The role of fanning behavior in water exchange by a nest‐guarding benthic fish before spawning

Abstract

In some species of fish, males provide parental care and fan within nests before eggs are deposited. Courtship fanning behavior by the male may serve to advertise to females both his ability to disperse reproductive chemical signals and to ventilate eggs once deposited in the nest. We used a technique, called particle image velocimetry, to visualize and characterize flow both in and out of a nest generated by courting Round Goby males, a bottom‐dwelling fish. Once flow was characterized, we conducted a laboratory experiment to determine if caudal and pectoral fanning behavior was influenced by the presence of a female near the nest. The presence of a single female in the vicinity of the nest had no effect on fanning rates. However, there were significant differences between fanning behavior between types of fins. Fanning by caudal fins (with the tail at the nest entrance) resulted in water being pushed out of the nest. We estimated that caudal fanning currents propagated over a distance of at least 34 cm from the nest entrance (2–3 times the body length of an adult Round Goby). In contrast, fanning by pectoral fins (with the head at the nest entrance) resulted in flow entering the nest. Flow entering the nest was much lower than flow being pushed out of the nest. We speculate that by pumping water out of the nest with their tail, males disperse odors to guide reproductive females to the nest; whereas pectoral fanning serves mainly to ventilate the nest, denoting specialization of these locomotive structures as pumping and water stirring appendages. Displacement fanning (fanning during courtship before eggs are deposited) by fish may be an indicator of parental ability. Fanning may be used for ventilation or for the dispersal of chemical messages. We used particle image velocimetry to visualize and characterize flow fields generated by courting male Neogobius melanostomus . Once flow fields were characterized, we conducted a laboratory experiment to determine if caudal and pectoral fanning behavior was modulated by the presence of a female near the nest. There were significant differences between caudal and pectoral fanning ( F 1,11 = 54.696, p < 0.001) as was the interaction term, fanning type x distance ( F 2, 22 = 9.870, p < 0.001), on water entering and leaving the nest. The presence of a single female near the nest entrance had no effect on fanning rates. The velocity of water exiting the nest induced by caudal fanning (with the tail at the nest entrance) was 6.0 ± 0.96 (mean ± SD), 5.6 ± 1.18, and 3.1 ± 1.05 mm s −1 at 5, 10, and 15 cm away from the nest opening, respectively. In contrast, the mean velocity of water entering the nest via pectoral fanning (with the head at the nest entrance) was −3.9 ± 0.69, −0.2 ± 0.45 and −0.1 ± 0.28 mm s −1 at the respective distances. Caudal fanning pushes water out of the nest and pectoral fanning pulls water into the nest. We speculate that these are specializations of locomotive structures that lead to water stirring (pectoral fins) and pumping (caudal fin) to disperse odors.