Mesomyzostoma okadai

Greg W. Rousea, Deborah Lanterbecq, Mindi M. Summersa & Igor Eeckhaut, 2016, Four new species of Mesomyzostoma (Myzostomida: Annelida), Journal of Natural History 50 (1), pp. 1-23 : 13-15

Mesomyzostoma okadai Rouse, Lanterbecq, Summers and Eeckhaut sp. nov.

( Figure 5 )

Mesomyzostoma n. sp. 2 in Lanterbecq et al. (2006, 2009) Mesomyzostoma n sp. 2 in Summers and Rouse (2014)

Material examined

Off Misaki Marine Biological Station, Sagami Bay (Japan), 35°9.788ʹ N, 139°36.732ʹ E, 10 – 15 m depth. Specimens from seven A. japonica individuals. Collector: Greg Rouse, 13 – 28 May 1998. One to five per host. Holotype (SIO-BIC A4079) in 70% ethanol following glutaraldehyde fixation. Paratypes: SIO-BIC A4080, A4081, A4082, seven intact or incomplete paratypes in 70% ethanol following formalin fixation; SIO-BIC A4083, A4084, two specimens in 70% ethanol following glutaraldehyde fixation; SIO-BIC A4085, one specimen fixed and preserved in 95% ethanol; SIO-BIC A4086, two specimens used for SEM observations; SAM E3406, one complete in 70% ethanol following formalin fixation. One incomplete specimen, from paratype lot SIO-BIC A4082, dissolved in bleach for observation of parapodial hook apparatus and another, originally from paratype lot SIO-BIC A4085, digested for DNA extraction and molecular phylogenetic analyses.

Etymology

Named for Dr Yaichiro Okada (deceased) in recognition of his important early work on Japanese myzostomes.

Diagnosis

Mesomyzostoma with elongated, flat, ribbon-like, crenulate body. Muscular pharynx present; eversibility unknown. No marginal cirri. Five chaetigers along body, chaetae close to trunk margin but parapodia absent or very low and then only anteriorly. Emergent hooks small with thin shaft, tip curving to 90° with respect to shaft. No replacement hooks. Aciculae as long as emergent hooks, but twice as thick at the base. Manubrium flag-like, developed on one side. Numerous ovoid lateral organs located on humps all along body margin. Mouth at extreme anterior end, cloaca posteriorly. Penes absent, but seminal vesicles at third chaetiger. Hermaphrodite. Parasite of crinoid coelom.

Description

Holotype 12 mm long, 0.8 mm wide and about 0.2 mm thick. Body flat, curled dorsally with anterior end sharper than posterior end, latter slightly broken. Body margin crenulate with about 37 lateral organs visible on left and 33 on right side. Openings of lateral organs circular, c.40 µm in diameter, often located on small humps 80 µm high and spaced regularly at 200 – 400 µm intervals. Pharynx in pointed, anterior tip of body, 300 µm long ( Figure 5 D). Mouth oval, 50 µm in diameter. First two chaetigers with low parapodia, others, if present, not observed. First chaetiger located 0.2 mm from anterior end, second 1 mm behind first. Remaining chaetigers distributed along body.

Entire paratypes ranging from 2 to 14 mm long and 0.8 to 1.5 mm wide ( Figure 5 A). Most paratypes with anterior and posterior ends very pointed, with mouth and cloaca respectively ( Figure 5 A, C, D). All paratypes with crenulate trunk margin due to presence of hump-shaped lateral organs ( Figure 6 E). Lateral organs present all along trunk margin, spaced 100 μm (in contracted portion of body) to 400 μm apart, specimens having 20 – 70 pairs ( Figure 5 A, D, E). Five chaetigers but only up to two pairs of low parapodia present, both of latter located anteriorly. Parapodia as small humps 0.1 mm high ( Figure 5 E). Each emergent chaeta with an acicula, but no replacement hook ( Figure 5 F). Emergent hook up to 0.4 mm long, shaft moderately thick, with distal third bowed slightly outward, tip curving 90° ( Figure 5 F). Aciculae of similar size to emergent hooks but, being c.80 µm thick at base, twice as thick as hooks. Manubrium visible as small projection on one side ( Figure 5 E).

Individuals found in coelom of pinnules close to gonads and in coelom of arms ( Figure 5 B). Live specimens with translucent margin, yellowish ovaries, orange digestive diverticula. Individuals in life less translucent than M. katoi , but still allowing observation of some internal organs ( Figure 5 A – D). Some specimens with pair of white seminal vesicles at level of third chaetiger, full of sperm/spermatophores.

Remarks

Mesomyzostoma okadai sp. nov. is found in the same host ( A. japonica ) and same geographic location as M. katoi . It also occurs in the same body region of the host and both species could be found in the same individual host. Mesomyzostoma okadai sp. nov. is morphologically distinct from M. katoi in its: (1) crenulate body margin from multiple lateral organs located on small humps; (2) its greater body size as adults and relative opacity. The two species are also very distinct in terms of DNA data, with M. okadai sp. nov. being the sister taxon to the other sequenced Mesomyzostoma species (see below, Figure 8 ).

Figures

Figure 5. Mesomyzostoma okadai sp. nov. (A) Paratype (SIO-BIC A 4080) in life. (B) Same specimen still partly inside arm of host crinoid. (C) Posterior end of paratype in A showing chaetigers 4 and 5, posterior cloaca and uterus, both fi lled with eggs. (D) Anterior end of paratype shown in A showing small fi rst chaetiger, pharynx and digestive diverticulae. (E) Detailed scanning electron micrograph view of anterior margin of paratype (SIO-BIC A 4086), showing lateral organs and low parapodia. (F) Micrograph of a parapodium digested in bleach showing the parapodial hook apparatus. Abbreviations: 1 – 5 refer to chaetigers; ac, acicula; c, cloaca; dt, gut diverticulae. go, arm gonad of host; h, hook; lo, lateral organ; ma, manubrium; pi, pinnule of host; u, uterus; ph, pharynx; m, mouth.

Figure 6. Mesomyzostoma botulus sp. nov. (A) Live adult being dissected from oral disc of host. (B) Juvenile stage with fi ve chaetigers. (C) Juvenile stage with four visible chaetigers and possible everted pharynx. (D) Larger specimens in life, one on left adult holotype (SAM E 3964) with possible everted pharynx. One on right paratype (SAM E 3409), arrow pointing to emergent chaeta visible via scanning electron micrograph (SEM) in E. (E) SEM view of paratype (SAM E 3409) showing only a single visible emergent chaeta. (F) Light micrograph of anterior end of juvenile specimen showing small hooks emerging from the body margin. (G) SEM view of smaller paratype on same stub (SAM E 3409) also showing only single visible emergent chaeta on its cylindrical body. (H) Micrograph of a parapodium digested in bleach showing the acicula. (I) Micrograph of a parapodium digested in bleach showing the emergent hook. (J) Detailed SEM view of the single emergent hook of larger paratype on stub SAM E 3409. Abbreviations: 1 – 5 refer to chaetigers; ac, acicula; ch, chaeta; h, hook; ma, manubrium; ph?, everted pharynx?

Figure 8. Phylogenetic relationships of the new Mesomyzostoma species: single most parsimonious tree resulting from the combined dataset alignment. Numbers above and below branches indicate support values> 50 % obtained in the maximum likelihood (100 bootstrap pseudoreplicates) and maximum parsimony analyses (1000 jackknife replicates), respectively. An asterisk (*) indicates> 95 % support for both analyses. There was low support for the relationships among the various Mesomyzostoma species and the maximum likelihood and maximum parsimony results showed some topological di ff erences. Uncorrected pairwise distance percentage values for COI sequences for Mesomyzostoma species from di ff erent localities or with obvious divergences are given. These are specimens of M. botulus sp. nov. from Papua New Guinea and Australia, M. cf. reichenspergeri from Papua New Guinea (PNG) and Australia, M. lanterbecqae from PNG. Unfortunately COI comparisons were not possible between M. katoi (Japan) and M. cf. katoi (PNG) and between Mesomyzostoma B and M. lobus sp. nov. Hypomyzostoma fasciatum was used as outgroup in the analyses, but excluded from the fi gure.