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Abstract
The first cases of anomalies in the number of shell-plates in Tonicia atrata (Sowerby II, 1840) are herein reported on the basis of three specimens from Zaratiegui Bay, Tierra del Fuego, Argentina (54°50’53’’ S, 68°29’7’’ W): two cases of coalescence (one symmetrical and another one asymmetrical) and a case of hypomerism. The abnormal specimens bore less ctenidia than physiological conspecific specimens of similar size. The symmetrical coalescent specimen and the hypomeric individual were significantly wider (higher width/length ratio) than physiological specimens. Nevertheless, the reported abnormalities seem to have no effect on general growth, since the body size of the abnormal specimens were close to the largest physiological ones found in the same locality.
Highlights
Members of the Class Polyplacophora normally bear eight dorsal shell-plates
The abnormal specimens were sampled by the second author through scuba diving in Zaratiegui Bay, Tierra del Fuego National Park, Argentina (54°50’53’’ S, 68°29’7’’ W) (Fig. 1), 2–4 m deep, in March 2010
Abnormal individuals The abnormalities of the three specimens of Tonicia atrata found at Zaratiegui Bay were identified as: Coalescence
Summary
Members of the Class Polyplacophora normally bear eight dorsal shell-plates. This ordinary number can be affected by infrequent abnormalities that can be classified as: Hypomerism (less than eight shell-plates), Hypermerism (more than eight shell-plates), Coalescence (two or more shell-plates fused), and Splitting (at least one shell-plate split), according to Dell’Angelo & Schwabe (2010).About 380 anomalous polyplacophoran specimens are known worldwide, belonging in 130 different species, from the first records by Crozier (1919) and Pelseneer (1919) to the recent study by Torres et al (2018), among others.The only teratological records from South America came from the southeastern Pacific (Peña, 1977; Peña & Castro, 1982; Gálvez Herrera, 1991; Torres et al, 2018). Members of the Class Polyplacophora normally bear eight dorsal shell-plates. This ordinary number can be affected by infrequent abnormalities that can be classified as: Hypomerism (less than eight shell-plates), Hypermerism (more than eight shell-plates), Coalescence (two or more shell-plates fused), and Splitting (at least one shell-plate split), according to Dell’Angelo & Schwabe (2010). About 380 anomalous polyplacophoran specimens are known worldwide, belonging in 130 different species, from the first records by Crozier (1919) and Pelseneer (1919) to the recent study by Torres et al (2018), among others. This paper is the first contribution to this topic from the southwestern Atlantic. Despite their pathological shell-plates, the abnormal specimens seem to be almost physiological regarding their growth
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