Phototactic behavior in cave populations of Nemacheilus evezardi with special reference to light intensity and feeding schedules

Abstract

Individuals of adult hypogean fish, Nemacheilus evezardi were caught in their natural habitats and were transported to the laboratory inside light-proof plastic containers. They were maintained in the laboratory under complete darkness prior to studying their phototactic responses under different light intensities and feeding regimens. During the period of acclimation the fishes were fed (Tokyo floating fish food) at least twice in a week and fresh water supplied a day after each feeding at random timings of the day. A choice-chamber, consisting of a light (photic) zone and a dark (aphotic) zone, was used to gauge the response to 50 or 250 or 1250 lux of achromatic light. The group of fishes in each light intensity was provided with Tokyo floating fish food ad libitum for a period of 24 hours either in the light zone or dark zone or was subjected to starvation. The choice made by the group consisting of 10–12 individuals was recorded at five minute intervals over an experimental session of 60–90 minutes. The response was computed as phototactic index, PI (ø) by using the formula given by Camassa (2001). Six experimental sessions over a period of two consecutive days (three sessions per day) were performed on the group of fishes exposed to three different light intensities under three different feeding regimens. The water in the choice-chamber was replenished before the initiation of the study and food was not provided during the experimental sessions. Individuals of N. evezardi showed mild photophobia (ømean = −0.34), when food was supplied in the light zone. Starvation made them markedly photophobic (ømean = −0.96). In contrast, they displayed phototaxis (ømean = 0.38), when food was supplied in the dark zone. The results indicate that the feeding schedules may modulate the phototactic behavior of N. evezardi drastically. In another experiment a population of N. evezardi was fed once for 24 hours in both the zones and their phototactic responses were monitored every day for seven consecutive post-feeding days. The fishes showed phototaxis until end of the second day, became photophobic on the third day and continued to remain photophobic thereafter. Probably individuals became photophobic to reduce their locomotor activity as soon as they experience that food is no longer available. This behavioral trait might be helping the individuals to reduce their metabolic rate whenever they encounter food-deficient situations. Similar studies on the epigean populations would help us to ascertain if this behavior is a pre-adaptive trait aimed at achieving optimization of energy expenditure in the putative ancestor of N. evezardi.