Evidence and Modification of Non-Visual Eyestalk Organs in Troglobiont Mysida and Stygiomysida (Crustacea)

Why do the eyes remain comparatively large, whereas the cornea is reduced in most troglobiont mysids? Are there any important organ size-dependent functions served by non-visual eye rudiments? This issue was approached by measuring eye structures in five troglobiont species of Mysida and two Stygiomysida compared with 14 troglophiles and 49 trogloxene Mysida species. The Organ of Bellonci (OB) was found in all Mysida and, as first records, also in Stygiomysida. The length of OBs increased with individual body length (BL) and eye length (EL) in four examined species: from postnauplioid larvae to adult stage in a troglophile and a trogloxene mysid species examined; and from juveniles to adults in a troglobiont mysid and a troglobiont stygiomysid. At the interspecific level, EL was, on average, 26% shorter at a given BL, while the OB was 40% longer at a given BL and 54% longer at a given EL, respectively, in adult troglobionts compared with trogloxene mysids. The OB is clearly proliferating, while the cornea is reduced in troglobionts. This points to sensory functions (possibly together with other functions). The sensory pore organ was found in all 15 Mysida species whose eyes were mounted on slides, and the first record of this organ was also found in Stygiomysida.