Mesomyzostoma botulus Rouse, Lanterbecq, Summers and Eeckhaut

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 : 15-17

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

( Figure 6 )

Mesomyzostoma n. sp. 3a in Lanterbecq et al. (2006, 2009) Mesomyzostoma n. sp. 3b in Lanterbecq et al. (2006, 2009) Mesomyzostoma n. sp. 3 in Summers and Rouse (2014)

Material examined

North Point, Lizard Island Great Barrier Reef (Australia), 14°38.655ʹ S, 145°27.267ʹ E; 10 – 15 m depth. Collector: Greg Rouse, 18 November 2001. One to three individuals found in several Comaster schlegelii ( Carpenter 1881) (Comatulidae) . Holotype (SAM E3964) in 70% ethanol following formalin fixation. Paratypes: SIO-BIC A4087, seven specimens in 70% ethanol following formalin fixation; SAM E3409, two specimens used for SEM observations. One individual, from paratype lot SIO-BIC A4087, dissolved in bleach for observation of parapodial hook apparatus. One juvenile and one adult from the type locality but not excluded from type series, digested for DNA extraction and molecular phylogenetic analyses. Other material: Madang Lagoon, Papua New Guinea, 5° 8.16' S, 145°48.6ʹ E; 3 m deep; One specimen in Comaster audax Rowe, Hoggett, Birtles and Vail, 1986 (Comatulidae) . Collectors: Greg Rouse and Mindi Summers, 11 December 2012 (SIO-BIC A3721).

Etymology

Noun from Latin botulus , sausage, referring to the general shape of the specimens.

Diagnosis

Small Mesomyzostoma with flattened (juvenile) or nearly cylindrical (adult) body. Narrow anterior protrusion (eversible pharynx?) present in some specimens, but no lateral organs or cirri. Adults with none to three pairs of chaetae ventrally along anterior part of body, close to trunk margin; five pairs visible along body in juveniles. Emergent hooks small with thin shaft, tip curving to 90° with respect to shaft. No replacement hooks. Penes absent. Adults hermaphroditic. Parasitic in coelom of crinoid oral disc.

Description

Holotype curled, 4 mm long (6 mm in life). Body ovoid in cross-section, thicker laterally than dorso-ventrally, thickest at mid-body, 0.5 mm (1 mm in life), tapering anteriorly and posteriorly ( Figure 6 D). Possible small pharynx, 0.1 mm long, everted anteriorly. No lateral organs, parapodia, chaetae or cirri visible. Five larger complete paratypes, all curled in fixation, 2 – 6 mm long, 0.8 – 1.2 mm wide ( Figure 6 A – E, G). No external organs visible except for possible everted pharynx ( Figure 6 C). One small paratype with five chaetigers, no parapodia ( Figure 6 B), another with four visible pairs of chaetae and possible everted pharynx ( Figure 6 C). Two paratypes studied with SEM both cylindrical, 2 and 2.2 mm long, no external cilia, only one visible emergent hook each ( Figure 6 D, E, G).

Emergent hooks inconspicuous ( Figure 6 F), with very thin shaft, tip curving 90° with respect to shaft ( Figure 6 I, J), no replacement hooks. Aciculae 50 µm long, 5 μm wide basally. Manubrium very small, developed on one side ( Figure 6 H, I).

Colour of individuals in life variable, adults opaque, yellow to orange with translucent body margins ( Figure 6 A, D), smaller juveniles white and translucent, some with orange gut diverticulae ( Figure 6 B, C, D). Spermatocysts and eggs observed in all parts of trunk.

Remarks

A juvenile and adult of M. botulus sp. nov. found in Comaster schlegelii in Australia are morphologically different from the other specimens as described above, yet do not show any variability in the analysed gene sequences, confirming that they belong to the same species. The possibility that M. botulus sp. nov. has an eversible pharynx needs further confirmation, as it was not clear in the available specimens. A specimen of Mesomyzostoma from Papua New Guinea, which differed in COI sequence from two Australian specimens by a distance of 5.7% or 6.4%, respectively, is provisionally referred to here as M. botulus sp. nov., although it was found in a different host, Comaster audax ( Rowe et al. 1986) .

Figures

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?