Contributions to Zoology, 79 (2) – 2010Bastian T. Reijnen; Bert W. Hoeksema; Edmund Gittenberger: Host specificity and phylogenetic relationships among Atlantic Ovulidae (Mollusca: Gastropoda)

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Discussion

Associations

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Reported host preferences of Cyphoma gibbosum (Bertsch, 1984; Lasker et al., 1988; Botero, 1990; Nowlis, 1993; Chiaponne et al., 2003) are partly confirmed and supplemented with new observations of associations of this species, showing once more that C. gibbosum is a generalist parasite (Table 2). Cymbovula acicularis turned out to be another generalist.

It remains unclear why 43% of the encountered alcyonacean species did not appear as hosts for ovulids at the time of our fieldwork (Table 3). It is known that Octocorallia may produce secondary metabolites as protection against predation (Ciereszko and Schneider, 1987; Chiaponne et al., 2003), but the effect of this defence strategy on ovulids is still largely unexplored. Other factors, such as the nutritional value of the corals (O’Neal and Pawlik, 2002) and the unpalatability of sclerites (Alstyne and Paul, 1992), may also influence host choices.

An overview of natural products produced by West Indian gorgonian octocorals reveals that many types of secondary metabolites are found. The largest class of metabolites encountered in Caribbean alcyonaceans are diterpenoids, followed by the sesquiterpenes (Rodriguez, 1995). For some of these compounds that are obtained from alcyonaceans, such as Erythropodium caribeorum, feeding experiments were performed, resulting in the observation that coral extracts are deterrent to fish. Also, crude extracts from the gorgonian Gorgonia ventalina, containing terpenoids, were used in feeding experiments with C. gibbosum. As a result, C. gibbosum consumed only 49% of an artificial diet containing terpenoids (Alstyne and Paul, 1992).

The sclerites of the gorgonian species that we found as hosts for Simnialena uniplicata and Cymbovula acicularis turned out to be relatively small. The sclerites of Gorgonia spp. and Pseudopterogorgia spp. have average sizes of 0.10 mm and 0.10-0.15 mm, respectively, whereas 0.30-1.00 mm is common for other Octocorallia species, like Eunicea and Pseudoplexaura species (Bayer, 1961). For Cyphoma gibbosum,however, the sclerites’ size seems to be irrelevant for host selection, since this species is found on corals containing either small or large sclerites. By analysing faecal pellets which all contained sclerites, we conclude that C. gibbosum at least ingests sclerites instead of eating around them.

Paedomorphosis

In the phylogeny reconstruction, Simnia patula is the sister group of the combined Simnialena uniplicata and Cyphoma species group. This is surprising because, at first sight, fully grown individuals of Cyphoma species clearly differ from S. uniplicata in shell morphology. However, when shells of S. uniplicata are compared to a series of shells of C. gibbosum in various growth-stages (Fig. 4), it turns out that S. uniplicata closely resembles juvenile C. gibbosum (as well as juvenile ovulids in general). Both species lack a clear funiculum (narrow ridge of callus at the ventral side of the shell close to the aperture) and have rounded, tapering ends, character states that are absent in fully grown C. gibbosum. Identification of juvenile ovulid shells is difficult, if not impossible, due to a lack of diagnostic shell characters. However, some specimens that were collected alive showed a mantle colour pattern diagnostic for C. gibbosum, viz. bright orange spots, encircled with a black line at a whitish background (Fig. 1). Cymbovula acicularis had a nearly transparent mantle, sometimes with white protuberances. In Simnialena uniplicata the mantle is entirely black, whereas in Cyphoma signatum it had a distinct yellow/black fingerprint pattern. This leads to the conclusion that the mantle colour and pattern may be diagnostic in Ovulidae (Mase, 1989) and that this character can be used to distinguish fully grown Simnialena uniplicata from juvenile C. gibbosum. Therefore, we hypothesize that S. uniplicata exemplifies paedomorphosis.

FIG2

Fig. 4. Shells in dorsal and ventral view. Simnialena uniplicata (d) compared to a series of shells of Cyphoma gibbosum (a-c, e-k) in different developmental stages. Scale bar 1 cm.

Systematics, biogeography and nomenclature

According to the principles of phylogenetic systematics, the species referred to as Simnialena uniplicata should be called Cyphoma uniplicata (Fig. 3). Meanwhile, the status of the nominal genus Simnialena Cate, 1973, with its insufficiently known type species Simnialena marferula Cate, 1973, remains unclear. According to Lorenz and Fehse (2009: 105), ‘S. marferula is a close relative of S. uniplicata’. This conclusion, on which we cannot elaborate here, is based on similarities in shell morphology.

The DNA sequences of specimens belonging to Cymbovula acicularis and specimens that agree with the description of C. bahamaensis (Figs 2c-d) are almost identical. As a consequence, these nominal taxa should most probably be considered synonyms, as has also been suggested by Lorenz and Fehse (2009) based on morphological data.

There is a moderately strong supported dichotomy between the Atlantic and the Indo-Pacific taxa, with Neosimnia arcuata from the East-Pacific having an aberrant, but poorly supported, position in the cladogram, where it clusters with the Atlantic taxa. Together, these species represent the Simniinae Schilder, 1927. Furthermore, two undisputed clades were found among the Indo-Pacific taxa, supporting the occurrence of the subfamilies Prionovolvinae Fehse, 2007, and Ovulinae Fleming, 1822, respectively.

In order to get a better understanding of the phylogeny and parasite/host associations of the Atlantic Ovulidae, additional shells and DNA material are needed. DNA obtained from other ovulids occurring in the Atlantic area (e.g. Cyphoma macumba Petuch, 1979; C. versicolor Fehse, 2003; C. mcgintyi Pilsbry, 1939 ) may elucidate the taxonomical position of the genus Cyphoma as a monophyletic group. However, several ovulid species are rare and generally only their shells are found, which hampers further investigations.