Mesomyzostoma lobus 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 : 11-13

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

( Figure 4 )

Mesomyzostoma n. sp. 1 in Lanterbecq et al. (2006, 2009, 2010) Mesomyzostoma n. sp. 1 in Summers and Rouse (2014)

Material examined

Horseshoe Reef, Lizard Island Great Barrier Reef (Australia), 14°38.655' S, 145°27.267ʹ E; 5 m depth. Collector: Greg Rouse, 17 November 2001. Many body parts of several specimens in several Clarkcomanthus albinotus Rowe, Hoggett, Birtles and Vail, 1986 (Comatulidae) . Holotype (SAM- E3963) mostly fixed in formalin and preserved in 70% ethanol, part placed in 95% ethanol and used for DNA sequencing. Paratypes: SAM- E3412, E3413, fixed in formalin and preserved in 70% ethanol; SIO-BIC A4076, 3 (juveniles) fixed in formalin and preserved in 70% ethanol; SIO-BIC A4077, fragments fixed in formalin and preserved in 70% ethanol; SIO-BIC A4078, fragments fixed in 95% ethanol. One incomplete specimen, from paratype lot SIO-BIC A4076, dissolved in bleach for observation of parapodial hook apparatus. Another specimen, from paratype lot SIO-BIC A4078, destroyed for DNA extraction and molecular phylogenetic analyses.

Etymology

Noun from Latin, lobus , referring to the lobate nature of the specimens.

Diagnosis

Large Mesomyzostoma with body having central trunk giving rise to multifurcating lobes. No introvert, no lateral organs or cirri. Number of chaetigers unknown, inconspicuous chaetae present, located close to trunk margin. Emergent hooks small with thin shaft, tip curving 80° with respect to shaft. No replacement hooks. Mouth at end of anterior part of trunk. Simultaneous hermaphrodite. Parasitic in coelom of crinoid oral disc.

Description

Holotype in coelom of swollen oral disc of crinoid host ( Figure 4 A, B). Extracted incomplete, anterior piece 3 mm long, 2 mm wide ( Figure 4 C). Central part of trunk 1 mm wide and 0.2 mm thick, flattened with mouth anterior, and numerous curled multifurcating lobes ( Figure 4 C – E) extending from lateral and ventral sides of trunk ( Figure 4 C). No introvert, no lateral organs, no cirri. One chaetiger present in holotype just behind mouth, no obvious parapodia ( Figure 4 C). Lobes with digestive diverticulae, many eggs and spermatocysts. Simultaneous hermaphrodite. Emergent hooks 0.2 mm long with thin shaft, tip curving 80° with respect to shaft ( Figure 4 F). No replacement hooks. Aciculae 0.35 mm long and 0.06 mm wide at base, upper part with manubrium same width as acicula, lower part funnel-shaped with base three times thicker than hook. Manubrium triangular, developed on both sides ( Figure 4 F). Colour of individuals in life white and orange ( Figure 4 B – D). Incomplete specimens 1 to 4 mm long ( Figure 4 D).

Remarks

Mesomyzostoma lobus sp. nov. was observed filling the coelom of crinoid discs ( Figure 4 B). The morphology of this species is unusual for myzostomes and our attempts to extract an intact parasite from the host coelom failed. It was not impossible to determine whether one individual or more infested one crinoid. The multi-lobed, folded body that characterizes M. lobus sp. nov. also occurs to a lesser degree in Asteromyzostomum scotiae Summers and Rouse, 2014 in Summers, Al-Hakim, et al. 2014 , which is ectoparasitic on asteroids. Heinzeller et al. (1995) documented a myzostome that invaded connective tissue of Comanthus parvicirrus ( Müller, 1841) collected near Lizard Island. The authors did not name the species and their description was based on sections of oral discs of C. parvicirrus . The worm they described presented presumably two stages, a protandric hermaphroditic ectosymbiont and female endosymbiont ( Heinzeller et al. 1995). Mesomyzostoma lobus sp. nov. differs from Heinzeller et al. ’ s (1995) species by (1) its simultaneous hermaphroditic condition and (2) its localization in the coelomic channels of the host disc. Mesomyzostoma lobus sp. nov. was first reported (as Mesomyzostoma sp. 1) as being a parasite of Clarkcomanthus littoralis Rowe, Hoggett, Birtles and Vail, 1986 in Lanterbecq et al. (2006) , but subsequent work on the hosts ( Summers and Rouse 2014) has shown that this was Clarkcomanthus albinotus . The two specimens of M. lobus sp. nov. sequenced for DNA did not show any differences for any gene.

Figures

Figure 4. Mesomyzostoma lobus sp. nov. (A) Swollen oral disc of host crinoid Clarkcomanthus albinotus. (B) Oral disc dissected open, revealing coiled gut of host and emerging parts of a specimen or specimens of M. lobus sp. nov. (arrows). (C) Anterior end of holotype showing mouth and anterior gut (arrow) and fi rst chaetiger (arrowhead). (D) Parts of a specimen or specimens live after extraction from the host. (E) Scanning electron micrograph view of a specimen fragment. (F) Micrograph of a parapodium digested in bleach showing the parapodial hook apparatus. Abbreviations: anc, anal cone of crinoid oral disc; ac, acicula; h, hook; m, mouth of crinoid oral disc; ma, manubrium; tr, central trunk from which lobes arise.