How do cave fish find food

Blind cave fish show a remarkable ability to find their way around, despite their lack of eyes, as Rupert Collins explains.

The Astyanax mexicanus species comprises an eyed epigean (surface) form and at least 29 different pink, eyeless, hypogean (cave) forms, many of which are believed to have independent evolutionary origins over the last 10,000 years.

How the eyes were lost remains unclear; either as a result of random mutations in seemingly redundant eye-making genes, there being specific selective advantage in not having eyes — or that other cave-orientated adaptations have forced the indirect loss of them. Studies have indicated the latter idea as the best explanation so far.

Blind cave fish navigate, feed and reproduce with enhanced senses of smell, taste and feel. They do not shoal like surface cousins and do not feed in the same way either — by not feeding from the water column. They substrate feed at a different angle too, at 45 rather than 90°.

The cave form has also been shown to out-compete surface fish for food in complete darkness.

Keys adaptations for cave life are larger and increased neuromast (lateral line) cells around the head which sense pressure differences in water movement.

The lateral line organ senses pressure changes caused by underwater objects and the fish builds up and remembers a complicated spatial map.

These fishes have even been shown to respond to and ‘re-map’ a changing environment with an increased swimming rate.

This article was first published in the Christmas 2009 issue of Practical Fishkeeping magazine. It may not be reproduced without written permission. 

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.