Prosthiostomidae Lang, 1884

Prosthiostomidae Lang, 1884 View in CoL

Prosthiostomidae View in CoL are characterized by an absence of tentacles, a tubular, anteriorly located pharynx and a male complex characterized by a pair of spherical, free prostatic vesicles. Generic differentiation is based on the presence or absence of a main frontal intestinal branch, varying amounts (complete, partial or none) of a muscular enclosure of the prostatic vesicles, the arrangement of cerebral and marginal eyes and body shape (oval, elongate) ( Kato, 1938; Marcus & Marcus, 1968; Faubel , 1984). However, eye arrangement and body shape are highly plastic and thus, their taxonomic use is questionable at best ( Jokiel & Townsley, 1974; Poulter, 1975).

Using representatives of five currently recognized genera [i. e. Enchiridium View in CoL , Prosthiostomum View in CoL , Euprosthiostomum View in CoL , Lurymare View in CoL and Amakusaplana View in CoL (a sixth genus, Enterogonimus View in CoL , is monospecific)], we recovered a strongly supported monophyletic Prosthiostomidae View in CoL , in which Enchiridium View in CoL and Prosthiostomum View in CoL are well-supported taxa. According to Faubel (1984), a muscular sheath completely enclosing the prostatic vesicles and the seminal vesicle of Enchiridium View in CoL distinguishes the genus morphologically from Prosthiostomum View in CoL , in which neither the prostatic nor the seminal vesicles are enclosed by a muscular bulb.

Including Prosthiostomum siphunculus (Della Chiaje, 1822) View in CoL , Amakusaplana acroporae Rawlinson et al., 2011 View in CoL , Enchiridium evelinae Marcus, 1949 View in CoL plus two unidentified species of Enchiridium View in CoL , Bahia et al. (2017) also found a monophyletic Prosthiostomidae View in CoL . Despite the fact that currently all prosthiostomids are placed in a single family ( Faubel , 1984; Prudhoe, 1985), Bahia et al. (2017) elevated the family to superfamily status (Prosthiostomoidea). We contend that such a taxonomic addition is not warranted until at least another prosthiostomid family is recognized.

The genus Amakusaplana View in CoL was erected for a single species ( Amakusaplana oshimai Kato, 1938 ), separating it from Prosthiostomum View in CoL based on the lack of a cotyl, a more oval rather than elongated body shape and cerebral eyes that are scattered fan-like over the brain region rather than arranged into two distinct clusters ( Kato, 1938). However, the latter two characteristics are highly variable and co-vary with the age of the specimen ( Jokiel & Townsley, 1974; Poulter, 1975). Since then, a second species, Amakusaplana acroporae View in CoL , has been described, its species epithet reflecting the fact that its main prey items are several species of the stony coral Acropora ( Rawlinson et al., 2011) View in CoL . The authors based their specific separation mostly on the number and arrangement of cerebral and marginal eyes and on other plastic traits associated with the reproductive system (e.g. size of the seminal vesicle, the female atrium and the cement gland pouch). Recognizing the variability of these traits, Rawlinson & Stella (2012) accepted the possibility of synonymy between Amakusaplana oshimai and Amakusaplana acroporae View in CoL .

Regardless of it being a separate species, our analysis places Amakusaplana acroporae firmly into Prosthiostomum with strong support. Hence, rather than recognizing a separate genus based on highly variable traits, we here support Hyman (1959a) and Faubel (1984), who proposed that the genus should be eliminated and synonymized with Prosthiostomum . By synonymizing the two genera, Amakusaplana acroporae becomes Prosthiostomum acroporae comb. nov. The absence of a cotyl then represents a secondary loss, probably attributable to a highly specific lifestyle on madreporarian corals. In addition to established generic prosthiostomid traits (e.g. elongate body shape, absence of marginal tentacles, cylindrical pharynx and a pair of muscular accessory prostatic vesicles), synonymy of Amakusaplana with Prosthiostomum is further supported by: (1) the presence of a ventral eye in each cerebral eye cluster of both genera; (2) an anteriorly extending median intestinal branch; and (3) a cleft pharynx, which has also been described in another corallivorous species, Prosthiostomum montiporae Poulter, 1975 . According to Poulter (1974, 1987), Prosthiostomum montiporae is also an obligate symbiont of stony coral ( Montipora spp. ), and thus, a cleft pharynx might be an adaptation of the feeding mode in this genus.

The taxonomic affinity of Prosthiostomum utarum Marcus, 1952 has been equivocal ( Marcus, 1952; Marcus & Marcus, 1968; Faubel , 1984; Bahia et al., 2014). Originally described as Prosthiostomum utarum by Marcus (1952), it was moved to the newly erected genus Lurymare by Marcus & Marcus (1968). The amount of muscles surrounding the prostatic vesicles has been used to separate Lurymare from Prosthiostomum . However, the trait is highly variable and depends on the age of the specimen. Molecular data now support the original placement of this species in Prosthiostomum . The placement of the remaining species currently in Lurymare awaits further testing.