Pycnogonum gibberum du Bois-Reymond Marcus and Marcus, 1962

Lucena, Rudá Amorim, 2021, Discovery and re-description of the holotype of Pycnogonum gibberum (Pycnogonida: Pycnogonidae), Journal of Natural History 55 (1 - 2), pp. 61-66 : 62-65

Pycnogonum gibberum du Bois-Reymond Marcus and Marcus, 1962

( Figs 1A–E , 2A–D )

Pycnogonum gibberum du Bois-Reymond Marcus & Marcus, 1962, p. 3 , figs 1–5; Müller 1993, p. 286; Bamber 2008, p. 818; Lucena and Christoffersen 2018a, p. 114, 2018b, p. 95. Pycnogonum (Retroviger) gibberum – Stock 1992, p. 136.

Material examined

Holotype – ( MZUSP –016) 1 ♂, 23.iii.1961, Est. 16, Laje de Santos , São Paulo, Brazil, 31 to 32 m, col. Seção de Necton.

Diagnosis

Trunk with reticular ornamentation, completely segmented. Tubercles elongated at the distal margin of segments 1 to 3, and a short tubercle in segment 4. Lateral processes contiguous, lateral process 1 with two tubercles, 2 and 3 with three tubercles, and 4 with a dorsal tubercle. Proboscis conical, 2/3 of trunk length. Oviger with eight articles and one terminal claw. Legs with small irregular bifid setae concentrated mainly on the ventral region of tibia 2, tarsus and propodus. Terminal claw almost half propodus length.

Description

Trunk robust, reticular ornamentation in the cuticle, covered with small papillae and sparse setae, completely segmented. Elongated tubercles at distal margin of segments 1 to 3, taller than the ocular tubercle and with small setae distributed along their length, slightly facing forward and with a rounded apex. Segment 4 with a very small tubercle near the base of the abdomen ( Figures 1 (a,b) and 2(a,b)). Lateral processes contiguous, with dorsal tubercles, median ones being largest. Lateral process 1 with one median and one posterior tubercle, lateral processes 2 and 3 with one anterior, one median and one posterior tubercle, lateral process 4 only with one median tubercle. Proboscis approximately 2/3 of the length of the trunk, conic. Anterior half slightly curved downwards, anterior margin slightly dilated, approximately 1/3 of the width of the proboscis base. Ocular tubercle short and rounded. Four median eyes. Abdomen facing backwards, widened at the distal margin, with one dorsal-distal tubercle.

Oviger with eight articles and one terminal claw. Articles 1, 2 and 3 wider than long, being the smallest articles. Article 3 with one small ventral seta. Article 4 with one dorsal seta. Articles 4, 5 and 6 longest, article 5 being slightly larger than the others. Article 8 with three small spines in the ventral region. Terminal claw about 4/5 of the length of article 8 ( Figures 1 (c) and 2(c)).

Legs robust, with small tubercles along their length and small irregular bifid setae, sparsely on all the articles ( Figure 1 (d)). Coxa 1 broader than the length and wider than the other coxae, with three dorsal tubercles, median tubercle more robust. Coxa 2 narrower and longer than coxa 1, with two dorsal tubercles. Coxa 3 smaller and narrower than the coxa 1 and 2. Femur as the longest major article, slightly longer than tibia 1. Femur with more setae than the other articles, distal region with one bifid tubercle and apical seta. Tibia 1 distally dilated, with one terminal seta. Tibia 2 about 5/8 of the length of tibia 1. Tarsus small and trapezoid. Propodus slightly shorter than tibia 2 and tarsus combined. Tibia 2, tarsus and propodus with many bifid setae concentrated in the ventral region. Propodus setae with proportionally more elongated bifurcation than the setae of the other leg articles ( Figures 1 (e) and 2(d)). Terminal claw slightly curved, almost half the length of the propodus. Genital pores not observed.

Measurements of holotype (mm)

Length of trunk (measured from the anterior margin of the cephalic segment to distal margin of the lateral process 4), 3.22; trunk width (across second lateral processes), 2.11; length of abdomen, 0.83; length of proboscis, 1.94; length of oviger, 1.24; length of oviger articles (first to eighth), 0.07, 0.09, 0.07, 0.18, 0.24, 0.18, 0.13, 0.12; terminal claw, 0.16; third leg – coxa 1, 0.47; coxa 2, 0.60; coxa 3, 0.57; femur, 1.2; tibia 1, 1.12; tibia 2, 0.7; tarsus, 0.27; propodus, 0.82; main claw, 0.32.

Distribution

Brazil: off São Paulo (Laje de Santos), Rio de Janeiro and Espírito Santo ( du Bois-Reymond Marcus and Marcus 1962; Stock 1992).

Depth

32–56 m ( du Bois-Reymond Marcus and Marcus 1962; Stock 1992).

Remarks

Three main differences were observed between the original description and the analysed specimen. The first was the number of articles in the oviger. Although du Bois-Reymond Marcus and Marcus (1962) illustrated eight articles, they cited the presence of seven articles plus the terminal claw. In the analysed holotype, I observed the presence of eight articles plus the terminal claw, in addition to three small setae in the ventral region of the article 8, which were not previously described. The second difference concerned the shape of the proboscis, which was illustrated as having a swelling close to the base. However, I observed it to be conical, without such a swelling. Moreover, the mouth region had a slight swelling and a width that was approximately 1/3 of the width of the base of the proboscis, while in the original description the mouth region had slightly narrower than half the length of the base (40–45%). Finally, I observed the presence of bifurcated setae not only in the propodus, but also along the whole leg, with a slight difference between these setae. While the setae on the propodus have a more evident bifurcation (with almost half the length of the seta), those of the leg have a smaller bifurcation (with 1/3 of the length of the seta).

These characteristics bring P. gibberum morphologically even closer to Pycnogonum elephas Stock, 1966 . These are two sympatric species, despite the uncertain typelocation of P. elephas ‘between Rio de Janeiro and Rio de la Plata’ ( Stock 1966). Despite their great similarity, they can be distinguished by their size and the proboscis, which is thinner and more elongated (larger than the trunk) in P. elephas ; the ornamental pattern in the lateral processes and coxae 1, with only one dorsal tubercle in the lateral processes and no tubercles in the coxae in P. elephas ; and the shape of the tubercles of the trunk, which are thinner and slightly pointed in P. elephas . When analysing specimens of P. giberrum, Stock (1992) proposed that both species could be distinguished by the presence of an ‘oviger with seven (or eight?) joints in the males in P. gibberum ’, and its complete absence in P. elephas . We did not have access to P. elephas specimens to confirm this information (only via the literature), but we can state that the male of P. gibberum has eight articles in the oviger.

Figures

Figure 1. Pycnogonum gibberum (MZUSP–016 ♂), holotype: (a), habitus dorsal; (b), habitus lateral; (c), oviger; (d), first leg; (e), propodus. Scale bars (in mm): (a, b), 1; (c), 0.5; (d), 0.25, details of (d), 0.1; (e). 0.4, detail of (e), 0.1.

Figure 2. Pycnogonum gibberum (MZUSP–016 ♂), holotype: (a), habitus dorsal; (b), habitus lateral; (c), oviger; (d), propodus.