Vanadis minuta Treadwell, 1906

Vanadis minuta Treadwell, 1906 View in CoL

( Figs 1 View FIGURE 1 , 4–6 View FIGURE 4 View FIGURE 5 View FIGURE 6 )

Vanadis minuta Treadwell, 1906: 1158–1159 View in CoL , Figs 25–28 [original description (colored specimen); Hawaii, Pacific Ocean].— Day 1967: 184-185, Fig. 7.2 k–m View FIGURE 7 [description (both colored and unpigmented specimens); southwest Indian Ocean].— Orensanz & Ramírez 1973: 40–42, Pl. VI [description (unpigmented); subtropical Atlantic].— Fernández-Álamo 1983: 64–67, Fig. 15 View FIGURE 15 [description (weakly pigmented); Pacific Ocean off Mexico and Central America].— Støp-Bowitz 1992: 48–49 [description (faintly pigmented); Atlantic ocean off central Africa south of Gulf of Guinea].— Rozbaczylo et al. 2020: 159 [brief description (uncolored or weakly pigmented); Pacific Ocean northwest of Juan Fernández archipelago, Chile]; non Vanadis minuta Dales 1957: 119–121 View in CoL , Figs 29–30 [belong to Vanadis fuscapunctata Treadwell, 1906 View in CoL = Vanadis studeri Apstein, 1893 View in CoL according to the author’s later correction ( Dales & Peter 1972)]

Vanadis crystallina View in CoL (non Greef, 1876) Apstein 1900: 10–11, Pl. 1 Fig. 7 View FIGURE 7 [brief description (unpigmented); tropical and subtropical Atlantic].— Fauvel 1923: 206–207, Fig. 77 d–e [description (weakly pigmented); Mediterranean and subtropical Atlantic].— Wesenberg-Lund 1939: 32, Fig. 22 [description; Mediterranean]

Alciopa longirhyncha Greeff, 1885: 453–455 View in CoL , Pl. XIV Fig. 37 [original description (unpigmented specimen); Gulf of Guinea, tropical Atlantic]

Material examined. Eighteen specimens: ZMMU WS20945 View Materials , ZMMU WS20946 View Materials , ZMMU WS20947 View Materials , ZMMU WS20948 View Materials , ZMMU WS20949 View Materials , ZMMU WS20950 View Materials , ZMMU WS20951 View Materials , ZMMU WS20953 View Materials , ZMMU WS20954 View Materials , ZMMU WS20955 View Materials (FA, Et); ZMMU WS20952 View Materials , ZMMU WS20956 View Materials , ZMMU WS20957 View Materials , ZMMU WS20958 View Materials , ZMMU WS20959 View Materials , ZMMU WS20960 View Materials , ZMMU WS20961 View Materials , ZMMU WS20962 View Materials (FA) .

Description (based on all our material). Material represented by anterior fragments 2–4.5 mm long comprised by 10–17 ch and middle fragments of various length. Width at 10 chaetiger 0.3–0.4 mm. Body long and very slender with length of parapodia in mid-body noticeably exceeding body width. Prostomium not extending in front of eyes ( Fig. 5 View FIGURE 5 ). Inferior pair of frontal antennae long, digitiform, superior pair stout and 2–3 times shorter. Median antenna transformed into more or less distinct ridge between eyes. Eyes large, directed sideways and slightly forward. Proboscis long, slender, with one pair of long pointed lateral horns ( Fig. 5A View FIGURE 5 ). No marginal lobes or folds between horns; entire membraneous flanges extend from each horns’ base and converge in center. Three pairs of tentacular cirri arranged as 1+1+1 ( Fig. 5B View FIGURE 5 ). First pair is the longest; cirrostyle is mounted on broad cirrophore attached to lower surface of eye; cirrostyle tip extends to eye’s lens but does not reach eye’s outer margin. Second pair markedly shorter, third one shorter yet. Parapodia of segments 4 to 9 are reduced; parapodial lobe lacking ( Figs 5A–B View FIGURE 5 , 6A–D View FIGURE 6 ). Their dorsal cirri are relatively well developed from first pair. Ventral cirri of first pair of parapodia absent or almost indiscernible ( Fig. 6A View FIGURE 6 ). In segments 5–8, ventral cirri have appearance of small papillae at bases of dorsal ones ( Fig. 6B–C View FIGURE 6 ). In segment 9, ventral cirri almost equal in length to dorsal ones ( Fig. 6D View FIGURE 6 ). In females, dorsal cirri of segment 5 are transformed into globular receptacula seminis ( Fig. 5A View FIGURE 5 ). Parapodial lobes with chaetae in parapodia from segment 10 onwards ( Fig. 6E–F View FIGURE 6 ). They extend only slightly beyond dorsal cirri in segment 10 ( Fig. 6E View FIGURE 6 ) but rapidly increase. Their length exceeds body width by segment 14–15 ( Fig. 6F View FIGURE 6 ) and still continues increasing up to middle body region. Each lobe bears cirriform appendage exceeding tip of protruding aciculum and tuft of fine compound spinigers with obliquely cut shaft-heads and short blades. Subdistal widening only slightly expressed ( Fig. 6F View FIGURE 6 ). Dorsal cirri (nearly) reach chaetigerous lobe’s tip in anterior half, extend slightly beyond in remaining part; their shape is ovoid to almost round. Ventral cirri up to two times shorter than dorsal ones, elongated and indistinctly pointed, slightly asymmetrical. Body highly transparent. Majority of specimens completely uncolored with no glands visible ( Fig. 5A View FIGURE 5 ). Fewer ones have pale brown coloration of segmental glands usually represented by large central spot accompanied by a few pigment granules ( Fig. 5B View FIGURE 5 ).

Remarks. Dales & Peter (1972) noted that Dales’ earlier figures ( Dales 1957) refer to Vanadis fuscapunctata Treadwell, 1906 = Vanadis studeri Apstein, 1893 . They did not specify whether it also refers to the text of the paper, but the details of description (primarily brightly colored segmental glands from segment 7 and 5 pairs of reduced parapodia) make us think that it belongs to V. studeri as well. However, this note has been largely overlooked and the material of Dales (1957) has been repeatedly included in synonymies as V. minuta (see e.g. Ushakov 1972; Fernández-Álamo 1983; Rozbaczylo et al. 2020). Vanadis minuta has been previously confused a lot with Vanadis crystallina Greeff, 1876 . However, the mentionings of only one pair of receptacula seminis and reduction of the median antenna show that some of such “ V. crystallina ” records and descriptions actually refer to another species; we agree with Støp-Bowitz (1992) that descriptions of Apstein (1900), Fauvel (1923), and Wesenberg-Lund (1939) actually refer to V. minuta . A more confused case is the relation between V. minuta and A. longirhyncha Greeff, 1885 . Dales (1957) listed them as synonyms. Although he did not discuss the reasons, this is apparently justified, because Greef (1885) observed only 1 pair of receptacula seminis in his type specimen, followed by 4 segments with reduced parapodia, which conforms to V. minuta . But in their later synopsis, Dales & Peter (1972) listed A. longirhyncha as a synonym of Vanadis formosa Claparède, 1870 without giving any reason for this. However, V. formosa has only 2 pairs of fully reduced parapodia with dorsal cirri of both pairs transformed into receptacula seminis in females. Podial lobe in the third pair is usually just smaller than succeeding ones, although may sometimes need a careful examination to be detected. Thus, maximum 1 pair of partially reduced parapodia are present after 2 pairs of receptacula seminis in this species. Also, V. formosa is characterized by boldly colored segmental organs starting immediately after the beginning of well-developed parapodia, whereas pl. XIV Fig. 37 in Greef’s description shows no pigmentation at all. Thus, we support the earlier Dales’ view. Still, despite the priority of the older name longirhyncha , we consider changing the name of the species undesirable for the following reasons. There are not so many long-accepted names in pelagic polychaetes, and, particularly, in Alciopini . V. minuta is one of the few, whereas A. longirhyncha is apparently one of “little-known, or even long-forgotten” names, as defined in the Code. Also, it has been long synonymized with V. formosa and its resurrection as a valid name in replacement of the long-accepted V. minuta could cause confusion. Thus, we consider that this is the occasion when the Principle of Priority is “to be set aside <…> when its application would be destructive of stability or universality or would cause confusion” ( ICZN 1999).

Treadwell (1906) in his description of V. minuta mentions “segmental organ which is the only trace of color to be seen behind the eyes”. On the contrary, picture of Alciopa longirhyncha by Greef (1885) shows no pigmentation. Day (1967) described segmental glands in V. minuta as either uncolored or possessing faint brown coloration from segment 12. The same is found in ( Orensanz & Ramírez 1973; Fernández-Álamo 1983). It is noteworthy that uncolored specimens are usually recorded from Atlantic, while (weakly) colored ones are more typical of Indo-Pacific. In our material from the Red Sea , both uncolored and colored specimens were present. In the phylogenetic tree, all sequences fell apart in two well supported lineages ( Fig. 4 View FIGURE 4 ) coinciding with these two coloration patterns. No other morphological differences between them could be found. Each lineage itself was quite homogenous. The distance between lineages estimated from nuclear markers (0.4% for 18S and 1% for 28S) was higher than within-species distances in Vanadis antarctica ( McIntosh, 1885) (0.1% for 18S and 0.2% for 28S), the only Vanadis species for which relatively abundant molecular data from different locations is available. At the same time, it was lower than the between-species distance (0.6% for 18S and 2.1% for 28S) in the closely related V. antarctica – Vanadis longissima ( Levinsen, 1885) species group ( Tebble 1960), and considerably lower than the majority of between-species distances in Vanadis (1 to 1.7%, 1.5% on average for 18S; 2.1–5.3, 3.4 on average for 28S; and 18–24% for COI). Interestingly, the distance between the V. minuta lineages estimated from the COI fragments amplified with the PolyLCO–PolyHCO pair (25%) was even higher than those between all other species in our material (18 to 24%). However, the doubtfulness of the reliability of the use of sequences obtained from pelagic polychaetes with universal COI primers is argued in the Discussion. Thus, the existence of two (sub)species, one with an Atlantic origin (referring to A. longirhyncha Greef, 1885 , tentatively associated with the uncolored lineage), and another one with a Pacific origin (referring to Treadwell’s V. minuta , colored lineage) is likely. However, a study involving additional sequences from Atlantic and Pacific material is needed to confirm their validity and actual distribution.

Distribution. Type locality of Vanadis minuta Treadwell, 1906 is Hawaii, Pacific. Type locality of Alciopa longirhyncha Greef, 1885 is Gulf of Guinea, Atlantic. Later records of Vanadis minuta s. l. include: tropical and subtropical Pacific, southwest Indian Ocean, tropical and subtropical Atlantic; Central Red Sea (this study).