New records of Tylokepon with the description of a new species (Epicaridea, Bopyridae, Keponinae)

Abstract The parasitic isopod genus Tylokepon is recorded for the first time from the Mariana Islands and Australia. Tylokeponmarianensissp. n. is described from the Mariana Islands, infesting Thusaenysirami (Laurie, 1906). The holotype female differs from other known Tylokepon females by the tri-lobed projection on pereomere 6, almost smooth lateral plates and pleopods, shape of oostegite 1, and widely opened brood pouch. The host is first recorded for bearing bopyrids. The new record of T.bonnieri Stebbing, 1904 from Australia on the type host extends the range of this species from China and India. A table of localities and hosts and a key to all species of Tylokepon are provided.


Introduction
Epicarid isopods remain greatly underdescribed, and Williams and Boyko (2012) predicted that the central Indian Ocean and east Asian seas hold a wealth of undescribed species. The keponine genus Tylokepon Stebbing, 1904 is a case in point. It currently contains four species (An 2009), three of which are known from single collections.
Here we add a fifth species and provide a new record for the only Tylokepon previously known from more than one record. Boyko et al. (2013) recently erected the Keponinae for most genera previously attributed to Ioninae, restricting the latter to the type genus, Ione Latreille, 1818, and raising Ioninae to the family level. The keponine Tylokepon is readily differentiated from related genera by the prominent middorsal projections on the last two pereomeres and the extremely swollen bilobed head. Stebbing (1904) erected the genus for T. bonnieri Stebbing, 1904 infesting the epialtid crab Tylocarcinus styx (Herbst, 1803) from the Maldives. An (2009) reported it from Beibu Gulf and South China Sea. Bonnier (1900) described Cepon naxiae infesting Hyastenus diacanthus (De Haan, 1839) from Hong Kong, based on a dried female specimen. After examining the type specimen, Markham (1982) transferred the species to Tylokepon. Shiino (1950) described a third species, T. micippae, infesting Micippa philyra (Herbst, 1803) from Japan, and compared Tylokepon with the related genera Grapsicepon Giard & Bonnier, 1887, Merocepon Richardson, 1910, Cancricepon Giard & Bonnier, 1887, Paracepon Nierstrasz & Brender à Brandis, 1931, and Scyracepon Tattersall, 1905. An (2009 added a fourth species, T. biturus, infesting Menaethius monoceros (Latreille, 1825) from Hainan, China. Recorded hosts of Tylokepon are crabs belonging to the majoid families Epialtidae and Majidae, with a single host record from a species of Parthenopidae (An 2009).
In the present paper, two epialtid crabs bearing bopyrids were examined and the parasites identified as T. bonnieri and a new species, T. marianensis. These are new records for the genus from Australia and Micronesia. A table (Table 1) and a key to the species of Tylokepon are provided.
Male: Head semicircular, with black eyes. Seven pereomeres with truncate margins. Pereopods subequal in size and structure. First five pleomeres with tuberculate, uniramous pleopods. Sixth pleomere with a pair of round uropods covered in scales and each ramus with stout terminal setae.
Head large, covering pereomere 1, wider than long, bilobed, comprised of two ellipsoid structures separated by a deep median groove. Frontal lamina narrow and not extending to margin of head. Eyes absent ( Figure 1A). First and second antennae rudimentary with two and three articles, respectively, terminally setose ( Figure 1C). Barbula ( Figure 1D) with two lateral digitate projections and a medial blunt projection on each side. Maxilliped ( Figure 1E) with a smooth, curved palp and a sharp plectron; anterior article much larger than posterior one.
Pereon broadest across pereomere 3. Head covering much of pereomere 1, with only median part visible in dorsal view. Dorsolateral bosses distinct on first four pereomeres on both sides. Coxal plates absent. Tergal projections present on pereomeres 2-4. Last two pereomeres with middorsal projections: pereomere 6 with a tri-lobed projection; pereomere 7 with a single, large median projection ( Figure 1A). Brood pouch widely open. Oostegite 1 ( Figure 1F, G) composed of two equally-sized articles, internal ridge smooth, jagged posterior margin with sharp projections, without posterolateral point. Pereopod 1 much smaller than others, but all pereopods subequal in shape, all articles distinct, carpi and propodi with setae on ventral surface ( Figure 1H, I). Pleon with six distinct pleomeres, first five with longer lateral plates and biramous pleopods ( Figure 1B). Lateral plates tuberculated on both sides of first pleomeres, but smooth on remaining pleomeres. Terminal segment without lateral plates. All pleopods with slightly undulating to smooth margins, gradually shorter toward posterior. Pleomere 6 small. Uropods uniramous, lobose, with smooth surfaces and entire margins ( Figure 1B).
Head semicircular, black eyes near posterior margin of head ( Figure 1J). First and second antennae of three and four articles, respectively ( Figure 1L), terminal articles fringed with setae. Third pereomere widest; all pereomeres with truncate margins. Pereopods subequal in size and structure ( Figure 1M), meri and carpi with setae on ventral surfaces, each pointed dactylus retracts into groove formed between parallel series of tubercles on each propodus.
Pleon of six segments, first five pleomeres with tuberculate, uniramous pleopods. Sixth pleomere with a pair of round uropods covered in scales and each ramus with stout terminal setae; medial anal cone smooth.
Etymology. The specific epithet, marianensis, refers to the type locality in the Mariana Islands.
Remarks. Shiino (1950) and Markham (1982) summarized the distinguishing features of Tylokepon as follows: females have a large head formed by two semi-spherical structures separated by a deep median groove, and the last two pereomeres possess prominent middorsal projections. The present specimens are referred to Tylokepon on the basis of these characters. The new species is distinguished from the other four nominal species of Tylokepon by the tri-lobed middorsal projection on the sixth pereomere of females and the jagged posterior margin of oostegite 1. Tylokepon marianensis is most similar to T. bonnieri, which was recorded from Beibu Gulf in China on the related host Hyastenus diacanthus (An, 2009). Both have the middorsal projections on pereonite 6 consisting of three lobes and a single projection on pereonite 7. However, while the three lobes are separated at the base in T. bonnieri, they are united basally to form a tri-lobed structure in the new species (compare Figs. 1A and 2A). They also differ in the sculpture of the lateral plates on pleomeres 2-5 and pleopods: these are almost smooth in T. marianensis but tuberculate in T. bonnieri. Tylokepon micippae differs from T. marianensis in having a closed, rather than widely open brood pouch, tuberculate rather than nearly smooth pleopods and lateral plates on pleomeres 2-5, a sharp and pointed posterolateral point on oostegite 1 rather than an entire margin, and a cluster of three rather than a single mid-dorsal projection on the seventh pereomere. The poorly known T. naxiae, which also infests H. diacanthus, differs from the new species in the shape of the middorsal projections of the sixth pereomere, which are separated at the base as in T. bonnieri, and having digitate, rather than entire margins on the lateral plates of the second pleomeres. The new species also differs from T. biturus in having three rather than two mid-dorsal projections on the sixth pereomere, having the lobes of the head united into a dumbbell shape rather than separated by a groove, and smooth, rather than tuberculate, lateral plates on the first pleomeres. Figure 2 Tylokepon bonnieri Stebbing, 1904: 716-717, pl. LIII, B, C;Shiino 1950: 166;Pillai 1954: 19;Pillai 1964: 187-188, figs. 7-11;An 2009: 96-  Description of female. Length 3.58 mm (excluding uropods), maximum width 2.28 mm across pereomere 3, head length 0.66 mm, head width 1.26 mm. (Figure 2A).

Tylokepon bonnieri Stebbing, 1904
Head large, covering pereomere 1, wider than long, completely bilobed, with two ellipsoid structures separated by a deep median groove. Frontal lamina narrow, visible in dorsal view. Small black eyes near frontal lamina (Figure 2A). First and second antennae rudimentary, with three and five articles, respectively, terminally setose (Figure 2B). Barbula ( Figure 2C) with two large falcate projections and a mediad sharp projection on each side. Anterior article of maxilliped ( Figure 2D) much larger than posterior one, with prominent, inwardly-directed and setose palp, plectron short.
Pereon broadest across pereomere 3. Head covering much of pereomere 1. Distinct dorsolateral bosses on first four pereomeres on both sides. Coxal plates absent. Tergal projection present on pereomeres 2 and 3. Middorsal projections on last two pereomeres: three parallel projections on pereomere 6, one large and posteriorly extended projection on pereomere 7 (Figure 2A). Brood pouch incompletely covered by oostegites. Oostegite 1 ( Figure 2E, F) composed of two subequal in size articles, with smooth internal ridge that bears three marginal projections; ending in extended posterolateral point. Pereopod 1 much smaller than others, all pereopods with short dactyli ( Figure 2G, H).
As for T. naxiae, Bonnier (1900) only gave a brief description, and Markham (1982) found that the holotype female is badly damaged, and only three useful characters can be defined. However, from the figure (Markham 1982: fig. 18), it can be seen that the species has an almost smooth surface to the pleopods and the lateral plates are not covered with tuberculate (while the pleopods and lateral plates of T. bonnieri are densely covered with tubercles). Therefore, T. bonnieri differs from T. naxiae.
The present specimens conform well to Stebbing's description (1904), except for some minor differences, such as the male specimen lacking eyes. There are slight differences between the present specimens and those from Beibu Gulf (An 2009) in the middorsal projections on pereopod 6 and frontal lamina of the head, but the key characters of the females, such as oostegite 1, maxilliped, barbula, lateral plates and pleopods, as well as of males, are similar.