Crayfish are aquatic invertebrates that possess compound eyes. This eye-type allows for a wider field of view and higher sensitivity than camera type eyes, found in vertebrates. However, the sharpness of vision, or visual acuity, of many species of crayfish is not well studied. By determining the visual acuity threshold, the relationship between vision and other important behaviors, such as sexual selection and communication, can be better understood. There are currently over 600 described species of crayfish, though the visual acuity is only known for two. This study uses an optomotor apparatus to elicit a behavioral response from two species of crayfish found in Schenectady County, NY (Faxonius virilis and Orconectes rusticus) upon the presentation of a striped stimulus. The point at which the crayfish exhibit a fifty-percent response rate was recorded for each species in air and in water; this point is defined as the visual acuity threshold. The relationships between body length and visual acuity as well as eye diameter and visual acuity was also investigated.
In addition to determining the visual acuity threshold of the two species, it was observed that crayfish visual acuity is greater in water than in air. Furthermore, O. rusticus threshold values were greater than F. virilis threshold values in both air and water. Resulting visual acuity threshold values, calculated using experimental and estimated values, are similar to previously reported values indicating low crayfish visual acuity, suggesting that body length and eye diameter have a relationship with visual acuity.