Nereis ambigua Treadwell 1937

Nereis ambigua Treadwell, 1937 reinstated

Nereis ambiguus Treadwell, 1937: 149–150, pl. 2 figs 19–24.

Nereis riisei – Monro 1933: 43–44 (living pigmentation). — Hartman 1940: 221–222, pl. 33 fig. 37; 1956: 279 (synonymy). — Reish 1968: 77. — Fauchald 1977b: 31, fig. 8c–e (partim, only Pacific Panama specimens). — de León-González & Solís-Weiss 2000: 560–561. — Dean 2001: 57–58, figs 44–48 (non Grube & Örsted in Grube, 1857).

Diagnosis

Nereis with posterior eyes completely exposed; tentacular belt 2 × as long as first chaetiger; pharyngeal areas VII–VIII with a single row of large denticles; median and posterior parapodia with ventral ligules reaching tip of neuracicular ligules.

Material examined

Syntypes MEXICO • 1 juv.; Revillagigedo Islands, Clarion Island, Sulphur Bay, Clarion Island; 18°20′40″ N, 114°44′20″ W; Templeton Crocker Expedition, stn 163 D1; depth 36 m; 11 May 1936; AMNH 36595 • 1 heteroneris; same collection data as for preceding; AMNH 36883 (not seen).

Other material

MEXICO – Gulf of California • 3 specs; Baja California, Bahía de Los Angeles, fiscal harbour; 28°56′49.8″ N, 113°33′25.8″ W, 18 Apr. 2010; J.A. de León leg.; UANL 8146 • 3 specs; Baja California Sur, Puerto Escondido Marina, 25°48′52″ N, 111°18′41.1″ W; 2 Apr. 2011, J.A. de León leg.; UANL 8147 • 3 spec.; same locality as for preceding; 2 Sep. 2011, J.A. de León leg.; UANL 8148 • 6 specs; same locality as for preceding; 5 Nov. 2013, J.A. de León leg.; UANL 8149 • 3 specs; Baja California Sur, Santa Rosalia Marina; 27°20′13.4″ N, 112°15′47.6″ W; 12 Nov. 2013, J.A. de León leg.; UANL 8150 • 2 specs; 24 Jun. 2017; J.A. de León leg.; UANL 8151.

Description (abridged from several sources)

BODY. Dark brownish pigmentation on dorsal surfaces of prostomium and tentacular belt, often continued as two transverse bars per segment along anterior chaetigers.

PROSTOMIUM. Pyriform, longer than wide; eyes round, of similar size, in a rectangular arrangement. Antennae slightly surpassing palp tips. Palpophores globose, as long as wide, palpostyles rounded.

TENTACULAR BELT. 2× as long as first chaetiger, anterior margin middorsally projected anteriorly, not covering posterior eyes. Tentacular cirri smooth, longest one reaching chaetiger 5–9.

PHARYNX. Jaws brownish with 9–10 teeth. Paragnaths formula: I: 2 in tandem; II: 33–34 in an arch; III: 36 in an oval; IV: 33, 35 of different sizes; V: 0; VI: 8, 11 in a cluster; VII–VIII: single row with 4–5 small cones.

PATTERN OF PARAPODIAL LIGULES. Anterior chaetigers with ligules blunt, slightly longer than wide, of similar length and width, becoming tapered, blunt, slightly longer than wide in median and posterior chaetigers; ventral ligule progressively smaller, thinner, 2× as long as wide.

CHAETIGERS 1–2. With neuraciculae only. Ligules without glandular masses.

MEDIAN PARAPODIA. With dorsal cirrus longer than ventral one, both thin; dorsal cirrus almost 2 × as long as dorsal ligule; dorsal ligule slightly longer and wider than median ligule, both with dark inner compact glandular masses. Neuracicular ligule blunt, subconical, about as long as wide. Ventral ligule digitate, 2× as long as wide. Ventral cirrus not reaching tip of ventral ligule. Notochaetae: homogomph spinigers and falcigers from chaetiger 39, blades 3 × as long as wide, margin denticulate, distal tooth incurved. Neurochaetae: homogomph spinigers and heterogomph falcigers in supracicular and subacicular fascicles; falcigers with blades slightly more than 2× as long as wide, with margin denticulate, distal tooth incurved.

POSTERIOR REGION.Tapered. Pygidium with anus terminal with two anal cirri, as long as last 11 chaetigers.

Variation

Gulf of California specimens (n = 14) showed some variation in their paragnath formula which is not size-dependent. Pharyngeal area I: 0–3 cones (50% of specimens with 2; 25% with only one; 15% without cones; 10% with 3 teeth and when the third cone is present, it is minute). Area II: right side 7–23 cones, left side 10–26 cones. Area III: oval patch with 13–30 cones in three lines, proximal line with larger cones (20% of specimens with 21 cones). Area IV: right side 12–30 cones, left side 12–34 cones. Area V: none. Area VI: right side 3–10 cones, left side 3–9 cones (25% of specimens with 5–5 cones, respectively). Area VII–VIII: 2–7 in a row (60% of specimens with 5 cones). Longest tentacular cirri reach chaetiger 2–6 (in almost 25% of specimens reaching chaetiger 4).

Remarks

The specific epithet must be modified because the original one was masculine (' ambiguus ’, 'uncertain’), but the gender of Nereis is feminine. Therefore, the epithet must be used in the feminine form (ambigua).

There are some problems regarding the type material of N. ambigua. Treadwell (1937: 149) selected and described as “ type ” a juvenile collected in the Revillagigedo Islands. It was 25 mm long, 1 mm wide, without the posterior region, and he recorded another, smaller specimen. Surprisingly, he indicated “the type retained only one anal cirrus” (Treadwell 1937: 150), and this might correspond to the heteronereis he referred to in the last line of his description. Treadwell (1937) described the largest specimen, 25 mm

long and 1 mm wide, and its pharynx was not exposed, preventing him to clearly count the numbers of paragnaths.

Hartman (1940) synonymized this species with N. riisei on the basis of parapodial ligules and homogomph falciger shape only, but no further comments were made. Later, Hartman (1956) studied one of the type specimens, but she failed to indicate which one it was or whether there was only a single specimen remaining in the jar, the largest one being destroyed after dissection. She dissected the pharynx and found a paragnath distribution different from the one originally indicated by Treadwell (1937). Treadwell indicated that areas I and II were smooth, whereas Hartman found 3 paragnaths in I and a patch of larger paragnaths in II; furthermore, larger differences relate to areas VI and VII–VIII, because Treadwell indicated 3–4 cones in VI, 7–8 cones in VII and a circular patch with about 20 paragnaths in VIII, but Hartman noted 12 cones in VI and a single continuous band with about 10 alternating larger and smaller cones in VII–VIII. On the other hand, Hartman (1940: 222) found a different pattern in her Eastern Pacific specimens, with 4–5 cones in VI and a single band with 4–5 in VII–VIII. A similar pattern, matching Hartman’s account, was recorded by Dean (2001) for his Costa Rican specimens. Hartman (1956) also detailed 9 teeth on the jaws, dark aciculae and homogomph falcigers (presumably notopodial ones) appearing from chaetiger 26.

Because of this series of differences, which were more or less corroborated in the Gulf of California material, and because Treadwell (1937) indicated he had two specimens, the chances are that those two specimens belong to different species. An alternative is that Treadwell confused the pharyngeal areas in the non-everted pharynx, as he indicated, and that would explain the specific epithet. The specimens that Hartman (1940) and Dean (2001) studied were more similar to N. riisei than the specimen Hartman (1956) studied. Regretfully, subsequent collecting trips to the Revillagigedo Islands (Hartman 1939; Rioja 1960) did not provide topotype specimens of N. ambigua.

Consequently, the key below indicates that N. ambigua resembles N. riisei because both have posterior eyes completely exposed and a single row of paragnaths in areas VII–VIII, after the careful characterization by Dean (2001). Based on the original description, N. ambigua differs from N. riisei mainly by having larger eyes, tentacular cirri reaching chaetiger 9 and dorsal cirri in posterior chaetigers shorter than dorsal ligules. These two species further differ because in N. ambigua the ventral ligule in median and posterior parapodia reach the tips of the neuracicular ligules, and in fresh specimens the tentacular belt is pale. On the contrary, in N. riisei the ventral ligules in median and posterior parapodia surpass the neuracicular ligules, and fresh specimens have a blackish tentacular belt. Nevertheless, these differences must be taken as preliminary pending the finding of topotype specimens and confirming their diagnostic features.

Distribution

Originally described from the Revillagigedo Islands, the potential distribution of Nereis ambigua might extend along the Eastern Pacific, from the Gulf of California to the Galápagos Islands and Ecuador, in shallow water.