Stylet jaws of Chrysopetalidae (Annelida) Author Watson, Charlotte Author Faulwetter, Sarah text Journal of Natural History 2017 2017-11-24 51 47 - 48 2863 2924 http://dx.doi.org/10.1080/00222933.2017.1395919 journal article 10.1080/00222933.2017.1395919 1464-5262 5184119 Genus Shinkai Miura and Laubier, 1990 ( Figures 1e , 20a–g ; Tables 1 , 2 ) Type species: Shinkai sagamiensis Miura and Laubier, 1990 Material examined One specimen Shinkai longipedata Miura and Ohta, 1991 : SIO-BIC A1360, Costa Rica Margin, East Pacific (mCT-00401). Distribution Western and eastern Pacific Ocean. Habitat Shinkai species are considered obligatory symbionts of the bivalves Vesicomyidae Dall and Simpson, 1901 and Bathymodiolus Kenk and Wilson, 1985 from hydrothermal vents and cold seeps, ~ 1000–2000 m depth ( Miura and Laubier 1990 ). Shinkai longipedata specimens were collected from inside vesicomyid clams from active rock and bacterial mat seep sites off the Costa Rica Margin. A male and female adult pair were found to often inhabit a single clam between the gill lamellae and foot ( Aguado and Rouse 2011 ). General morphology Shinkai longipedata currently comprises the largest of all chrysopetalid species documented, with individuals attaining 150–200 mm in length and 300–400 segments ( Figure 1e ). Shinkai longipedata displays a simplified morphology of the anterior end with a blunt, lobe-shaped prostomium completely fused with segment 1 and with undifferentiated lateral antennae and palps (both short, cirri-like structures) ( Figure 20a, d ). Notopodia are very elongated and comprise large dorsal cirrophores with distal, short bulb-shaped dorsal cirri, lacking notoacicula and notochaetae. Neuropodia are shorter with distal, very short bulb-shaped ventral cirri and a small number of robust, short neuropodial hooks ( Figure 20a, b ) used for anchoring within gill tissues of the bivalve host. Simple neurochaetae exhibit internal longitudinal striations that are considered possible evidence of transformation from a fully camerated state, but internal horizontal diaphragms are lacking ( Aguado et al. 2013 ). Additional sensory structures appear minimal or are unknown; there is no identifiable body ciliation or nuchal development and eyes are absent. Figure 20. (a – d) Anterior end of Shinkai longipedata (mCT-00401), imaged through micro-CT and rendered in 3D. False-colour volume rendering: warm colours – higher densities; cold colours – lower densities. (a) dorsal view; (b) lateroventral view; (c) lateral view, virtually dissected; (d) dorsal view, virtually dissected; (e) micro-CT image of jaws; (f) micrograph of jaws; (g) line drawing (camera lucida), jaws (dotted lines indicate indistinct form). Pharynx and jaws Shinkai longipedata has an extremely short buccal organ in comparison to the overall length of the body. It extends between the first six anterior segments and consists of a short proboscis, an undifferentiated pharynx with broad internal septa and small posterior caeca. The very small paired stylets are situated close to each other at the top of the pharynx ( Figure 20c, d ) and exhibit a modified platelet/stylet form. The micro-CT scanned jaws have a pointed, distal-most extension and an anterior, curved platelettype jaw extending to a posterior elongation ( Figure 20e ). Micrographs resolve their form a little further with the distal extension forming a triangular ‘peak’ and the anterior jaw forming a fang that curves around to a grooved mid-jaw section ( Figure 20f, g ).