Prionospio dubia Day, 1961

Hektoen, Martin M., Radashevsky, Vasily I. & Bakken, Torkild, 2025, Partial revision of northern European Prionospio Malmgren, 1867 (Annelida: Spionidae), European Journal of Taxonomy 1027, pp. 1-58 : 10-14

Prionospio dubia Day, 1961

Figs 4–5; Table 1

Prionospio malmgreni var. dubia Day, 1961: 489–490 , fig. 3j–n.

Prionospio steenstrupi – Day 1963: 418. — Day 1967: 489, fig. 18.9o–r.

Prionospio dubia – Wilson 1990: 243–274, figs 9–15 (partim). — Sigvaldadóttir & Mackie 1993: 211– 215, figs 6–8 (partim).

Non Prionospio cf. dubia – Hektoen et al. 2024: figs 1, 4.

Diagnosis

Prostomium narrow, anteriorly rounded, posteriorly extending to end of chaetiger 1 as a narrow caruncle. Eyes present or absent. Four pairs of branchiae on chaetigers 2–5; those of chaetiger 2 up to 5 × as long as other pairs; branchiae of chaetigers 2 and 5 with many long, digitiform pinnules almost to tip of branchiae. Dorsal crests absent. Neuropodial postchaetal lamellae of chaetiger 3 upwards pointing. Sabre chaetae in neuropodia from chaetigers 16–21. Hooded hooks in neuropodia from chaetigers 18–22.

Material examined

SOUTH AFRICA • 13 specs (mix from nine different stations); 1960; J. Day leg.; BMNH 1961.19.635/662.

Description (based on adults from South Africa)

Largest anterior fragment 25 mm long, 0.7 mm wide, with 50 chaetigers. Color in alcohol pale to brownish. Prostomium narrow, anteriorly rounded, posteriorly extending to end of chaetiger 1 as a narrow caruncle. Nuchal organs U-shaped ciliary bands lateral to caruncle. Median eyes faint spots in one specimen ( Fig. 4A), absent in others ( Fig. 4B); lateral eyes present in one specimen. Both specimens with eyes of approximately 0.6 mm width. Peristomium fused with notopodial postchaetal lamellae of chaetiger 1, forming ear-shaped structures lateral to prostomium. Palps missing in all specimens.

Branchiae on chaetigers 2–5. Branchiae of chaetiger 2 elongated, cylindrical, up to 5 × as long as of remaining branchiae, with digitiform pinnules on posterior side, with short apinnate tip. Branchiae of chaetiger 5 cylindrical, similar in length to notopodial postchaetal lamellae on same chaetiger, with pinnules on outer lateral sides, leaving short distal tip apinnate ( Figs 4A–B, 5A–C). Branchiae of chaetigers 3 and 4 triangular, flat, with surface perpendicular to body axis, apinnate, with dense lateral ciliation, similar in length to notopodial postchaetal lamellae of same chaetigers. Nototrochs transverse ciliary bands between branchial bases on chaetigers 3 and 4. Dorsolateral longitudinal ciliation absent.

Notopodial prechaetal lamellae small, rounded in anterior chaetigers. Notopodial postchaetal lamellae of chaetiger 1 broadly rounded, fused with peristomium, of chaetigers 2–5 large and subtriangular, largest on chaetiger 3. Lamellae rapidly diminishing in size in postbranchial chaetigers, becoming lower, assuming low oval shape from chaetigers 12–15. Dorsal crests absent, but notopodial postchaetal lamellae extending slightly onto dorsum between chaetigers 10–16 ( Fig. 5A–B). Dorsolateral longitudinal ciliation absent. Neuropodial prechaetal lamellae inconspicuous. Neuropodial postchaetal lamellae of chaetiger 1 small, oblong and rounded; of chaetiger 2 trapezoidal, with rounded edges, not elongated downwards; of chaetiger 3 subrectangular, often with dorsally projecting tip; of chaetiger 4 quadrangular. Lamellae on subsequent chaetigers more evenly rounded, low and oval by chaetiger 10 ( Fig. 5C). Interneuropodial pouches absent.

Notopodial capillaries on anterior chaetigers arranged in up to five rows, unilimbate and granulated. Anterior row shorter than posterior row. Notopodial capillaries alimbate, long, thin, sometimes from approximately chaetiger 20. Neuropodial capillaries arranged in two rows on anterior chaetigers, unilimbate, granulated, anterior row shorter than posterior row. Sabre chaetae in neuropodia from chaetigers 17–21, one to two per fascicle, with heavy granulation on distal part. Hooded hooks in notopodia not present in any examined specimen. Hooded hooks in neuropodia from chaetigers 18– 22, up to 9 in a series, alternating with capillary chaetae; alternating capillaries limbate in anterior neuropodia, alimbate in posterior chaetigers; hooks with 3–4 pairs of upper teeth arranged in two vertical rows above main fang, with only outer hood, secondary inner hood absent. Chaetae in all ways similar to those of P. multisetosa ( Fig. 7G–H).

Pygidium missing in all examined specimens.

Remarks

Prionospio dubia was originally described as a variety of P. malmgreni from a shelf area off the southern coast of South Africa by Day (1961). Later, Day (1963, 1967) considered it as a junior synonym of P. steenstrupi , a species described from Iceland. Maciolek (1985) recognized P. dubia as a valid species.

Day (1961) described P. dubia as having indistinct eyes but later noted that eyes were likely only visible in juveniles, and disappeared with age ( Day 1963, 1967). Faint eyespots were observed in two of 13 specimens examined in the present study: one specimen with the lateral pair and another with the median pair of eyes; both specimens relatively large, 0.6 mm wide at chaetiger 10, excluding parapodia. Like Sigvaldadóttir & Mackie (1993), we found no essential morphological differences other than the presence of eyes in some specimens between South African P. dubia and specimens from the Northeast Atlantic now assigned to P. multisetosa . Additional material from the type locality is needed to clarify the morphological variation and obtain genetic characteristics of the species. However, P. dubia has not been found in South African waters since 1961 (Carol Simon pers. comm.). No molecular data is available for the species, as the Prionospio cf. dubia in Hektoen et al. (2024) is here identified as P. multisetosa .

Outside of South Africa, P. dubia has been reported from both sides of the north Atlantic ( Maciolek 1985; Sigvaldadóttir & Mackie 1993), Japan ( Imajima 1990), Australia ( Wilson 1990), and the East Pacific ( Blake 1996). All reports noted characters slightly different from those of specimens from South Africa, especially regarding the presence/ absence of eyes and distribution of sabre chaetae and hooded hooks in neuropodia. Blake (1996) noted that these differences were likely due to different sizes of studied worms, as many morphological characters are size dependent. However, considering quite similar size ranges reported by these authors, this assertion seems unlikely. It is more likely that these reports include more than one cryptic species. An overview of the main morphological characteristics of worms referred by different authors to P. dubia is given in Table 1. The worms from Australia studied by Wilson (1990) are most similar to South African P. dubia , with no apparent morphological differences between them.

Habitat and distribution

Pending further study of additional specimens from the type locality and a comprehensive revision of the data obtained from outside, it can be confirmed that P. dubia is found only in the waters of South Africa, at depths of 84– 183 m.