LYCAEIDAE Claus, 1879

Zeidler, Wolfgang, 2021, Review of the hyperiidean amphipod family Lycaeidae Claus, 1879 (Crustacea: Amphipoda: Hyperiidea), Zootaxa 5081 (1), pp. 1-59 : 3-4

Family LYCAEIDAE Claus, 1879

Diagnosis. Body length 5–16 mm, with slightly inflated pereon, especially in females. Head of females rounded, relatively large; slightly smaller in males but also rounded ( Lycaea ) or slightly pointed ( Simorhynchotus ); as long as first 3–5 pereonites. Eyes large, occupying most of head surface. A1 of females with 2-articulate peduncle, and enlarged first flagellar article (callynophore), followed by two small, terminal articles.A1 of males with 2-articulate peduncle, and enlarged, curved first flagellar article (callynophore) with two-field brush of aesthetascs medially, and three smaller, slender articles inserted on, or near, antero-distal corner. A2 absent in females. A2 of males with relatively short, slightly enlarged basal article, three slender articles folded back on one another and one short terminal article, tucked underneath head and pereon, between the pereopods. Mandibles with 3-articulated palp in males, absent in females. Maxillae 1 very reduced in size, consisting of tiny rounded lobes, or absent. Maxillae 2 absent. Coxae 1–6 not fused with pereonites, although suture with pereonites very faint or difficult to discern. Coxa of P7 seems to be fused with the pereonite. G1 and G2 simple, weakly chelate or sub-chelate. P5 the longest. P5 and P6 often with moderately enlarged basis, distinctly shorter than remaining articles combined, which are inserted terminally. P7 reduced in size, with full complement of articles, basis enlarged, longer than remaining articles combined. U1 usually with articulated rami; endopod rarely fused with peduncle. U2 endopod sometimes fused with peduncle. U3 endopod always fused with peduncle. Telson triangular, apex rounded, usually extending to about limit of U3, fused with double urosomite. Gills with folds on pereonites 2–6. Oostegites on pereonites 2–5.

Genera. Lycaea Dana, 1852 and Simorhynchotus Stebbing, 1888 .

Remarks. The systematic limits of this family are in a state of flux, with some authors including such diverse genera as Pseudolycaea (= Lycaea ), Tryphana , Brachyscelus and Thamneus , besides Lycaea (e.g., Bowman & Gruner 1973, Shih & Chen 1995). Authors who have accepted this arrangement usually place Simorhynchotus in the family Oxycephalidae , based on the erroneous assumption that the maxillae are present in Lycaeidae and absent in Oxycephalidae and Simorhynchotus . While the maxillae are not discernible in Simorhynchotus , this is also only true for two genera of Oxycephalidae : Oxycephalus and Rhabdosoma . In all other genera of Oxycephalidae , the first maxillae are reduced to a small rounded lobe, and the second maxillae are absent, or so reduced that they cannot be distinguished from the buccal mass, as found in Brachyscelus and Lycaea . Thus, there is no valid reason to include Simorhynchotus in the family Oxycephalidae based on the absence of maxillae. The same applies to Metalycaea , a junior synonym of Lycaea , which Nair (1993) resurrected as a valid genus of Oxycephalidae based on the absence of maxillae. In Thamneus the maxillae are present as small rounded lobes and in Tryphana the maxillae are relatively well developed ( Zeidler 2016).

Thus, following the review of Zeidler (2016), the family Lycaeidae is restricted to the genera Lycaea and Simorhynchotus . Simorhynchotus more closely resembles Lycaea , rather than any genus of Oxycephalidae , in the general habit and the morphology of A2 of males, and the gnathopods, pereopods, urosome and coxae. Also, in both Lycaea and Simorhynchotus , the A2 of males extend posteriorly for almost the entire length of the pereon. In Oxycephalidae the A2 of males usually extend posteriorly to about pereonite 2, and only in Tullbergella do they extend beyond pereonite 2, to about pereonite 5. Similarly, in other families of Platysceloidea the A2 of males do not extend posteriorly much further than about pereonite 2, except for some genera of Platyscelidae and Parascelidae where they can extend to pereonite 3 or 4.

Clearly the family Lycaeidae is most closely related to Oxycephalidae . Browne et al. (2007) and Hurt et al. (2013), utilising molecular techniques, also found strong support for the inclusion of Lycaea within the Oxycephalidae .

One of the unusual characters of this family is that the coxae are very difficult to discern in preserved specimens. Upon initial observation they seem to be fused with the pereonites or a very faint suture is present. In order to resolve this problem a specimen of L. bovallii was carefully dissected and cleared so that the pereopods remained attached to the pereonites. As a result, I was able to determine that, at least for this species, the coxae of G1 and G2 have a very faint suture, those of P3 and P4 a faint suture, and those of P5 and P6 a slightly more defined suture with the pereonites, while the coxa of P7 is fused with the pereonite and has a small posterior notch where a suture might have been. And this seems to be the condition of the coxae in all other species of Lycaea and Simorhynchotus .

Key to the genera of the family LYCAEIDAE Claus, 1879

- Gnathopods sub-chelate or simple. Head rounded in both sexes, or with slight anterior knob in some males.................................................................................................... Lycaea Dana, 1852

- Gnathopod 1 simple. Gnathopod 2 sub-chelate. Head rounded in females, slightly produced and pointed in males................................................................................... Simorhynchotus Stebbing, 1888