Thrausmatos Watson, 2001

Watson, Charlotte & Faulwetter, Sarah, 2017, Stylet jaws of Chrysopetalidae (Annelida), Journal of Natural History 51 (47 - 48), pp. 2863-2924 : 2890-2891

Thrausmatos Watson, 2001

( Figure 16a–d ; Tables 1, 2)

Type species: Thrausmatos dieteri Watson, 2001

Material examined

One specimen Thrausmatos sp. : SMF 24382, Pakistan, NW Indian Ocean (mCT-00089); 2 specimens Thrausmatos dieteri: NTM W . 26201, Fiji, SW Pacific (mCT-00040; mCT-00093) .

Distribution

Thrausmatos dieteri was originally recorded from the Lau and North Fiji Basins, and Lihir south of New Guinea, South Western Pacific ( Watson 2001). An undescribed species has recently been found at the Makran subduction zone, between the Eurasian Plate in the north and Arabian Plate in the south, North-Western Indian Ocean (CW, unpub.). A North-Eastern Atlantic species, Thrausmatos senghorensis Watson in Watson et al., 2014, is now considered to belong to a new generic taxon (Watson and Rouse, in prep.).

Habitat

Thrausmatos inhabits hydrothermal vents and cold seeps to ~ 2700 m in areas associated with a speciose molluscan and vestimentiferan fauna and is especially common in washings of Bathymodiolus brevior Von Cosel, Métivier and Hashimoto, 1994 ( Watson 2001; Pleijel, pers. comm.). Vent geomorphology consists of basalt with massive sulphides and fissures, overlain in part with sediment, and hydrocarbon enriched sediments and indurated muds present at cold seep sites ( Watson 2001).

General morphology

Thrausmatos species are relatively broad, elongate chrysopetalid polychaetes possessing multi-layered, symmetrical main paleae that overlap and cover the dorsum; a discrete

median fascicle is lacking ( Figure 16a ). Individuals are rarely collected entire and fragment easily. Sensory structures include a small prostomium (not fused with anterior segments) that lack complex eyes (though simple eye spots are sometimes present), a ciliated ovoid caruncle with posterior notch and pigment spots and basally ovoid shaped palps with cirriform, distally retractile tips. The epidermis is thickened and covered in bacteria and appears to lack ciliation; glandular ventral pads are prominent ( Watson 2001; CW pers. obs.).

Pharynx and jaws

Thrausmatos possesses a proboscis which in some scans appears most clearly as two large lobe-like structures that may be integral to the proboscis itself or consisting of overlying glandular lobes ( Figure 16b ). The pharynx is long, slender and partially differentiated with a muscular constriction and extending to ca. segment 20; caeca are difficult to distinguish. The pharynx anterior to the constriction, wherein the short pair of elongate jaws is situated, appears rounded ( Figure 16b, c ). The jaws sit close to one another and interlink along most of their length, diverging slightly at the distal quarter. Jaw margins are straight with an inner raised longitudinal ridge and an outer mid-way diagonally sloping, rounded projecting ridge is present ( Figure 16d ). Thrausmatos stylets are slightly triangular at the basal end; overall jaws appear less strongly calcified ( Figure 16d ). The particularly small size of jaws does not allow fine focus with CT imaging.

Figures

Figure 16. Anterior end of Thrausmatos dieteri (mCT-00040), imaged through micro-CT and rendered in 3D. (a) dorsal view; (b) lateral view, virtually dissected to expose pharyngeal structures. (c) dorsal view, virtually dissected to expose pharyngeal structures; (d) jaws, left: dorsal view, middle: lateral view, right: cross sections through the jaw along its length.