Scleral Ossicles of Teleostei: Evolutionary and Developmental Trends

Scleral ossicles are bones within the sclera of the eye. A total of 547 teleost species (744 specimens) from 36 orders and 163 families were investigated with respect to scleral ossicle presence/absence and number. This is the first extensive investigation into the distribution of scleral ossicles in living teleosts. Derived orders were found to have the most variable scleral ossicle numbers (zero, one, or two per eye), while more basal groups tend to have no ossicles. Whereas more data on the activity level of individual families and on family-level interrelationships is needed, significant findings were nevertheless made. Ninety-four percent of the families investigated have a consistent ossicle number, indicating that family level is a reliable predictor of scleral ossicle presence/absence. In a subgroup analysis of 28 families, additional trends were observed with regard to activity level, namely that 80% of the families that are described as sluggish have no scleral ossicles while 100% of those that are very active have two ossicles per eye. In addition, fish that inhabit deep sea environments are the most likely ones to lack scleral ossicles. The analysis also supports the hypothesis that scleral ossicle number declined from the basal condition of four elements per eye seen in Cheirolepis to no ossicles in the eye, early in teleost evolution. At least 24 evolutionary steps are needed to account for the scleral ossicle distribution seen in teleost orders today. This study describes the variation of scleral ossicles in the most diverse group of vertebrates, Teleostei, and provides the first step in understanding the evolvability of these elements in bony fishes.