Scleral Ossicles of Teleostei: Evolutionary and Developmental Trends

Abstract

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.

Novel characteristics of the cultured Lumpfish Cyclopterus lumpus eye during post‐hatch larval and juvenile developmental stages

“…The spherical lens was also typical of teleosts. The scleral cartilage shell we observed is consistent with the wide variability of scleral ossicle tissues in teleosts (Franz‐Odendaal, ). The two distinct sizes of nuclei we observed in the inner nuclear layer in developing stage fish may represent either different stages of differentiation or different types of retinal neural cells as found in other teleosts (Hitchcock & Raymond, ).…”

The sclerotic ring of squamates: an evo‐devo‐eco perspective

“…chondrichthyans, crocodiles, some basal mammals, and most actinopterygians) whereas others have both scleral cartilage and scleral ossicles (i.e. testudines, avians, most squamates, and many teleosts and dinosaurs) (Walls, ; Franz‐Odendaal & Hall, ; Franz‐Odendaal, ). In extant reptiles [i.e.…”

“…In extant reptiles [i.e. Curtis & Miller, (birds); Underwood, , (lizards); Franz‐Odendaal, ; (turtles); Hall, ,b, (birds and lizards)], the scleral cartilage is present as a cup that forms around the posterior portion of the eye, whereas the scleral ossicles are positioned at the corneal‐scleral limbus (the anterior portion of the eye) and form the sclerotic ring (Fig. , de Beer, ).…”

Early Lens Ablation Causes Dramatic Long-Term Effects on the Shape of Bones in the Craniofacial Skeleton of Astyanax mexicanus

“…Circumorbital bones are late-forming bones which begin to ossify at approximately 22 mm body length, which is around the age when scales develop [11]. In addition, the surface fish (as in many other teleosts) have two endochondrally formed scleral ossicles in the eye [12], [13], while cavefish have a single cartilage element in the sclera [11], [13].…”