Genus Horaiclavus Oyama, 1954

Type species: Mangilia splendida A. Adams, 1867 (by original designation).

Remarks. The taxonomic position of the genus Horaiclavus Oyama, 1954 has been controversial. Powell (1966) assigned it to the Turridae but considered the possibility of placement in Buccinidae. Subsequent authors (Sysoev, 1993; 1996; Higo, Callomon & Goto, 1999) assigned Horaiclavus to Drilliidae Olsson, 1964, while Fedosov & Kantor (2008) placed it in the Crassispirinae (= Pseudomelatomidae Morrison, 1965). More recently, Bouchet et al. (2011) designated Horaiclavus as the type genus of the new family Horaiclavidae which differs from Pseudomelatomidae mainly on molecular grounds. Species assigned to Horaiclavus are characterized by a claviform shell sculptured with axial folds with a very shallow to virtually absent anal sinus.

Sysoev in Fedosov & Kantor (2008) noted that there are likely to be numerous additional undescribed Horaiclavus species and that the genus, as presently construed, is most probably polyphyletic and includes species that actually belong to distinct genera. Horaiclavus phaeocercus and H. anaimus (both described by Sysoev in Fedosov & Kantor, 2008) differ from H. splendidus (A. Adams, 1867), the type species of Horaiclavus, in lacking a radula and in foregut anatomy. These species also differ from typical members of Horaiclavus in their smaller dimensions and proportions (“lower spire and larger aperture and a somewhat longer siphonal canal”). Τhe tw0 new speʗIes desʗrIbed beI0w resemble the relatively small-sized species described by Sysoev (in Fedosov & Kantor, 2008) but otherwise differ in having peculiar morphological characters. It is thus possible that they may actually belong to one, or possibly two, still undescribed genera. This consideration is particularly pertinent in the case of Horaiclavus ordinei sp. nov. which differs from all species currently assigned to Horaiclavus in possessing relatively strong spiral sculpture. The presence of a relatively well developed spiral sculpture is a feature in apparent contrast even with the original diagnosis of the family Horaiclavidae proposed by Bouchet et al. (2011: 293), which includes genera with spiral sculpture typically obsolete to feeble on spire whorls. For this reason, the position of H. ordinei is problematic even at the familial level; the possibility that it actually belongs to Pseudomelatomidae cannot be excluded on the basis of shell morphology alone. Kantor et al. (2008) recently pointed out that shell features of members of Turridae are useful for species-level identification but are inadequate for supraspecific classification. Thus, in the absence of soft parts for anatomical and/or molecular studies, we refrain from proposing new, doubtful genera or subgenera; the two species here described are hence provisionally assigned to the genus Horaiclavus, within family Horaiclavidae. Under SEM, Horaiclavus adenensis sp. nov. was found to show a microsculpture of squamiform pustules in the interior part of the inner lip. Judging from our preliminary SEM observations, similar microsculptural elements are widely present within “turriform” gastropods and seem to differ according to families. In the present case, the microscopic pustules found in H. adenensis sp. nov. are very similar to those occurring in Crassispira tuckerana Bonfitto & Morassi, 2011, and may represent further morphological evidence of the close relationship between the families Horaiclavidae and Pseudomelatomidae (= Crassispiridae). The occurrence of this feature has not been investigated in the second new species, H. ordinei sp. nov., because the type material is represented by only few shells and these microscopic pustules, occurring in the inner part of the lip, may require breakage of the outer lip to be adequately shown. Horaiclavus adenensis sp. nov. represents the second record of the genus Horaiclavus in the Gulf of Aden. Sysoev (1996) reported the occurrence of H. splendidus (A. Adams, 1867) in this area, but the material was subsequently described as Horaiclavus sysoevi by Smriglio & Mariottini, 2003.