Identical entry (though see n.4 below) in Photius; broadly comparable ones in Hesychius s.v. and Erotian 82.7.
The headword, generated by a passage in Aristotle (see n. 1 below), is a neuter noun in the nominative/accusative singular; see generally LSJ s.v. A diminutive of se/laxos, and most often in the plural (cf. sigma 195), the name applies to the cartilaginous or elasmobranch fishes: sharks and rays. The name may derive from se/las (light, gleam; cf. sigma 193), as some of these fish species emit phosphorescent light (Chantraine, s.v. se/laxos, and LSJ s.v.).
[1] From Aristotle, History of Animals 6.9-10 (564b15-18).
[2] This verb, registered in the Supplement to LSJ, occurs only here and Photius.
[3] LSJ s.v. kopai=on seems to assume that this is the sophist of the C4 CE, but there are other possibilities, notably FGrH 348; cf. (e.g.) alpha 3223.
[4] In her critical apparatus Adler notes that mss G and M (after correction) and Photius' Lexicon transmit the alternative spelling mala/xion; cf. mu 98.