Echinoderes remanei (Blake, 1930)

Grzelak, Katarzyna, Yamasaki, Hiroshi, Mincks, Sarah, Phillips, Anna J. & Sørensen, Martin V., 2023, Revision of an Arctic kinorhynch species: Echinoderes svetlanae and E. tubilak are junior synonyms of E. remanei, Zoologischer Anzeiger 302, pp. 75-89 : 86-88

4.2. Diagnostic features of E. remanei

Echinoderes remanei belongs to the most common group of Echinoderes regarding its spine pattern. Five middorsal spines and lateroventral tubes/spines on segments 5 to 9 are shared with 46 congeners ( Yamasaki et al., 2020). However, when we combine these characters with the presence of glandular cell outlets type 2 in subdorsal, laterodorsal, midlateral/sublateral and ventrolateral positions on segment 2, we shorten the possibilities to 10 species, i.e., Echinoderes angustus Higgins & Kristensen, 1988 , Echinoderes aquilonius Higgins & Kristensen, 1988 , Echinoderes beringiensis Adrianov & Maiorova, 2022 , Echinoderes cernunnos Sørensen et al., 2012 , Echinoderes galadrielae Grzelak and Sørensen, 2022 , Echinoderes juliae Sørensen et al., 2018 , Echinoderes obtuspinosus Sørensen et al., 2012 , Echinoderes pennaki Higgins, 1960 , Echinoderes romanoi Landers & Sørensen, 2016 , and Echinoderes xalkutaat Cepeda et al., 2019 ( Higgins 1960; Sørensen et al., 2012; Landers & Sørensen 2016; Grzelak & Sørensen 2018; Herranz et al., 2018; Sørensen et al., 2018; Cepeda et al., 2019; Adrianov & Maiorova 2022; Grzelak & Sørensen 2022). Nevertheless, E. beringiensis , E. cernunnos , E. galadrielae , E. romanoi and E. xalkutaat can immediately be distinguished from E. remanei by the absence of glandular cell outlets type 2 on segment 4. Instead, E. cernunnos shows a pair of glands on segment 7 and very characteristic long and spiniform tergal extensions, while E. romanoi and E. xalkutaat are (in addition to their lack of gco2 on segment 4) much smaller and show different shapes of tergal extensions ( Sørensen et al., 2012; Landers & Sørensen 2016; Cepeda et al., 2019). E. juliae and E. obtuspinosus , although possessing a subdorsal pair of glandular cell outlets type 2 on segment 4, have an extra pair of glands on segment 3 ( E. juliae ) or can be easily differentiated from E. remanei by the presence of very short and stout lateral terminal spines ( E. obtuspinosus ) ( Sørensen et al., 2012; 2018).

Of the abovementioned species, E. remanei shows a much closer general resemblance with E. aquilonius , E. angustus and E. pennaki . Interestingly, the first two have been described from Disko Island, western Greenland, along with E. tubilak ( Higgins & Kristensen 1988) . The latter one, on the other hand, has been described from the San Juan Archipelago in northwest Washington, USA, and belongs to one of the first Echinoderes species described by Higgins (1960). Several years later, the species was redescribed by Higgins (1977) and recently by Herranz et al. (2018). Echinoderes remanei share identical spine patterns, distribution of glandular cell outlets type 2, and even midventral partial articulation of segment 2 ( Grzelak & Sørensen 2018; Herranz et al., 2018) with E. aquilonius , E. angustus and E. pennaki ; the only exception is the absence of laterodorsal glandular cell outlets type 2 on segment 10 in E. pennaki ; the lack of this trait has been confirmed using SEM by Herranz et al. (2018). Therefore, the easiest and most reliable way to Abbreviation s: LA lateral accessory; LD laterodorsal; LV lateroventral; MD middorsal; ML midlateral; PD paradorsal; PV paraventral; SD subdorsal; SL sublateral; VL ventrolateral; VM ventromedial; ac acicular spine; gco1/2 glandular cell outlet type 1/2; ltas lateral terminal accessory spine; lts lateral terminal spine; pe penile spine; pr protuberance; si, sieve plate; ss sensory spot; tu tube; ♀ female condition of sexually dimorphic characters; 6 male condition of sexually dimorphic characters.

distinguish E. remanei from its three congeners is by using morphometric differences in combination with a comparison of the pectinate fringe on segments 1 and 2. Unlike E. remanei , none of these species is characterized by the middorsal spine on segment 8 being significantly shorter than the one on segment 7 ( Table 2; Fig. 8 ) ( Higgins & Kristensen 1988; Herranz et al., 2018). This feature, which is unusual among Echinoderes , is consistently found in all individuals in E. remanei , irrespective of sampling location. Moreover, E. angustus and E. pennaki , in contrast to E. remanei , do not show significant differences in lateral terminal spine lengths between males and females ( Higgins & Kristensen 1988; Herranz et al., 2018). Even if only the length of spines for males would be compared, both species have longer spines in comparison to those of E. remanei (avg. LTSm:> 170 μm vs 149 μm, respectively). Echinoderes aquilonius is, however, similar to E. remanei in sexual dimorphism expressed in longer lateral terminal spines in males than in females ( Higgins & Kristensen, 1988; own data), but still both species can be distinguished from one another since the lateral terminal spines in E. aquilonius are shorter than those in E. remanei (avg. LTS ♀: 70 μm vs 90 μm, respectively; avg. LTS 6: 135 μm vs. 149 μm, respectively). In addition, all three species differ from E. remanei by being larger (avg.TL: 405–460 μm vs. 369 μm), and having markedly less conspicuous and more uniform midventral pectinate fringe on segment 1 and 2.

It seems that the morphometric pattern for middorsal and terminal spines itself represents a rather unique feature for E. remanei . In species of Echinoderes segment 8 usually has the longest middorsal spine, or is at least similar in length to the spine on segment 7. Therefore, a significantly shorter spine on segment 8 certainly stands out and distinguishes E. remanei among other Echinoderes species. Also, the sexual dimorphism expressed in lateral terminal spine lengths is not a very common trait. According to our knowledge, this trait has only been observed for five other species so far, i.e., E. aquilonius , Echinoderes bengalensis ( Timm, 1958) , Echinoderes blazeji Grzelak & Sørensen, 2022 , Echinoderes coulli Higgins, 1977 , and Echinoderes lusitanicus Neves et al., 2016 ( Higgins 1977; Higgins & Kristensen 1988; Neves et al., 2016; Grzelak & Sørensen 2022; Sørensen, unpubl. obs). The difference in length due to sexual dimorphism observed in E. remanei is very conspicuous ( Table 2; Figs. 2 and 9 ) and the size ranges between the sexes never overlapped ( Fig. 9 ); males were characterized by considerably longer lateral terminal spines than those in females, and this pattern was very consistent for all localities. Consequently, when all morphometric data were taken into account, regardless of sampling location males were plotted closer together and form a distinct group ( Fig. 10 , left side of the plot) in comparison to females ( Fig. 10 , right side of the plot). This result only emphasizes that the specimens, despite the large geographic distance between their locations, are indeed morphologically highly similar, and that differences between the sexes are consistent and driven by the length of lateral terminal spines.

Figures

Fig. 8. Box plot showing middorsal (A) and lateroventral (B) spines length of Echinoderes remanei collected in different regions.

Fig. 2. Line art illustrations of Echinoderes remanei. A male, dorsal view; B male ventral view; C Female, segments 10–11, dorsal view; D Female, segments 10–11, ventral view. Abbreviations: ldgco2 laterodorsal glandular cell outlet type 2; ltas lateral terminal accessory spine; lts lateral terminal spine; lvs lateroventral spine; lvt lateroventral tube; mds middorsal spine; mlgco2 midlateral glandular cell outlet type 2; mva midventral articulation; pdgco1 paradorsal glandular cell outlet type 1; pe penile spines; pr protuberance; sdgco2 subdorsal glandular cell outlet type 2; sdss subdorsal sensory spot; si sieve plate; slgco2 sublateral glandular cell outlet type 2; vmss ventromedial sensory spot.

Fig. 9. Box plot showing lateral terminal spines length for males and females of Echinoderes remanei collected in different regions.

Fig. 10. Non-metric multidimensional scaling (nMDS) of Euclidean distance similarities of Echinoderes remanei specimens morphometric data for each of the investigated regions; open circles indicate female specimens, filled circles indicate male specimens.