Echinoderes bengalensis
Creators: name: Sørensen, Martin V.
Publisher: name: Zenodo
Year: 2023
Subjects: Biodiversity
Taxonomy
Animalia
Kinorhyncha
Cyclorhagida
Echinorhagata
Echinoderidae
Echinoderes
Echinoderes bengalensis

description: 3.2. Echinoderes bengalensis (Timm, 1958)
3.2.1. Emended diagnosis
Echinoderes with lateral accessory tubes on segment 5, sublateral tubes on segment 8, and sexually dimorphic laterodorsal tubes on segment 10; spines only present on terminal segment. Large, elongate nephridial sieve plate present in sublateral positions on segment 9. Glandular cell outlets type 2 not present. Females with strongly reduced lateral terminal accessory spines. Males with well-developed laterodorsal tubes on segment 10, and three pairs of slender penile spines on segment 11.
3.2.2. Material examined
Topotypes collected by Dr R. P. Higgins on Jan. 9, 1967 from intertidal sediment on the north side of Sonadia Island, Bangladesh (position ca: 21 o 30 ′ N, 91 o 53 ′ E), mounted most likely in Hoyer’ s medium, and deposited at the Smithsonian Institution, USA, under catalogue numbers USNM-23552 Specimens 1-70 and USNM-1182922 Specimens 1-30.
Of the one hundred mounted topotype specimens, a vast majority was unfortunately in a condition that made them unsuitable for detailed examination. The mounting medium had hardened to a point where many specimens started to develop wrinkles or cracks in the trunk cuticle, which obscured observation. Bleaching had furthermore made it difficult to observe minute structures in the trunk cuticle. Among the dorso-ventrally mounted specimens the five most well-preserved males and females, respectively were selected for measuring and further observations: USNM-235522 Sp. 01, 04, 24-26; USNM-1182922 Sp. 08, 47, 48, 50, 55. In addition, morphological observations from three laterally mounted specimens contributed to the redescription: USNM-235522 Sp. 27, 42, 69.
3.2.3. Description
Due to the condition of these 55 years old specimens, it was difficult to obtain detailed information about minute cuticular structures, such as sensory spots and glandular cell outlets type 1. The ongoing deterioration, combined with presence of numerous minute marks due to deterioration artefacts, made it extremely hard to visualise these structures. To minimize the risk of confusing actual structures with artefacts, sensory spots and glandular cell outlets type 1 are only reported if they appeared as a pair on the segment, or if they could be observed in same positions on two or more specimens. Following this approach, it was not possible to identify any sensory spots, and glandular cell outlets type 1 were only observed on the sternal plates of segments 3 to 10. Thus, the reader should expect the species to have more sensory spots and type 1 outlets than reported in this redescription. On the opposite, it is my impression that all tubes are reported, and that the complete lack of acicular spines (except on terminal segment) and glandular cell outlets type 2 are confirmed. Morphometric data for the ten most well-preserved specimens are summarised in Table 3, but due to the shortage of observed cuticular structures it did not seem necessary to provide a summary table for this.
Details on head morphology are unavailable. The neck has 16 placids, measuring 14 μm in length. The midventral placid is broadest, measuring 14 μm in width at its base, whereas all other are narrower, measuring 7 μm in width at their bases. The trichoscalid plates are small, and oval, without any distinct difference between the four dorsal and the two ventromedial ones. The trunk is stout, and nearly parallel sided (Fig. 5A and B; 6 A-B). Maximum sternal width is reached at segment 8, but sternal widths of segments 4 to 9 are basically the same, which contributes to the cylindrical shape of the trunk. Cuticular hairs are lightly scattered over the dorsal side of segment 1, whereas the ventral side has even fewer hairs and only at the posterior half of the segment (Fig. 6C and D). Segments 2 to 10 are covered with hairs (except in ventromedial and paraventral positions), but the hair covering appears to be less dense compared to other congeners.
Although most examined specimens were well-stretched, some of them showed that up to 90% of segments 10 and 11 could be retracted into segment 9 (Fig. 6B, I). This retractability is also reflected in the morphometrics (Table 3), where total trunk lengths show quite a bit of variability, whereas individual segment lengths are nearly constant across the measured specimens.
Posterior segment margins terminate into straight, well-developed pectinate fringes on segments 1 to 9. Segment 10 also terminates into pectinate fringes, but with smaller fringe tips; dorsal margin is straight, whereas ventral margin is concave. Terminal segment has very minute fringes along the sternal margins, and between the tergal extensions. Tergal extensions are short, forming not very well-defined projections with blunt tips (Fig. 5A, C; 6G).
Glandular cell outlets type 1 were only observed on sternal plates of segments 3 to 10. They appear in ventromedial positions on segment 3 to 8 (Fig. 5B), but move gradually closer to the midsternal line, and appear ultimately in paraventral positions on segments 9 and 10. Large, elongate nephridial sieve plates are present in sublateral positions of segment 9 (Fig. 5B, D; 6F). It was not possible to detect any circular pore field posterior to the plates, but the sieve area is always very distinct, and represents about 40% of the total segment length. Long, well-developed tubes are present in lateral accessory positions on segment 5, and in sublateral positions on segment 8 (Fig. 5B; 6 D-F). Males furthermore have long laterodorsal tubes on segment 10 (Fig. 5C; 6 I-J). Similar tubes were not observed in any females, thus female tubes are either strongly reduced or lacking completely. Sexual dimorphic acicular spines are present on segment 11. Lateral terminal spines are present in both sexes, but spines in females appear to be about 16% longer than in males (Table 3). Strongly reduced lateral terminal accessory spines are present in females only (Fig. 5A; 6 G-H), whereas males have three pairs of penile spines (Fig. 5C and D; 6I). Opposite to most male congeners that tend to have penile spines attaching midlaterally, the penile spines in E. bengalensis have been shifted to more dorsal positions. Dorsal- and ventral most penile spines are flexible and tubular, whereas the median one is rigid and tubular.
In addition to the morphology, it was noted that several specimens carried various epibionts. Some had filamentous bacteria attached to their posterior segments (Fig. 7A), whereas others had colonies of rodshaped protists attached to their tergal plates (Fig. 7B).
Another noteworthy observation regards the gut contents. Many of the examined show a few to numerous ingested diatoms in their guts (Fig. 7C, E-F). At least three different kinds of diatoms could be observed, and they were present in all parts of the guts, inclusive the hindgut gut and anal opening (Fig. 7E).
3.2.4. Notes on morphological and diagnostic characters
In their phylogeographic study of E. coulli group species, Randsø et al. (2019) suggested that the poorly described E. bengalensis perhaps should be considered as part this group. Although only limited data could be obtained from the topotypes, there is sufficient information to confirm that E. bengalensis is part of the E. coulli species group. The enlarged sieve plate, the lack of acicular spines on all segments except the terminal, the cylindrical trunk and the strongly reduced lateral terminal accessory spines, are all character traits that point towards the E. coulli species group.
With this subgeneric affinity established, it is relevant to look into diagnostic traits that distinguish E. bengalensis from other species of the group. Its lack of glandular cell outlets type 2 separates E. bengalensis from a majority of the E. coulli group species, and comparison can be restricted to the six species without these outlets, i.e. E. applicitus, E. cyaneafictus Cepeda et al., 2022b, E. guianensis Cepeda et al., 2022a, E. marthae Sørensen, 2014, E. strii Pardos et al., 2016b and E. zacharyi Kennedy et al., 2022. For this, spines and tubes are useful. Of the seven species without type 2 outlets, E. bengalensis included, only E. cyaneafictus and E. guianensis have acicular spines on segments 6 and 7 (Cepeda et al., 2022a; 2022b). Both E. marthae, E. strii and E. zacharyi lack acicular spines, but all three of them have an extra pair of tubes on segment 8, in laterodorsal or midlateral positions (Sørensen, 2014; Pardos et al., 2016b; Kennedy et al., 2022). This leaves us with E. applicitus as the species showing closest resemblance with E. bengalensis, and the two species are indeed very similar. They share the lack of glandular cell outlets type 2, the presence of single tube pairs in the lateral series of segments 5 and 8 (and no other tubes or spines on segments 1 to 9), and the elongate shape of their sieve plates (Ostmann et al., 2012). In addition, new observations on types and non-types of E. applicitus revealed that segment 11 can be completely retracted into segment 10, and segment 10 can be partly retracted into segment 9. These traits are also shared with E. bengalensis. However, there are still several minute details that enable us to distinguish the two species. One trait regards the exact positions of tubes on segments 5 and 8. In E. applicitus the tubes on segment 5 attach in lateroventral positions, whereas they are lateral accessory in E. bengalensis. Likewise, tubes on segment 8 are lateral accessory in E. applicitus, whereas they are sublateral in E. bengalensis. This might seem like rather subtle, and perhaps even subjective differences, but a direct comparison of the two species demonstrate that the differences are quite distinct. However, these minor differences can obviously not stand alone, and for further distinction we need to compare segments 10 and 11. The easiest way to distinguish between the two species is by the posterior segment margins of these segments. E. applicitus is characterised by a conspicuous mid- and subdorsal extension from segment 10, and few, but very strong fringe tips from the margins of the sternal plates of this segment (Fig. 7D, but see also Ostmann et al., 2012). The species furthermore has well-developed, pointed tergal extensions, with an extra denticle on each inferior margin (Fig. 7D). Such distinct characters are lacking in E. bengalensis, which has very fine fringe tips along the posterior margin of segment 10, and clearly no middorsal extensions. Likewise, its tergal extensions of segment 11, are blunt, poorly defined, and without conspicuous points or denticles (Fig. 6 G-J).
After re-examination of E. applicitus, another potential difference is left as more uncertain. Females of E. applicitus are reported as lacking lateral terminal spines (Ostmann et al., 2012), but the re-examination revealed that reduced lateral terminal accessory spines are actually present, and even documented (see Fig. 4F in Ostmann et al. (2012)). Its reduced lateral accessory spines look very similar to those in E. bengalensis and E. zacharyi. However, the shape of the segment margins of the two posterior-most segments, still makes it fairly easy to distinguish E. bengalensis and E. applicitus.
Another interesting, although not taxonomic, similarity between E. bengalensis and E. applicitus is the relatively dense populations of bacterial and protist epibionts on the trunk. Occasional occurrence of epibionts has been reported from numerous kinorhynch species (see Ostmann et al., 2012 for references), but the density of these epibionts in E. applicitus is extreme. The densities in E. bengalensis might not be as high as in E. applicitus, but the frequency of specimens with epibionts is still noteworthy.
A final note on E. bengalensis regards the high contents of diatoms in the guts of the examined specimens. Neuhaus (2013) lists several studies suggesting that diatoms are a part of the kinorhynch diet, and the high concentration of diatoms in the gut of E. bengalensis agrees with this.
description: Published as part of Sørensen, Martin V., 2023, New data on the Echinoderes coulli-group (Kinorhyncha: Cyclorhagida: Echinoderidae): A new species from New Caledonia mangroves, and a redescription of E. bengalensis (Timm, 1958), pp. 90-101 in Zoologischer Anzeiger (Zool. Anz.) (Zool. Anz.) 302 on pages 96-100, DOI: 10.1016/j.jcz.2022.11.009, http://zenodo.org/record/8164187