Peltogaster unigibba Yoshida & Naruse, 2016, n. sp.
Creators: name: Yoshida, Ryuta
name: Naruse, Tohru
Publisher: name: Zenodo
Year: 2016
Subjects: Biodiversity
Taxonomy
Animalia
Arthropoda
Maxillopoda
Kentrogonida
Peltogastridae
Peltogaster
Peltogaster unigibba

description: Peltogaster unigibba n. sp.
[Japanese name: Katakobu-nagahukuromushi] (Figs. 1–4)
Material examined. Holotype: RUMF-ZC-3067 (transverse sections, six slides), Osawa Beach in Kita-kou Bay, Haha-jima, Tokyo, Japan; depth 10 m; coll. T. Sasaki et al., 26 February 2015 (host, RUMF-ZC-4039, SL 2.0 mm). Paratype: RUMF-ZC-3068 (whole externa 3.0 mm in length, 1.1 mm in width); Kominato Bay, Chichi-jima, Tokyo, Japan; coll. T. Naruse, 18 March 2014 (host, RUMF-ZC-4040, SL 1.8 mm).
Description. External morphology: Externa irregularly elongated and ellipsoidal, approximately three times longer than maximum diameter (Figs. 1, 2). Externa appearing sinuous owing to vermicular movement when alive (Fig. 1), but appearing nearly straight after fixation in Bouin’s solution (Fig. 2). Embryos visible through mantle (Fig. 1). Left lobe incurved, prominently projecting 1.0 mm in front of mantle aperture. Mantle aperture slightly elevated as tube-like projection, opening at anterior end, slit-like in contracted specimens (Fig. 2). Shield elongated, with growth rings extending from stalk antero-posteriorly (Fig. 2).
Anatomy of externa: Mantle aperture not attached to ovary (Fig. 3 a). Left lobe projecting 905 µm long, not containing ovary (Fig. 3 a). Right lobe slightly projecting 200 µm. Tube-like mantle aperture 290 µm long (Fig. 3 a). Tube-like aperture wall 150 µm thick. In serial section, ovary semicircular (Fig. 3 b). Mantle cuticle of lobes, dorsal side, and ventral sides 25 µm, 35 µm, and 10 µm thick, respectively. Mesentery nearly as broad as ovary. Inner surface of mantle without septa.
Left and right colleteric glands 420 µm and 375 µm long, respectively. In serial section, anterior end of left colleteric gland reaching beyond level of anterior end of right colleteric gland (Fig. 4). Anterior and posterior parts of colleteric glands with unwrinkled walls, 6 µm thick (Fig. 3 b). Middle part of colleteric gland with few wrinkled walls, 20 µm thick (Fig. 3 c). Colleteric glands overlapped by stalk and receptacles (Fig. 3 d). Posterior end of left colleteric gland just reaching posterior end of stalk (Fig. 4). Posterior end of right colleteric gland reaching beyond posterior end of stalk (Fig. 4).
Ganglion 70 µm long, 88 µm wide, 42 µm high (Fig. 3 c). Ganglion having appearance of squat triangle, appearing in same section containing middle portions of colleteric glands, but never overlapped by stalk and receptacles.
Stalk 250 µm long and 220 µm wide, radiating from holdfast at point of insertion into body wall of host (Fig. 3 d). Stalk directly connected to ovary (Fig. 3 d).
Anterior ends of two receptacles emerging posterior to that of stalk (Fig. 4). Two receptacles 170 µm long, present on stalk, walls comparatively thin up to 10 µm thick, internal layer of polygonal cells 33 µm in diameter, shaped like straight tubes, gradually passing into receptacle ducts (Figs. 3 d, 4).Left and right receptacle ducts 355 µm and 340 µm long, respectively. Receptacle ducts about twice as long as receptacles. Receptacle ducts beginning at level of stalk, spaced well apart from each other, c. 65 µm. Anterior part (c. 200 µm) of receptacle ducts 150 µm in diameter, with thick walls 30 to 88 µm thick, shaped like straight tubes (Fig. 3 e). Remaining posterior part of receptacle ducts 80 µm in diameter, with walls 30 µm thick, slightly coiled, directed posteriorly on lateral surface of ovary (Fig. 3 f).
Retinacula absent.
Coloration. Mature externa pink (Fig. 1). Interna light green.
Position on host. On left side of pleon between second and third pleopods.
Etymology. The specific name unigibba is derived from a combination of the Latin uni, meaning one, and gibba, meaning hump, alluding to the left lobe that projects in front of the mantle aperture.
Distribution. Known from Haha-jima (type locality) and Chichi-jima, Bonin Islands, Japan.
Remarks. Peltogaster unigibba n. sp. clearly belongs to the genus Peltogaster, as it possesses the following characteristics: parasitism on hermit crabs, mature externa with a cuticular shield around the stalk, the absence of septa on the inner surface of the mantle, and the absence of lateral semicircular expansions in cross-sections of the ovary.
TABLE]. Character states of species of Peltogaster Rathke, 1842. Abbreviations: CG, colleteric glanđ; GA, ganglion; RE, receptacle; ST, stalk.
Species Externa shape Arrangement of Anterior lobes Position of anterior top of Reference (Length: Wiđth) mantle aperture internal structures
Peltogaster unigibba n. sp. Irregulatery elongateđ Anterior enđ Left lobe projecting CG> GA> ST> RE This stuđy ellipsoiđal (3:1)
Peltogaster aelaniticus Boschma, 1969 Little curveđ (2:1) Anterior enđ Lobes not projecting CG> RE> ST Boschma (1969)
GA unknown
Peltogaster boschmai Reinharđ, 1944 Right curveđ (2.5:1) Anterior enđ Lobes not projecting CG> RE> GA> ST Reinharđ (1944)
Peltogaster contorta Boschma, 1958 Little curveđ (2:1) Right siđe Left lobe projecting CG> RE> ST Boschma (1958)
GA unknown
Peltogaster curvata Kossmann, 1874 Right curveđ (2-3:1) Anterior enđ Both lobes projecting CG> RE> ST Høeg & Lützen (1985)
GA unknown
Peltogaster depressa Reinharđ, 1944 Little curveđ (2:1) Anterior enđ on đorsal Both lobes projecting GA> CG> RE> ST Reinharđ (1944)
siđe
Peltogaster lata Van Baal, 1937 Little or None curveđ (2:1) Anterior enđ Lobes not projecting CG> RE> ST Van Baal (1937)
GA unknown
Peltogaster lineata Shiino, 1943 Little curveđ (2.5:1) Anterior enđ Lobes not projecting CG> RE> ST Shiino (1943)
GA unknown
Peltogaster naushonensis Reinharđ, 1946 Little curveđ (2.5:1) Anterior enđ Lobes not projecting CG> GA> ST> RE Reinharđ (1946) Peltogaster reticulata Shiino, 1943 Strongly right curveđ Anterior enđ Both lobes projecting CG> RE> ST Shiino (1943)
(4:1) GA unknown
Peltogaster rugosa Boschma, 1931 Little curveđ (2:1) Anterior enđ Both lobes projecting CG> RE> ST Van Baal (1937)
GA unknown
Peltogaster senegalensis Guérin-Ganivet, 1911 Elongateđ (3:1) Anterior enđ Lobes not projecting ST>RE>CG Guérin-Ganivet (1911)
GA unknown
The external morphology of Peltogaster unigibba n. sp. is characterized by the anterior end of the left lobe being anterior to the mantle aperture and right lobe, and by the right and left lobes being asymmetric with respect to the mantle aperture (Fig. 2). Of the16 known species of Peltogaster now currently recognised,, P. unigibba n. sp. and P. contorta Boschma, 1958 share the feature of a left lobe that projects beyond the mantle aperture (Table 1; Boschma 1958: figs. 1, 2). However, P. uni gibba n. sp. can be easily distinguished from P. contorta as the mantle aperture of the former opens at the anterior end (Fig. 2), whereas that of the latter opens on the right side (Boschma 1958: figs. 1, 2).
The anterior end of the internal structure in Peltogaster unigibba n. sp. is arranged from the front as follows: colleteric glands, ganglion, stalk, and receptacle, successively (Fig. 4). Although the arrangements of the internal structures of P. ovali s Krüger, 1912 and P. purpurea (Müller, 1862) have not yet been described, that of P. u ni g i b ba n. sp. is similar to that of P. naushonensis Reinhard, 1946 (Table 1). Peltogaster unigibba n. sp. also shares the following features with P. naushonensis: live coloration, a slit-like mantle aperture, receptacle ducts about twice the length of the receptacles, and a lack of retinacula. These two species, however, differ in their internal structure: the colleteric glands of P. u ni g i b ba n. sp. reach beyond the posterior end of the stalk (Fig. 4), whereas those of P. naushonensis reach the middle of the stalk (Reinhard 1946).
Among fourteen Peltogaster species for which the internal structure has been studied, Peltogaster senegalensis Guérin-Ganivet, 1911, P. naushonesis, and P. unigibba n. sp. have anterior ends of the receptacle that are posterior to that of the stalk (Table 1; Fig. 4). In the other 11 species, the anterior end of the receptacle is anterior to that of the stalk. Peltogaster unigibba n. sp. and P. naushonesis differ from P. senegalensis in that the anterior ends of the colleteric glands are located beyond the posterior end of the stalk (Guérin-Ganivet 1911: Fig. 3). Peltogaster unigibba n. sp. can be differentiated from P. naushonensis by the above-mentioned characters.
The paratype (RUMF-ZC-3068) and holotype (RUMF-ZC-3067) share the same live coloration, host species, and left lobes whose distal ends project beyond the ends of the right lobes. The mantle aperture of the paratype, however, was barely visible because it was retracted into the mantle. The retraction of the mantle aperture might have been caused by the area surrounding the mantle aperture expanding owing to the absorption of water between the cuticle and epidermis during the thawing process.
Nauplius larvae of Peltogaster species are known to lack their nauplius eye in four out of 14 species (see Yoshida et al. 2011). Yoshida et al. (2011) mentioned the possibility that the absence of the nauplius eye may be a synapomorphy of Peltogaster species. Although the present study recognized embryos in the externa of the holotype of P. unigibba n. sp. (Fig. 1), we were unable to observe the condition of the eye as the embryos are still in the two-cell stage (Fig. 3 a).
The hermit crab genus Pagurixus includes approximately 40 species that live in tropical and warm temperate waters of the Indo-Pacific region (Osawa et al. 2013). Because it is difficult to find Pagurixus species in their preferred habitats (e.g. under stones, in crevices of dead coral, and in underwater caves), their ecology is not yet well-characterized. Similarly, little is known about the ecology of species that parasitize Pagurixus. Indeed, P. unigibba n. sp. is only the second species of rhizocephalan parasite that has been reported for Pagurixus (McDermott et al. 2010; Yoshida et al. 2015). Further study of host hermit crabs, which are difficult to collect, may lead to the discovery of additional parasite species.
description: Published as part of Yoshida, Ryuta & Naruse, Tohru, 2016, A new species of the genus Peltogaster Rathke, 1842 (Cirripedia: Rhizocephala: Peltogastridae) parasitizing the hermit crab Pagurixus boninensis (Melin, 1939) from the Bonin Islands, Japan, pp. 209-220 in Zootaxa 4139 (2) on pages 210-216, DOI: 10.11646/zootaxa.4139.2.5, http://zenodo.org/record/261479