Anatomical comparisons of neural systems in sighted epigean and troglobitic crayfish species

The activity of visual systems is known to affect development of the neural tissue associated with vision in both vertebrates and invertebrates. Three species of crayfish were compared for variations in the gross structures of the eye and of the underlying neural tissue of the optic system that were associated with environmental adaptation. The troglobitic crayfish Orconectes australis packardi and two epigean crayfish, Cambarus tenebrosus and Procambarus clarkii, were used. Cambarus tenebrosus raised in the cave are functionally blind although ommatidia develop, indicating that the primary sensory structures still develop without normal input. Troglobitic crayfish have lost the genomic ability to forma functional visual system. Electrophysiological records from neurons within the optic stalk of O. australis packardi showed no response to light. The neuronal ganglia within the eye stalk of C. tenebrosus are disorganized which could be the reason for the lack of a behavioral response related to sight. Second order neurons associated with olfaction arise in the central brain and send processes to lobula within the eye stalk via the protocerebral tract. Cross sections of this tract revealed that the troglobitic crayfish have more olfactory projection neurons and fewer large axon profiles than the other two crayfish, suggesting that O. australis packardi has more neural processing devoted to olfaction as an adaptation to cave life.