Humming Fish and Hooked Seeds at the Tide Pools

A Low-Tide Field Trip

1Before dawn on a June morning, Sefa Mbatha's marine biology elective arrived at the rocky shore near the Seal Rock Marine Station. The tide was very low, so pools and boulders that are usually underwater were open to the air. Marine biologist Dr. Yuki Halloran led the group and reminded everyone to step only on bare rock and to put back every rock they turned over.

2Under one large flat rock, the class found a fish about 25 cm long lying in a damp hollow. It was a male plainfin midshipman. In late spring, males of this species move into the shallow rocky shore and dig out a nest under a rock. At night, each male makes a long, steady hum by quickly squeezing muscles around an air-filled sac in his body. People living on boats nearby sometimes hear the humming at night. Females that visit a nest lay their eggs on the underside of the rock and then leave. The male stays with the eggs for about a month, chasing off crabs and other fish and fanning fresh water over the eggs with his fins. Even at low tide, he usually stays in his wet nest.

3Dr. Halloran showed the class two studies done at the station. In the speaker test, each of 20 females was placed, one at a time, in the middle of a long tank with an underwater speaker at each end. One speaker played a recorded sound for 10 minutes and the other stayed silent. In the nest survey, scientists checked 20 nests through one summer and counted how many of the eggs in each nest hatched.