In some ways, stingrays are the Cinderellas of the elasmobranch world. Compared with their better-studied cousins, the sharks, little is known about the ways that stingrays sense their environment. Coupled with that, stingrays seem to have a major disadvantage relative to the majority of other fish; their eyes are on the opposite sides of their bodies from their mouths. This probably makes snatching a snack tricky so stingrays must rely on senses other than vision when searching for food. Curious to find out how stingrays sense their surroundings, a pair of scientists from UCLA, Laura Jordan and Malcolm Gordon, and Stephen Kajiura from Florida Atlantic University, decided to investigate how three stingray species sense and react to signals that their prey may send (p. 3037, p. 3044).
First, Jordan had to find some stingrays. Heading to Santa Catalina Island off the California coast, Jordan went fishing, collecting round stingrays with a seine net and pelagic stingrays on a long line. But to collect bat rays she and a buddy donned SCUBA gear and went fishing beneath the waves with a supersized hand net. Jordan explains that she had to be stealthy and quick to capture bat rays resting on the seabed. `Once a fish had swum off, I had little or no chance of catching it,' remembers Jordan.
Returning to the Wrigley Marine Science Center with the stingrays, Jordan began testing the fish's responses to jets of water (p. 3037). Jordan explains that bat rays spend most of their time on the seabed searching for buried clams and bivalves, so she decided to simulate the telltale jets of water produced by the bat ray's favourite molluscs to see how all three species react to fluid movements.
Releasing individual hungry rays into a pool, Jordan filmed all three species' reactions. Not surprisingly, the bat rays reacted most enthusiastically as they swam over the jets, stopping and biting at the jet as if it were produced by a tasty mollusc. The least responsive of the three fish was the pelagic ray, which is the only stingray that hunts in open water and dines on squid and fish. Despite coming across the jets more often than the other two species, the pelagic ray only reacted to 32% of the jets, where as the round and bat rays reacted to 40% and 60% of the jets they encountered.
Knowing that all fish detect fluid movements with an organ known as the `lateral line' (a series of pores at the skin surface that are linked by fluid-filled channels just beneath the skin), the team related the distribution of lateral line pores at the skin surface to the animals' reactions to their encounters with the jets. The jet-sensitive bat ray had the highest density of lateral line pores along its skin, where as the less responsive pelagic and round rays had low pore densities. However, the underlying canals that link the pores are more branched in the round ray than the pelagic ray, which was the least responsive of them all. And even though the flow sensitive pores were only distributed across 70% of the disc of the round and pelagic rays' bodies, the rays were still able to respond to jets that touched the tips of their fins despite lacking pores at the outer perimeter of their bodies. `This was a big surprise for me,' says Jordan.
Having tested the fish's reactions to jets of water, Jordan and her colleagues turned their attention to the fish's sensitivity to electric fields (p. 3044). Knowing that all elasmobranches can detect electric fields and that the distributions of electrosensitive pores across the skin surfaces of all three rays differed significantly, the team decided to test the fish's reactions to weak electric dipole fields, similar to the fields generated by the small crustaceans beloved by round and bat rays.
This time Jordan and Kajiura designed a plate with four dipole electrodes attached to it that could be placed in the bottom of the rays' pool. Back at the Wrigley Marine Science Center, Jordan switched on each dipole randomly, varied the electric fields from 5.3 to 9.6 μVcm–1 and filmed the rays as they homed in on, and bit at, the tempting electric field. Analysing the fish's reaction to the fields, it was clear that all of the fish were extremely sensitive to the electric fields and were able to detect fields as weak as 1 nV cm–1. However, the bottom-dwelling bat and round rays, both with higher densities of electrosensory pores around their mouths, attacked the dipole more enthusiastically than the pelagic ray, which has lower densities of electrosensitive pores. The round and bat rays were also able to pinpoint the dipole's location more accurately than the pelagic ray, stopping precisely over the dipole at the end of their single approach run, while the larger pelagic ray often overshot the dipole before reversing into place.
Jordan suspects that the differences in the stingrays' performances are related to their different lifestyles. As round rays are confirmed seabed residents, and bat rays spend much of their time foraging for buried critters, both species probably rely heavily on their sensitivity to electric fields and jet-like fluid flows when searching for a meal. However, pelagic rays probably rely more on their vision when homing in on a tasty fish, and switch to their other senses once lunch is within reach.