Hyalorisia Dall 1889

Genus Hyalorisia Dall, 1889

Type species: Capulus (Hyalorisia) galea Dall, 1889 (by monotypy), off Barbados.

Shell: Size average for the family, limpet-like, thin, depressed, white, rounded to ovoid, with yellowish periostracum. Glossy lamella inside posterior part of aperture.

Soft parts: Foot rounded; head large, swollen; cephalic tentacles subulate; pseudoproboscis elongated, flattened, rounded at extremity, longitudinally curved to form a dorsally open canal.

Remarks: Species of Hyalorisia are similar to other limpet-like capulids in their shell shape and in the presence of an extension of the oral area, the pseudoproboscis; they are diagnosed by the shell, which is thin, depressed and regular in shape (from rounded to ovoid), and shows a glossy lamella inside the posterior part of the aperture.

Dall (1889) described the type species as Capulus galea from Barbados (holotype USNM 508724; Fig. 8 A–C) but, probably noticing some differences with congeners, included it in the new ‘ Hyalorisia section’. Several Tertiary species from Italy and France were classified in Hyalorisia by Sacco (1896), but Cossmann (1896) treated Hyalorisia as a synonym of Plesiothyreus (family Phenacolepadidae). Sacco (1897) reconsidered his previous assignments and reclassified the fossils he had earlier placed in Hyalorisia in Williamia (family Siphonariidae) and Plesiothyreus. Subsequently, Otuka (1939) described a second extant species from Japan, Capulus tosaensis (holotype; Fig. 8 D–F), also including it in Capulus (Hyalorisia). He also suggested that two other species of Capulus may belong to Hyalorisia: Capulus fragilis E. A. Smith, 1904 (Laccadive Sea, 192 m and Bay of Bengal, 749 m; syntype NHMUK 1904.6.15.136; Fig. 8 G–I) and Capulus fallax S. V. Wood, 1842 (fossil from the Coralline Crag of England, Lower Pliocene; Fig. 8J: holotype). More recently, another fossil, Capulus (Hyalorisia) nettlesi J. E. Robinson, 1983 (Moodys Branch Formation, Mississippi, USA, Upper Eocene), was included in Hyalorisia (holotype PRI 30058; Fig. 8 K–N).

Based on the original description and drawings, and on photographic examination of a syntype (NHMUK 1904.6.15.136), C. fragilis may indeed belong to Hyalorisia: the shell is limpet-like, thin, ovoid, depressed, with the internal lamella (internal septum of Smith, 1904: 2) still present in the NHMUK syntype (Fig. 8H). We had no material available for molecular systematic analysis from that part of the Indian Ocean that could be referred to as C. fragilis and we were, therefore, unable to assign this name to any of the unnamed MOTUs in our dataset. Incidentally, we note that the name Capulus fragilis E. A. Smith, 1904 is a junior homonym of Capulus fragilis Meek & Hayden, 1856 from the Cretaceous of South Dakota. The allocation of C. fallax to Hyalorisia is more doubtful due to the apparent absence of lamella and to the shape of the aperture, which is flat in the posterior part and thus more similar to that of Capulus or hipponicids. However, C. fallax also shows similarities with Hyalorisia, such as a slightly pointed anterior part of the aperture and the presence of a dorsal shell ridge (Fig. 8J). Finally, C. (Hyalorisia) nettlesi, showing the general features of the genus (ellipsoidal shell with an internal lamella; Fig. 8 K–N), may be the only known fossil of the genus. However, it is larger in size, compared with all the 11 extant species, and with the dorsal margin projected further than the apex (Fig. 8M); this may suggest that either C. (Hyalorisia) nettlesi was living on a different host or that it did not belong to the same Hyalorisia lineage as the extant species.

Because of the presence of a pseudoproboscis similar to that of Capulus, it has been suggested that Hyalorisia galea is a kleptoparasitic filter feeder (Bouchet & Warén, 1993). Prior to the present study, almost nothing was known about the hosts of Hyalorisia. The only data consist of records of H. tosaensis attached to the auger shell Terebra pretiosa Reeve, 1842 (Otuka, 1939) and to the scallop Propeamussium sibogai (Otuka, 1939, as Propeamussium sp.; Matsukuma, 1978), and a photo of H. galea attached to a living Propeamussium sp. (García, 2012: fig. 4). Otuka’s record parallels what is known for C. ungaricus, that is that it may also live in association with burrowing gastropods, such as Turritella communis (Thorson, 1965). However, it should be noted that while Turritella communis as a ciliary or suspension feeder (Yonge, 1946; Fretter & Graham, 1962) has a feeding strategy consistent with the hypothesized kleptoparasitic behaviour of the associated capulids, Terebra pretiosa as a predator (carnivore) certainly does not. It is in fact not clear whether Otuka actually examined the specimen of H. tosaensis reported to have been found by Torao Yamamoto, and we are inclined to believe that it was a hipponicid rather than a Hyalorisia. As previously reported by Matsukuma (1978), we found that Hyalorisia specimens are found (alone or in pairs) on propeamussiid scallops on the anterodorsal part of the left valve and close to the edge of this valve. Hyalorisia attachment generally produces a small semilunar notch on the valve edge (i.e. caused by the insertion of the pseudoproboscis) and a scar on the shell on the site of attachment (Fig. 9). These traces are very similar to those left by C. ungaricus on its hosts (e.g. Fassio et al., 2015: fig. 1).