Abstract: Chama pulchella is characterized by having a flat, free, right valve with commar- ginal lamellae, a radial row of wide spines at each side of the ...
Chama pulchella (Bivalvia: Heterodonta) with Transposed Shell and Normal Dentition Akihiko MATSUKUMA*,Naoto HAMADA*and Paul H. SCOTT** (*Paleobiology, Kyushu University, Fukuoka 812-81, Japan and **Santa Barbara Museum of Natural History, Santa Barbara, California, USA)
Abstract: Chama pulchella is characterized by having a flat, free, right valve with commarginal lamellae, a radial row of wide spines at each side of the postero-dorsal sulcation, and two reddish brown radial streaks o n the valve. The early dissoconch is 0.7- 1.1 mm long and ornamented with both punctations umbonally and radial striae distally. The majority of specimens of Chama pulchella attach themselves t o substrate by the left valve ("normal" form), but some individuals occasionally attach by the right valve ("inverse" form). The species differs from Pseudochama similis in having a smooth postero-dorsal inner margin. Sixteen specimens of Chama pulchella from Fraser Island, southern Queensland, have the "normal" form with the "normal" dentition, and one specimen with the right valve attached form ("inverse" form) with the "normal" dentition. We describe the two forms precisely and report the second example of members of the Chamidae with a transposed shell but without a transposed hinge. This evidence shows that in Chama pulchella the transposition of the shell and the hinge are independent of one other and an attached valve does not always have a wide anterior cardinal. Keywords:
Chama, transposed shell, hinge, normal dentition
Introduction Mutations with effects in early stages of development can have large phenotypic consequences. Homeotic mutations and regulatory gene macromutations are examples of hopeful monsters. Whole transposition in gastropods may bring a reproductive isolation between the normal form and the transposed form, while partial transposition in bivalves may produce genetical polymorphism within a species. Since Munier-Chalmas (1882), it has been well-known that there are left valve attached forms ("normal" forms) and right valve attached forms ("inverse" forms) in the Chamidae and that dentition of the attached valve of the both forms is similar to each other and the free valves of the both forms also have a similar dentition in both forms. The "normal" forms are usually placed in the genus Chama Linnaeus, 1758 and the "inverse" forms
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are in the genus Pseudochama Odhner, 1917. The "inverse" form species are considered to be results of a transposition from the "normal" form species (Davies, 1935; Yonge, 1967). Although some investigators consider that the "inverse" specimens in a "species" represents another species (Odhner, 1917, 1919 ; Bernard, 1976), several taxa have both forms in the same species (Healy et a/., 1993; Matsukuma, 1996a). Many authors have recorded both "normal" and "inverse" form specimens in the same species, but they have not described both specimens precisely. For example, Taylor and Kennedy (1969) said that "normal" specimens of Pseudochama exogyra closely resemble Chama peNucida ( = C. arcana Bernard, 1976) and "inverse" specimens of C. pellucida closely resemble P. exogyra. Taylor and Kennedy (1969) and Kennedy et al. (1970) noted that the similarity of the unusual shell structure together with the similar geographical range of these two species, taken together with the problems of inversion in the Chamoidea, made them suspect that C. arcana and P. exogyra may be synonymous. Pseudochama exogyra clearly differs from C. arcana in having smooth inner margin and size and ornamentation of the early dissoconch (Hamada & Matsukuma, 1995) and the transposition of shell of these species is very rare. We suspect that their "normal" specimens of P. exogyra may be true C. arcana and the "inverse" specimens of C. pellucida may be true P. exogyra. If both forms belong to the same species, their shells show similar characters, including the ornamentations and size range of the early dissoconch, form of adductor scars and pallial lines, and the inner ventral margin. Although a number of investigators have noted transposition of the bivalve hinge, only a few have considered transposition of the shell (Lynge, 1909; Pelseneer, 1920; Cosel, 1995; Matsukuma, 1996a, b). Matsukuma (1996a) showed an example of a chamid that has the right-valve-attached specimen ("inverse" form) with the "normal" dentition and emphasized that the transposition of the shell and hinge is independent of one other. The reason why so few have recorded the transposition of the shell may be due primarily to the difficulty of recognizing the phenomenon in bivalve shells. The transposition of the hinge is one of the discontinuous variations impressed on molluscan shells, and may be genetically controlled (Matsukuma, 1996b). It has been found in the Palaeoheterodonta and the Heterodonta that usually have bilaterally symmetric shells. The Chamidae is one of the best bivalve families to study this transposition, because it has bilaterally asymmetric shells caused by the cementing mode of life. Studies of the transposition of shell and hinge may clarify a shell-mantle relationship of bivalves.
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Matsukuma et al.: Chama pulchella with Transposed Shell and Normal Dentition
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Institutional abbreviation: 'The following abbreviations indicate institutions cited in the text : AMNH: American Museum of Natural History, New York. AMS : Australian Museum, Sydney. BM(NH): Natural History Museum, London [ = British Museum (Natural History)]. GK: Kyushu University, Fukuoka. LACM: Natural History Museum of Los Angeles County, Los Angeles. MNHN: Museum National d'Histoire Naturelle, Paris. SBMNH: Santa Barbara Museum of Natural History, Santa Barbara. USNM: National Museum of Natural History, Washington, D. C. Transposed Chamid Shell with Normal Dentition
Yonge (1967) pointed out that a transposition of the hinge is of pallial origin alone and that the viscero-pedal mass is not affected. He did not distinguish transposition of the hinge from transposition of the shell, because it has been considered that in the Chamidae a specimen with a transposed shell always has a transposed hinge (Munier-Chalmas, 1882). However, the transposition of the shell defined by Matsukuma (1996a) is common in the Chamidae, especially in Amphichumu species, and the hinge and shell should be discriminated. Matsukuma (1996a) proposed one new Philippine chamid with the carditid hinge and the cementing mode of life based on a conjoined specimen (holotype, "normal" form, MNHN) and a left free valve (paratype, "inverse" form, MNHN). The left valve of the holotype is the attached valve and the paratype is the free valve, and both have two diverging cardinals and an anterior and a posterior lateral. So far as we know, this is the first example of a transposed shell with "normal" dentition. The specimens of Chama pulchella of Fraser Island, southern Queensland, represent both the "normal" and "inverse" forms (Figs. 1-4). The "normal" form has the deeper left valve with a wide anterior cardinal (2), a weak posterior cardinal (4b), and a posterior lateral (LPII) (Figs. 2b, e and 4a). The right free valve has a wide socket corresponding to the wide anterior cardinal (2) of the left valve, several weak denticles (3a) along the antero-dorsal border of the socket, a strong central cardinal (3b) just behind the socket, a weak crenulated posterior cardinal (5b) on the nymph, and a posterior lateral (LPI) (Figs. 2d, f and 4b). The commarginal lamellae of the attached valve are erected when they attach the substrate and form the attachment lamellae (Fig. 2a). The "inverse" form has the deeper right valve with the attachment lamellae. The right attached valve has a wide socket with several denticles (3a) along the antero-dorsal margin of the socket, a strong central cardinal (3b), a weak posterior cardinal (5b) on the nymph, and a weak posterior lateral (LPI) (Figs. Id, f and 3b). The convexity and ornamentation are similar to those of the left attached valve of the "normal" form, but the dentition is the same as that of the right free valve of the "normal" form. The left free valve of the "inverse" form with the two radial rows of wide spines is flat and has a wide anterior cardinal (2), a weak posterior cardinal (4b), and a posterior lateral (LPII) (Figs. Ib, e and 3a). The convexity, ornamentation and size of the posterior adductor scar of the
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VENUS: Vol. 56, No. 3 (1997)
Taxonomy Chama pulchella Reeve, 1846 (Figs. 1 - 7)
Chama pulchella (part.) Reeve, 1846, Conch. Icon., vol. 4 : Churna pl. 3, sp. 10a; Odhner, 1919: pl. 2, fig. 18 (one of syntypes); Habe & Kosuge, 1966: 155, pl. 60, fig. 1 (northern Australia); Lamprell & Whitehead, 1992: pl. 23, fig. 149 (Australia); Healy et al., 1993: 213, figs. 6 a - b (Torres Strait and Harvey Bay, Queensland).
Type Material: T w o o f three syntypes o f Chama pulchella Reeve, i.e. o n e figured specimen
Fig" 5-6. i\tltluctor \c;~r\0 1 ('lili~~i(i ~ ) ~ t l ~ ~ lI
