Plotohelmis cf. capitata ( Greeff, 1876 )

Plotohelmis cf. capitata ( Greeff, 1876) View in CoL

( Figs 1 View FIGURE 1 , 4 View FIGURE 4 , 7–8 View FIGURE 7 View FIGURE 8 )

Rhynchonerella capitata Greeff, 1876: 74 View in CoL , pl.5 Fig. 67–68 [original description, figures of head region and one of the tentacular cirri; Arrecife, Canary Islands]

Rhynchonerella fulgens Greeff 1885: 450–453 View in CoL , Pl. XIII Figs, 27–32, Pl. XIV Figs. 33–36 [original description; Gulf of Guinea, tropical Atlantic].— Apstein 1900: 15, Pl. II Figs 19–22, Pl. V Fig. 51 [brief description and comparison with Rhynchonerella capitata Greeff, 1876; tropical and subtropical Atlantic].—Izuka 1914: 8–9, Pl. I Figs 11–12 View FIGURE 11 View FIGURE 12 [description; Pacific Ocean off Japan] — Fauvel 1923: 2010, Fig. 79 a–d [description; Atlantic, Mediterranean off Algiers and between the Balearic Islands and Sardinia]

Plotohelmis capitata Støp-Bowitz 1948: 32 View in CoL [description almost missing; synonymy and discussion of distribution based on Atlantic and Mediterranean records], 1992: 57 [description almost missing, discussion of synonymy; Atlantic ocean off central Africa].— Dales 1957: 126, 128 [comparison of three Plotohelmis species based on records published elsewhere].— Day 1967: 195, Fig. 7.5 J–L View FIGURE 7 [description; southwest Indian Ocean].— Orensanz & Ramírez 1973: 47–49, Pl. IX [description; subtropical north Atlantic].— Hartman 1959: 171 [synonymy].— Fernández-Álamo 1983: 82–86, Fig. 20 [description; Pacific Ocean off Mexico and Central America]

Material examined. Seven specimens: ZMMU WS20963 View Materials , ZMMU WS20964 View Materials , ZMMU WS20965 View Materials , ZMMU WS20966 View Materials (FA, Et); ZMMU WS20967 View Materials , ZMMU WS20968 View Materials , ZMMU WS20969 View Materials (FA) .

Description (based on all our material). Material represented by fragments 2–3 mm long comprised by 13–20 chaetigers; width at 10 th chaetiger 0.4–0.5 mm. Prostomium projects in front of eyes. Frontal antennae tapering, subequal; median antenna hardly discernible, as round papilla ( Fig. 7A View FIGURE 7 ). Eyes directed sideways and slightly forward. Proboscis short with 12 marginal papillae. Tentacular cirri arranged as; dorsal ones increasing in size caudally with D3 being almost equal to body width without parapodia; ventral cirri minute, V2<V3 ( Fig. 7B View FIGURE 7 ). All parapodia well-developed, each with oviform and bluntly pointed dorsal cirrus and small ventral cirrus of elongated oviform shape 1.5–2 times smaller than dorsal one. Chaetigerous lobe bluntly conical, reaching approximately as far as dorsal cirrus, without cirriform appendage. Parapodia of segments 4–12 or 4–13 with several strong and stout acicular chaetae with slightly curved tips. In most well-preserved specimen, 3 acicular chaetae are present in parapodia of segments 4 and 5 ( Fig. 8A View FIGURE 8 ), 4–5 in segments 6–11 ( Fig. 8B View FIGURE 8 ), and 3–4 again in segments 12 and 13 ( Fig. 8C View FIGURE 8 ). In some individuals, number of last segments with multiple acicular chaetae lower, as well as their maximum quantity; but see Remarks. Acicular chaetae may be accompanied with single or few short spinigers whose occurrence increases posteriorly. Starting from segment 14 onwards each parapodium bears a tuft of long and fine compound spinigers with obliquely pointed shaft tips and short blades, and only one long and slender acicular chaeta with straight tip ( Fig. 8D View FIGURE 8 ). Latter decreases in length caudally. Three or four pairs of genital papillae present in males below parapodia within range of segments 10–15 ( Fig. 7B View FIGURE 7 ). Segmental glands as vertical ventro-lateral bars behind parapodia. Coloration: body slightly yellowish; small granules of brown pigment scattered across body. Anteriorly, they occur mainly around segmental glands and under parapodia, thus forming two indistinct longitudinal bars on ventral area. Posteriorly they group near segment boundaries. Dorsum pigmented much scarcer than ventrum. Location of pigment granules on dorsum seems irregular. Segmental glands pigmented entirely or contain granules of pigment similar to those scattered across body, but darker and larger, but no branching chromatophores. Pigmentation differs between individuals from rather dense to very poor one including only rare spots confined to segmental glands (especially in females).

Remarks. Acicular chaetae are easily broken in this species. Without cutting off parapodia and preparing a slide, their number and arrangement can be underestimated. But even studying parapodia preparations under a compound microscope allows tracking bases of broken chaetae ( Fig. 8B View FIGURE 8 ) only in well preserved specimens. Otherwise, the true number of acicular chaetae in a parapodium can be underestimated even after a careful examination. In our material, multiple acicular chaetae could be seen up to segment 9–13 in different individuals, but the ones with fewer acicular chaetae were also the most poorly preserved ones. In the literature, it is usually just mentioned that acicular chaetae predominate in anterior segments and a single one remains in posterior parapodia ( Dales 1957; Orensanz & Ramírez 1973; Fernández-Álamo 1983). Day (1967) noted that “Segments 4–10 have three to five acicular setae with curved tips but later ones have only one to two with straight tips”.

Instead of branching chromatophores extending to the ventrum from segmental glands as described by Day (1967) or Orensanz & Ramírez (1973), brown granules of pigment were scattered across the body in all our specimens, although apparently tending to group around segmental glands. Extent of pigmentation varied between individuals. The number and arrangement of genital papillae in males from our material were within the range described by other authors.

Specimens included in our phylogenetic analyses embraced as much morphological variability as was possible: genital papillae located within the range of segments from 10–12 to 13–15; different number and arrangement of acicular chaetae; and different extent of pigmentation. However, obtained sequences were exactly the same ( Fig. 5 View FIGURE 5 ). This confirms high intraspecific variability; exact numbers of segments with genital papillae and multiple acicular chaetae turn out to be of little importance and only general ranges should be taken into account.

The difference in pigmentation between our material and literature descriptions remains unclear. Pigmentation patterns in this genus are considered important ( Day 1967), so this inconsistency cannot be disregarded.Unfortunately, no other sequences from this species are available for comparison.

Distribution. Type locality: Arrecife, Canary Islands. Succeeding records included mostly subtropical and tropical Atlantic and Mediterranean, which led to a conception that it is a predominantly warm Atlantic form ( Støp-Bowitz 1948). Dales (1957) listed (Izuka 1914) as the only existing record outside Atlantic. However, later studies provided records from southwest Indian Ocean ( Day 1967), Indian Ocean between Java and Australia ( Peter 1974), tropical and subtropical Pacific ( Fernández-Álamo 1983) as well. Central Red Sea (this study).