Tylokepon marianensis , An, Jianmei, Zhang, Miao & Paulay, Gustav, 2018
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Tylokepon marianensis sp. n. Figure 1
UF 42220: holotype ♀and allotype♂, USA, Mariana Islands, Guam Island, Haputo, 13°34.74'N, 144°49.84'E, rubble, 8-10 m, 8 July 2003, Coll. G. Paulay, infesting the right branchial chamber of Thusaenys irami (Laurie, 1906) (host, UF 5935, identified by Dr. Amanda Windsor).
Female: Head large, swollen, and bilobed. Without eyes. Pereon with seven segments, sixth pereomere with tri-lobed projection and seventh pereomere with a single, large median projection. Pleon of six segments, first five with uniramous lateral plates and biramous pleopods. Lateral plates tuberculated on both sides of first pleomeres, but smooth on remaining pleomeres. Sixth pleomere small with uniramous uropoda.
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.
Description of holotype female.
Length 3.38 mm (excluding pleon and uropods), maximum width 1.9 mm across pereomere 4, head length 0.73 mm, head width 1.5 mm. All body regions and segments distinct; no pigmentation (Figure 1A, B).
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).
Description of allotype male.
Length 0.95 mm, maximal width 0.23 mm across pereon 3, head length 0.11 mm, head width 0.19 mm, pleonal length 0.31 mm. All body segments distinct with scattered pigmentation (Figure 1J, K).
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.
The specific epithet, marianensis, refers to the type locality in the Mariana Islands.
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.
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