A new study into Blind cave fish has increased our understanding of how the cavefish evolved from its surface-dwelling ancestor.
Biologists from the University of Maryland have identified how behavioral and genetic traits co-evolved to compensate for the loss of vision in the Mexican blind cavefish (Astyanax mexicanus) and to help them find food in darkness.
In a study published in a recent issue of the journal Current Biology, Masato Yoshizawa and co-authors studied vibration attraction behaviour (VAB) in the cavefish and its surface-dwelling cousins.
VAB is the ability of fish to swim toward the source of a water disturbance in darkness.
The authors placed individual cave and surface-dwelling forms of A. mexicanus in an assay chamber. Individual fish were subjected to either a single assay in the absence of a rod, in the presence of a non-vibrating rod, in the presence of a rod vibrating at 50Hz, or to three successive assays using a random sequence of these three conditions.
The authors found that the cavefish, but not the surface fish, were strongly attracted to the rod.
VABs are advantageous to cavefish, which live in environments where food is limited and large predators are absent. They are not as useful to surface-dwelling fish, because the vibrations are just as likely to indicate the presence of a predator as a food source.
The authors then demonstrated that the potential for showing VAB has a genetic component and is linked to the mechanosensory function of the lateral line.
The authors first confirmed the role of the lateral line system (and not the inner ear) in VAB by varying the frequencies (5–500Hz) of the vibrating rod in the assay. They found that VAB has a relatively low frequency range (10–50Hz) with a peak at 35Hz, suggesting that the lateral line system (with a detection range of 20–80Hz as opposed to 200–6000Hz for the inner ear) was involved. They confirmed the role of the lateral line by treating the fish lateral-line inhibitors, whereby the treated fish failed to show any VAB.
The lateral line system consists of canal neuromasts (CN) and superficial neuromasts (SN), with the numbers of CN roughly equal in surface fish and cavefish, but several-fold more SN present in the cavefish. The authors considered SNs to be the ideal candidates responsible for VABs because they displayed a peak sensitivity at 35Hz. In experiments it was found that the cavefish showed a significant reduction in VAB when they had their SNs ablated.
Lastly, to explore the role that the number and size of the SN played in VAB, the authors crossed cavefish and surface fish to produce hybrids and examined the presence of VABs and the size and number of SNs in the hybrids. They found that the hybrids showed intermediate VAB levels and exhibited SN numbers between those of their surface fish and cavefish parents, although the size of the SNs between the hybrids and the cavefish were similar (cavefish have larger and more numerous SNs than surface fish).
From their results, the authors concluded that the VAB and SN enhancement co-evolved to compensate for loss of vision and to help the cavefish find food in darkness.
For more information, see the paper: Yoshizawa M, S Goricki, D Soares and WR Jeffery (2010) Evolution of a behavioral shift mediated by superficial neuromasts helps cavefish find food in darkness. Current Biology 20, pp. 1631–1636.