Aphelochaeta zebra sp. nov.

Figures 3 F, 4C–D, 6B

Material examined. Golfo Dulce; Coral reef, Punta Islotes, 8°43ʹ41″N; 83°23ʹ8″W. Col: Ana Fonseca by SCUBA, Holotype (MCZ 132807).

Description. Holotype complete, in two pieces (anterior 17. 2 mm, posterior 2.2 mm), 19.4 mm long for 154 setigers, thoracic width 0.7 mm, abdomen 0.4 mm wide, swollen posterior end 0.6 mm wide. Thoracic region with 22 crowded segments 25–30 times wider than long, dorsum inflated, dome-like (Fig. 4 C, D), venter flattened with mid-ventral line, groove absent; abdominal segments of uniform width, approximately two times as wide as long, rounded dorsally, flattened ventrally, first 21 abdominal setigers with narrow mid-ventral groove, subsequent setigers with mid-ventral line, groove absent; posterior end inflated dorso-ventrally, with 29 setigers; pygidium with small dorsal and ventral lobes (Fig. 3 F). Color in alcohol brownish yellow.

Prostomium short, rounded, nuchal organs not apparent, prostomium retracted into peristomium; peristomium short, wider than long, with three annulations, anterior annulation approximately one-half length of two posterior annulations, with short, narrow, median dorsal ridge on posterior annulations (Fig. 4 D); dorsal tentacles emerging from posterior margin of peristomium. First pair of branchiae immediately lateral to dorsal tentacles on anterior edge of setiger 1; subsequent thoracic branchiae at posterior border of notopodial lobe at medial surface of shoulders, gradually moving closer to, and becoming adjacent to notosetae by setiger 10; abdominal branchiae emerging from conical notopodial lobe dorsal to notosetae. Thoracic parapodia swollen lobes forming weak shoulders along lateral surface of body, notosetae emerging from apex of elevated shoulder, neurosetae from slightly elevated cone ventral to notopodia; abdominal notosetae emerging apically from short, cone-like projections, neurosetae emerging from body wall at base of notopodial cone.

Setae long, smooth capillaries, with minute, ragged-edged scale-like structures visible with SEM (Fig. 6 B); thoracic parapodia with up to seven setae in each fascicle; abdominal segments with up to 24 notosetae and seven neurosetae, reducing to eight notosetae and four neurosetae in far posterior; swollen posterior end with up to five notosetae and three neurosetae, reducing in number posteriorly.

Methyl green staining pattern. Body staining dull greenish-grey, ventral groove of posterior thoracic region dark green, pygidium unstained; swollen posterior region with intersegmental lines staining dark green providing a striped zebra-like pattern (Fig. 3 F).

Remarks. Aphelochaeta zebra sp. nov. is similar to A. multifilis in having a broadly rounded prostomium, a relatively broad peristomium and a robust body throughout. However, the posterior end of A. multifilis is inflated with paired ventral grooves while that of A. zebra sp. nov. is more greatly inflated, possesses only a single midventral groove, and has a distinctive, striped methyl green staining pattern within the intersegment grooves. This striped staining pattern on the swollen posterior region of A. zebra sp. nov. is somewhat similar to that of A. tigrina Blake, 1996 but in A. tigrina the staining occurs on the entire ventral surface of individual setigers except for the intersegmental region while in A. zebra sp. nov. staining does not occur in the median area of each setiger but stains intensely in the intersegmental grooves. Aphelochaeta zebra sp. nov. shares a shortened anterior region, a triannulate peristomium, crowded thoracic segments and smooth setae with A. guttata Doner & Blake, 2009 but the first branchiae occur on the peristomium in that species while they occur on setiger one in A. zebra sp. nov.

Etymology. The species name is from the Portuguese zebra meaning wild ass and refers to the striking methyl green staining pattern of the swollen posterior end.

Distribution. Known only from a coral reef at Punta Islotes, Golfo Dulce, Costa Rica.