Morphological variation in a new Hawaiʻian kinorhynch species (Echinoderidae, Cyclorhagida, Kinorhyncha) challenges well-established species diagnostic characters

A new kinorhynch species, belonging to the genus Echinoderes, is described from re-examined material collected in Kaneohe Bay, Oahu, Hawaiʻi in May 2017, and since then stored at the Natural History Museum of Denmark. The new species is easily distinguished from all other congeners by its lack of middorsal spines combined with the presence of subdorsal tubes on segments 2 and 5. The absence of middorsal spines, combined with acicular spines in the lateroventral series, and regular small sieve plates suggest that the new species is part of the E. horni species group. Morphologically it shows most resemblance with Echinoderes wilberti and differs mainly by the presence of subdorsal tubes on segment 5. The new species is named Echinoderes imperfectus sp. n. because it displays unusual high levels of spine and tube variation in the lateral series of segments 8 and 9, often leaving a specimen with asymmetrical appearance of these cuticular structures. The 32 examined specimens showed six different cuticular patterns on segments 8 and 9, expressed as the presence or absence of single or multiple lateral accessory tubes and lateroventral acicular spines. Among the examined specimens, 50% possessed paired lateral accessory tubes on segment 8, but lacked acicular spines on segments 8 and 9; the other five potential tube/spine-patterns were always present in less than 15% of the specimens. These variations follow a gradient that goes from the presence of all these structures towards symmetrical or asymmetrical absence of one or more structures, until just a single tube is present. To our knowledge, this represents the most extreme case of intraspecific tube/spine variation in the genus.http://www.zoobank.org/urn:lsid:zoobank.org:pub:A3FBEE29-88DA-4319-A241-06BA1F0E603B