Scleral ossicles of teleostei: evolutionary and developmental trends

Area of Science:

• Ichthyology
• Comparative Anatomy
• Evolutionary Biology

Background:

• Scleral ossicles are bony elements found in the sclera of vertebrate eyes.
• Their evolutionary history and distribution across diverse fish groups remain incompletely understood.

Purpose of the Study:

• To conduct an extensive investigation into the presence, absence, and number of scleral ossicles in living teleost species.
• To analyze the correlation between scleral ossicle characteristics and factors such as evolutionary lineage, activity level, and habitat.

Main Methods:

• A comprehensive survey of 744 specimens from 547 teleost species across 36 orders and 163 families.
• Systematic recording of scleral ossicle presence/absence and counts per eye for each specimen.

Main Results:

• Scleral ossicle number is highly variable, with derived teleost orders showing the most variation (zero, one, or two per eye).
• Family-level ossicle number is consistent in 94% of investigated families, serving as a reliable predictor.
• Sluggish fish families predominantly lack ossicles (80%), while active families possess two ossicles (100%). Deep-sea fish are more likely to lack ossicles.

Conclusions:

• Scleral ossicle distribution provides insights into teleost evolution, suggesting a decline from an ancestral four-element condition.
• At least 24 evolutionary steps are inferred to explain current scleral ossicle diversity in teleost orders.
• This study establishes a foundation for understanding the evolvability of scleral ossicles in bony fishes.