Halichaetonotus sanctaeluciae Todaro, Dal Zotto, Bownes & Perissinotto 2011

The description is mainly based on an adult specimen, 146.5 μm in total length. Head rounded, slightly elongated along anterior/posterior axis, bearing a shallow cephalion but no pleural lobes or visible hypostomion; neck narrower than head, trunk sac-like, terminating in a furcate caudum. Body widths at the head/neck/trunk/caudum are 31/22.5/34.5/22 μm, at U11/27/58/81, respectively. Caudum of medium length (26.6 μm), paired laterally divergent adhesive tubes (20 μm in length) with a slightly swollen base (6.5 μm), covered by scales. Cuticular armature. Head, neck, and trunk covered dorsally and lateroventrally by alternating columns (7 dorsal, 1+1 lateral and 1+1 ventrolateral hydrofoil, 2+2 ventral) of 17–19 keeled scales, barely overlapping. On dorsal side, head and neck scales are round (3–5 μm in diameter), while trunk scales are oval to semi-elliptical (9.5 × 5.5 - 12.7 × 6.6 μm). In general, keel on dorsal scales extends beyond the edge of scales as short spiny process; however, on two lateral and one median trunk scales, at U63 and U71, respectively, keels form robust, very long spines projecting 26 μm beyond scales. On posterior trunk region are two oval, double keeled scales (5 × 4 μm) each anchoring a sensorial bristle at U79.5 and a couple of oval spiny scales (4 × 3.5 μm) bearing spines (4–5 μm long) protruding into the furcal indentation. Lateral and ventrolateral spines of hydrofoil scales bearing flattened lamellae, most of which taper into a long hairy process; lamellae bearing spines of the lateral scales are longer than related ventrolateral ones (up to 25 vs up to 19), while lamellae of a column are longest at mid trunk. On ventral side, up to five keeled scales, 3–4 μm long, cover the fleshy portion of each furcal branch; the interciliary field appears naked except for two pairs of oval keeled scales in the perianal region; scales of anterior pair are larger (9.5 × 4.5 μm) than posterior ones (6.0 × 3.5 μm). Ventral ciliation. paired longitudinal bands extending from U03 to approximately U77; each broadly club-shaped anteriorly, but narrowing considerably from the posterior pharyngeal region; bands approach each other immediately behind the mouth, but remain separate throughout their entire length; individual cilia are about 11 μm in length. Digestive tract:Mouth of medium size (ca.6 μm in diameter), projecting very slightly ventrally and leading progressively into a 32 μm long pharynx; pharynx muscular, roughly cylindrical (8 μm in diameter), showing a bulb anteriorly (12 μm in diameter); two cuticular teeth are visible within the bulb; pharynx connected to sack-like intestine at pharyngeo-intestinal junction at U25; intestine straight, narrowing posteriorly, anus ventral at U77. Reproductive tract. Three specimens were in parthenogenic phase, two of which with a large egg filling much of the trunk.