck clam or yellow-foot clam as Paphia malabarica (Dillwyn, 1817). It is noted that this name does not conform with current nomenclature and is now correctly referred to Protapes gallus (Gmelin, 1791). Furthermore, it is shown that the identification is also incorrect. Comparative shell morphology of venerid clams of the subfamily Tapetinae from the south Indian coast demonstrates that the short-neck clam in Ashtamudi Lake is Marcia recens (Holten, 1802). Small numbers of M. opima (Gmelin, 1791) were found in Ashtamudi Lake but appear not to be part of, or contribute significantly to, the fishery. The venerid clams Protapes gallus and P. ziczac (Linnaeus, 1758) are not found in Ashtamudi Lake but are inhabitants of the shallow coastal waters of south India. Descriptions of the four confused species M. recens, M. opima, P. gallus, and P. ziczac are given. On the basis of this study, the species involved in Marine Stewardship Council (MSC) certification may be better considered at the generic level of Marcia or at the species level as Marcia recens, the most dominant species in the Ashtamudi Lake clam fishery zone.
Fisheries management, India, Marcia , MSC certification, Paphia , Protapes , Venerid clam, Veneridae
India supports extensive bivalve fisheries, notably for mussels, oysters, and clams, with an estimated annual production of 84,483 tonnes (
During a bivalve training workshop in Kochi in 2016 (
In order to record the species diversity represented in clam fisheries, surveys were conducted in the clam fishing zones of Ashtamudi Lake (8°56'N, 76°30'E), during 2015–2017 (Figure
An initial review was undertaken by examining literature, primarily that of
The nomenclature was reviewed from all original sources including
Shell measurements such as shell length (maximum distance from anterior to posterior margin), shell height (maximum distance from dorsal to ventral margin), and shell breadth (maximum inflation of the valves when joined) were recorded based on
DABFUK Department of Aquatic Biology and Fisheries, University of Kerala, Trivandrum, Kerala, India
No specimens referable to the genera Paphia or Protapes were found among the Ashtamudi Lake samples, but were collected from the offshore sampling.
Venus opima Gmelin, 1791
Moderately sized, outline triangular-ovate to elongate-ovate. Hinge with three cardinal teeth in each valve; posterior and middle cardinal bifid in left valve, middle cardinal bifid in right valve; laterals absent. Ligament external, elongate. Pallial sinus moderately deep, horizontally aligned. Sculpture variable from smooth to commarginal lines to weak commarginal ridges. Often highly and variably patterned externally with bold geometric blotches and radial rays. Inner shell margins smooth.
The species within the genus Marcia are rather variable in form with the sculpture varying from almost smooth (M. opima) to finely ridged (M. japonica and M. hiantina).
Venus opima Gmelin, 1791
(from
As the name Venus pinguis Chemnitz, 1782 is unavailable, this species takes the name of Venus opima Gmelin, 1791, both names referring to Chemnitz, 1782 tab. 34, figs 355–357. The type locality is given as East Indian Seas by
Ashtamudi Lake, Kerala, 21 live collected specimens + 26 articulated conjoined valves.
Length 30.4–48.6 mm, mean L/H = 1.3, mean L/B = 1.6.
Shell equivalve, relatively thin; inflated, umbos prominent. Outline triangularly subovate, inequilateral, beaks slightly in front of midline. Lunule well defined, prominent, flattened, and broad. Escutcheon weakly defined. Shell surface smooth, glossy with faint growth lines. Adductor muscles of equal size. Pallial sinus horizontally aligned, broadly rounded, extending to midline of shell. External colouration variable and variously patterned, shades of brown, cream and dark grey with 3–4 radial darker bands. Internal colouration white.
Marcia opima is distributed throughout the Indian Ocean from the Red Sea to Indonesia. Authentic records are from East Africa, Kenya, Djibouti, Yemen, Arabian Gulf, Oman, Pakistan, India, Sri Lanka, Myanmar, Andaman Sea, West Malaysia, Penang, and Sulawesi (
Marcia opima was originally described from India and it has a wide distribution on the east and west coasts of India including the Andaman-Nicobar and Lakshadweep islands (
Venus recens Holten, 1802
(from
Tuticorin, 42 live collected specimens: Ashtamudi Lake, 217 live collected specimens: Mumbai (Bombay), Maharashtra, 6 empty articulated shells, as Tapes marmorata Lamarck, leg. J. C. Melvill,
Shells from Ashtamudi Lake and Tuticorin ranged in length from 12 to 51 mm. More detailed measurements are given for the morphotypes described below.
Shell robust, moderately thick, moderately inflated. Outline elongate subovate, inequilateral, beaks in front of midline. Lunule flattened, not well defined. Escutcheon weakly defined. Shell surface slightly glossy; sculpture commarginal, of weak lines and growth stops, some with more defined ridges especially over anterior area. Muscle scars weakly heteromyarian, posterior larger. Pallial sinus horizontally aligned, broadly rounded extending to one third of shell length. External colouration highly variable and variously patterned, cream, red, white or brown and patterned with 3–4 black radiating rays, or darker trigonal blotches over a light ground or with anastomosing narrow radial rays. Internal colouration white, some with pinkish umbonal cavity.
Variability The type locality of M. recens is given as Tuticorin but without any further precision. Shells collected for this study from Tuticorin can be considered to come from the type locality and are given topotype status.
Topotypes (Figure
Shells from Ashtamudi Lake show considerable variation in shell colour and pattern and these are defined as follows.
Morphotype 1 (Figure
Morphotype 2 (Figure
Morphotype 3 (Figure
Morphotype 4 (Figure
Morphotype 5 (Figure
The type locality is recorded as Tuticorin on the Coromandel coast (
The species was originally described by
The morphotypes differ primarily in colour pattern with some variation in relative timidity as shown by the L/B ratios that vary from 2.0 to 2.4. This ratio is distinctly different from the 1.6 for M. opima. The sampling regime was not precise enough to distinguish if there was any relationship between morphotype and distribution.
Venus gallus Gmelin, 1791
Moderately large, outline triangular ovate to oval, often posteriorly truncated and pronounced anteriorly; lunule margin excavated. Hinge with three cardinal teeth in each valve. Ligament external, opisthodetic. Pallial sinus steeply ascending towards the umbonal cavity. External sculpture strong, of raised commarginal ridges. External patterns predominantly of interrupted zig-zag lines.
The genera Protapes, Paphia, and Paratapes all share the character of the ascending pallial sinus. Paphia and Paratapes differ in outline and sculpture in being elongate, distinctly longer than high, and having a smooth shell.
Three species of Protapes are recorded from Indian waters, P. gallus (Gmelin, 1791), P. ziczac (Linnaeus, 1758), and P. monstrosus (Römer, 1870), and all are well illustrated by
Venus gallus Gmelin, 1791
(from
As the name Venus malabarica Chemnitz, 1782 is unavailable this species takes the name of Venus gallus Gmelin, 1791, both names referring to Chemnitz, 1782 tab. 31, figs 324–325. The type locality is given as the Malabar coast by
Neendakara, Kollam, 5 live collected specimens + 10 empty articulated shells; Dharmadam, Kannur, 6 live collected specimens + 4 empty articulated shells.
Shell to 60 mm in length, solid, compressed, inequilateral, beaks slightly to the anterior. Outline trigonal-subovate, lunule margin impressed, anterior margin pronounced, posterior ventral margin weakly truncated, posterior dorsal margin sloping steeply. Lunule lanceolate, demarcated by shallow groove. Escutcheon long, narrow, weakly striated. Sculpture of evenly sized, rounded, closely spaced, commarginal ribs separated by narrow grooves. Pallial sinus wide, deep, ascending steeply toward umbonal cavity. External colouration light brown with narrow and light zigzag or chevron streaks with four distinct brown rays radiating from the beak to the ventral margin. Shell interior white with yellowish tinge on the umbonal cavity.
Protapes gallus has an Indo-West Pacific distribution extending from India to China (
This species was described as Venus malabarica by
Venus ziczac Linnaeus, 1758
(from
Neendakara, Kollam, 5 live collected specimens + 18 empty articulated shells; Dharmadam, Kannur, 3 live collected specimens.
Shells to 50 mm, solid, inflated, equivalve, inequilateral, beaks slightly in front of midline. Outline sub-ovate, lunule margin impressed, anterior margin pronounced, posterior ventral margin subtruncate, posterior area strongly sinuous. Lunule lanceolate, weakly ridged. Escutcheon narrow, smooth. Sculpture of raised concentric ridges separated by nearly equal-sized grooves. Pallial sinus narrow, apex rounded, ascending steeply towards umbonal cavity. Shell external colour tan with bright zigzag streaks and four brown rays radiating from umbo to ventral margin. Shell interior colour white with yellowish tinge in umbonal cavity.
The species has an Indian Ocean distribution with records from the Red Sea, Aden, East Africa, Somalia, Mozambique, Maputo, Inhambane, Nacala, Natal, Madagascar, Oman, and Persian Gulf (
Protapes ziczac (Linnaeus, 1758) has an inflated, heavy, and solid shell with the external sculpture the strongest of any species of Protapes.
The species was recorded as Protapes sinuosa (Lamarck 1819) by
It is evident from the results that the Ashtamudi Lake short-neck clam fishery is based primarily on Marcia recens and not Paphia malabarica (= Protapes gallus). It is surprising that two such different shells should have become so confused. The literature citations for Paphia malabarica in India are extensive (
It would appear that a lack of illustrations in recent Indian literature have led to a misidentification and this has been carried into modern references referring to the Ashtamudi clam even where Marcia shells are illustrated:
No deleterious effects on the viability of the fishery have resulted from this error in identification but from a legislative context applying the incorrect name to the exploited species could undermine its certification and protection. On the basis of this study, the species involved in the Marine Stewardship Council certification would be better considered at the generic level of Marcia or at the species level for Marcia recens, the most dominant species in the Ashtamudi Lake clam fishery zone. We have limited this proposal to the Ashtamudi fishery as, at this time, we are unable to confirm the identity of clams from other fisheries, including those cited as exploiting Marcia opima. It is possible that the name “short-neck clam” is applied loosely to both Marcia species. Subba Rao gives the Tamil name “vazhukku matti” for M. opima but none for M. recens, perhaps indicating a lack of discrimination by fishermen. In this context it will be necessary to ascertain the relative abundance of the two Marcia species in any fishery and elucidate the ecology of these species especially their micro-habitat preferences.
Misidentification can undermine comparative biological studies. For example,
Marcia recens has now been confirmed from its type locality in Tuticorin and is conspecific with the Ashtamudi Lake population in Kerala. Similar shells are known from further north at Mumbai but its northern limit is not known nor where, or if, it overlaps with Marcia cordata, a widespread Arabian species. It would appear that M. recens extends throughout the Indo-Pacific although the molecular data from
While this study has correctly identified the Ashtamudi clam, many unresolved issues surround this species and the genus as a whole. Molecular studies are necessary to resolve the population differences within M. recens and the relationship between it and M. cordata and M. opima. Marcia recens is in many ways similar to some species of Tapes and Politapes and here too a molecular resolution is needed. Morphologically other species of Marcia have been separated into Hemitapes (
The authors thank the traditional fisher folk in Ashtamudi for their kind support in collecting the clam resources. One of the authors (Arathi) thanks the support of Kerala State Council for Science, Technology and Environment (KSCSTE) for the taxonomy fellowship offered for her PhD programme. We also acknowledge the constructive efforts of the reviewers and the subject editor Richard Willan (Museum and Art Gallery Northern Territory).