Scyracepon distincta, An & Boyko & Yu, 2012

An, Jianmei, Boyko, Christopher B. & Yu, Haiyan, 2012, A review of the genus Scyracepon Tattersall, 1905 (Crustacea: Isopoda: Bopyridae), with description of a new species from China, Journal of Natural History (J. Nat. Hist.) 46 (45 - 46), pp. 2889-2895 : 2890-2895

Scyracepon distincta sp. nov.

( Figures 1 , 2 )

Material examined

Infesting right branchial chamber of female Jonas distinctus (De Haan) (CL = 36.56 mm) ( Figure 1A ), a hole on the carapace of the host. Holotype ♀: South China Sea, Stn, 6062, 21 ◦ 45 ′ N, 113 ◦ 30 ′ E, 32 m, 15 July 1959, Coll. Xiutong Ma ( Figure 1B, C ) ( CIEC606201 ) . Allotype ♂: same data as holotype ( CIEC606202 ) . Paratypes: same data as holotype, but infesting left branchial chamber, 1♀ ( CIEC606203 ) , 1♂ ( CIEC606204 ) .

Description of holotype female (CIEC606201)

Length 5.85 mm, maximal width 4.28 mm across third pereomere, head length 1.38 mm, head width 1.97 mm, pleon length 1.54 mm. All pereon segments distinct. No pigmentation ( Figures 1B, C , 2A, B ).

Head wider than long with distinct frontal lamina, laterally expanded into lobes; anterior edge bilobate. Eyes absent ( Figure 2A ). Antennules and antennae of three and five articles, respectively ( Figure 2C ). Maxilliped ( Figure 2D ) subquadrate, upper edge with two tufts of setae, large palp with setae on inner margin. Barbula ( Figure 2E ) with two smooth falcate projections on each side and a pair of low-rounded triangle-shaped projections medially.

Pereomeres distinct, third one broadest. First four pereomeres with conspicuous dorsolateral bosses, second and third pereomeres with large tergal projections. Small mid-dorsal projections on last two pereomeres, other pereomeres without projections. Brood pouch completely covered by oostegites ( Figure 2B ). First oostegite ( Figure 2F, G ) with equal length articles, internal ridge bearing two large and two small projections on lateral half, posterolateral point blunt, posterior edge fringed with setae. Pereopods longer posteriorly, second pereopod ( Figure 2H ) with sharp dactylus and carinate basis. Seventh pereopod ( Figure 2I ) with reduced dactylus and non-carinate basis.

Pleon of six pleomeres, first segment distinct, second to fifth segments with fused strip in middle region but segments distinguishable laterally. First five pleomeres produced into digitate and tuberculate lateral plates on both sides. Five pairs of biramous digitate and tuberculate pleopods ( Figure 2J ), endopodites approximately half as long as exopodites. Uniramous uropods, digitate, slightly shorter than exopodites on pleomere 5.

Description of allotype male (CIEC606202)

Length 2.37 mm, maximal width (across fifth pleomere) 0.74 mm, head length 0.16 mm, head width 0.37 mm, positioned on the pleon of the female ( Figure 1C ). Pereon segments distinct, but pleomeres partially fused, no pigmentation ( Figure 2K,L ).

Head elliptical, lacking eyes ( Figure 2K ). Antennules and antennae of two and four articles respectively, without setae ( Figure 2M ). Third to sixth pereomeres subequal in width, margins truncate. All pereomeres and first pleomere with large ovate midventral projections ( Figure 2L ). All pereopods subequal in size and shape, with short dactyli, inflated propodi, reduced carpi and meri ( Figure 2N ).

Pleon of six pleomeres, first two segments distinct, other segments can only be distinguished by lateral indentations. First pleomeres with large ovate midventral projection. First five pleomeres with laterally placed uniramous tuberculate pleopods, uropods lacking ( Figure 2L ).

Description of paratypes

Female length 5.64 mm, maximal width across pereomere 3, 4.25 mm, head width 1.83 mm, head length 1.31 mm. Seventh pereomere with much smaller mid-dorsal boss, other characters same as in holotype. Male length 2.19 mm, head length 0.14 mm, head width 0.35 mm. All pleomeres partially fused and only distinguished from lateral indentations.

Etymology

The name distincta refers to the host’s species name, but formed as a feminine adjective to conform to the feminine genus, and also refers to the median part of the female pleon bearing a distinctly fused strip.

Distribution and hosts

Infesting Jonas distinctus (De Haan, 1835) , South China Sea.

Remarks

The genus Scyracepon cannot be distinguished from several other genera of ionine bopyrids (e.g. Trapezicepon Bonnier, 1900 , Allokepon Markham, 1982 ) by the number of the mid-dorsal projections present on the females ( Shiino 1936), but the coalescence of the male pleomeres places the new species in Scyracepon Tattersall, 1905 , which now contains seven described species ( Table 1) infesting seven different brachyuran families. The record of the new species from a host in Corystidae is the first record for any species in that family bearing parasitic isopods. Note that Scyracepon australiana Markham, 2010 , is provisionally retained in the genus but may not belong here, based on the lack of prominent mid-dorsal bosses as well as the shape of the first oostegite.

Scyracepon distincta sp. nov. can be distinguished from other species in the genus by females having reduced mid-dorsal projections only on pereomeres 6 and 7 and the pleomeres having a median fused strip. Scyracepon distincta sp. nov. appears most similar to Scyracepon hawaiiensis Richardson, 1910 but differs from it in the lack of prominent mid-dorsal projections on the pereomeres (small projections on pereomeres 6 and 7 versus small projections on pereomeres 5 and 6 and a large tapered projection on pereomere 7 in S. hawaiiensis ) and the more convex margin of the distal lobe of oostegite compared with the same in S. hawaiiensis . The males of the two species are very similar, but Richardson (1910) did not describe or illustrate the midventral region so a complete comparison cannot be made.

Collection depths for species of Scyracepon range from intertidal to 620 m (see Table 1) but in only five of the nine published records for this genus were the depths specified in which the hosts were collected. However, for the hosts Eriphia scabricula and Austroplax tridentata , their collection depth can be inferred as intertidal because that is the only habitat known for those species ( Davie 2002). For the hosts Maja japonica and Rochinia hertwigi , the potential depths of collection are of a much larger range (e.g. 68–120 m and 300–700 m, respectively, see Sakai 1976; Griffin and Tranter 1986; Macpherson 1991). Based on host data, only three Scyracepon species occur on shallow water hosts ( Scyracepon oceanicum , S. australiana and S. distincta sp. nov.) and all of the species parasitizing spider crab hosts (Majoidea), as well as from geryonid and goneplacid hosts, are from deeper waters.

The double branchial infestation seen in the corystid host for S. distincta sp. nov. is not unknown for bopyrids (e.g. Roccatagliata and Lovrich 1999; Kazmi and Boyko 2005; Vinuesa and Balzi 2010), but single infestations are the norm.

Key to the species of Scyracepon Tattersall, 1905

1. No mid-dorsal tubercles, keels or lobes....................... S. australiana Mid-dorsal tubercles, keels or lobes on some pereomeres.................. 2

2. Mid-dorsal tubercles, keels or lobes on all pereomeres.................... 3 No mid-dorsal tubercles, keels or lobes on pereomeres 1–3................ 4

3. Mid-dorsal keels on pereomeres 1–4................................ S. levis Mid-dorsal tubercles on pereomeres 1–5..................... S. tuberculosa

4. Mid-dorsal lobe on pereomere 7 much larger than that on 6............... 5 Mid-dorsal lobe on pereomere 7 subequal or smaller to that on 6.......... 6

5. Mid-dorsal lobe on pereomere 7 not reaching past posterior margin of pleomere 1................................................. S. hawaiiensis Mid-dorsal lobe on pereomere 7 well exceeding posterior margin of pleomere 5...................................................... S. quadrihamatum

6. Cephalon medially bilobed, longer than wide; pleopods reaching anteriorly to pereomere 5................................................. S. oceanicum Cephalon not medially bilobed, as long as wide; pleopods reaching anteriorly to pereomere 1........................................ S. distincta sp. nov.

Figures

Figure 1. Scyracepon distincta sp. nov. and its host. (A) Host: Jonas distinctus (De Haan, 1835), showing infestation of both branchial cavities. (B) Dorsal view of holotype female isopod (CIE606201). (C) Ventral view of holotype female isopod with male in naturally occurring position in pleopod region, oriented perpendicularly to female body. Scale bars 1.7 mm (A), 1 mm (B, C).

Figure 2. Scyracepon distincta sp, nov. Holotype female (CIEC606201) (A–J): (A) dorsal view; (B) ventral view; (C) left antenna and antennule; (D) right maxilliped, external view; (E) barbula; (F) right oostegite 1, external view; (G) right oostegite 1, internal view; (H) right pereopod 2; (I) right pereopod 7; (J) left first pleopods. Allotype male (CIEC606202) (K–N): (K) dorsolateral view; (L) ventral view; (M) antenna and antennule; (N) pereopod 1. Scale bars 1 mm (A, B), 0.10 mm (C, M, N), 0.41 mm (D), 0.36 mm (E), 0.60 mm (F, G), 0.19 mm (H, I), 0.64 mm (J), 0.47 mm (K, L).