The sharp shooters of marine life

The archerfish’s long distance spitting can fire a bug off of a branch and send it down to the water’s surface, and the nearly-blind pistol shrimp uses its gigantic claw to stun its prey with a bubble nearly as hot as the Sun. However, if the archerfish didn’t have keen eyes enabling it to detect an insect against a vegetative background, and if the pistol shrimp lacked its  protective eye covers, called orbital hoods, these animals might never have developed the ballistic mechanisms that characterize them.

Long-range archerfish

The archerfish, which lives in estuaries and mangrove waters, primarily hunts tree-dwelling insects by shooting water from its mouth—it uses its gills to fire a jet of water between its tongue and a groove in the roof of its mouth to knock terrestrial prey into the water. Archerfish are extremely accurate at targeting bugs more than a meter away by calculating the trajectory—the path of the water stream as it moves toward the bug—and refraction—the bending of light at the water-air intersection.

A 2002 study demonstrated the fish also predicts the bug’s point of impact on the water, enabling the striking fish to arrive before other predators do: “Only about 100 milliseconds after prey is dislodged, the fish initiate a quick turn that aligns their body axis right towards where the prey will later land, and not to the actual position of the prey at that moment.”

In a study published this week in Proceedings of the Royal Society B, Shelby Temple from the University of Queensland, Australia and colleagues explored the visual ecology of the archerfish. That is, the optical demands placed on the archerfish as it sees under and above water—aquatic and aerial fields of view.

It turns out that archerfish are able to view both fields simultaneously, despite the distortion of refraction. Their eyes are also adapted to pinpoint colorful objects, like the shiny exoskeleton of a bug, against a backdrop of green foliage, detect dark objects against the bright blue sky and distinguish between shades of brown in brackish waters.

The trichromatic color vision that these fish possess—the kind that enables them to spot prey against a background of overhanging foliage—is similar to humans’ trichromatic visual system, which is thought to be adapted for the same task. However, researchers still do not know which evolved first in the archerfish: the keen eyesight or the accurate spitting.

Quick draw pistol shrimp

On the other end of the spectrum, the nearly-blind pistol shrimp gets close to its prey and stuns it with a quick snap of its gigantic claw. The force causes water to shoot from the socket at speeds of up to 62 miles an hour, creating a low-pressure bubble in its wake. As the bubble stabilizes, it pops and with it generates a flash of light called sonoluminescence. As physicist Detlef Lohse reported in a 2001 Nature study, this is due to the bubble peaking in intensity with temperatures reaching at least 5,000 degrees Kelvin (8,540 degrees Fahrenheit) inside the bubble. To compare, the surface temperature of the Sun is about 10,000 degrees Fahrenheit. As described in a National Geographic article:

[Lohse] compared the heating inside the bubble at the time of collapse to that of a bicycle pump when it’s being used to pump a tire. When you pump your bike to get air, you feel the pump is getting hot. It is getting hot because you pump fast and the heat generated at compression cannot escape.

The entire process takes only 300 microseconds, but its intensity cannot be overlooked—even by the shrimp emitting the blow. Scientists believe pistol shrimp, like the Alpheus heterochaelis, developed elaborate, permanently closed eyelid-like coverings, called orbital hoods, prior to the snapping claw adaptation. In other words, since there are other non-snapping species of shrimp which also have orbital hoods, it is likely this thick protective lid allowed the pistol shrimp’s claw to evolve without harming the shrimp itself.

However, there is a downside to this protective layer: It impedes the shrimp’s vision and, as a result, the shrimp has a difficult time detecting prey or potential threats from predators from far distances. To compensate for this characteristic, some species of pistol shrimp have developed a symbiotic relationship with certain species of gobies. Take the shrimp Alpheus bellulus and goby Cryptocentrus cinctus relationship for example. The shrimp creates a habitat of tunnels underneath the seafloor where it can hide and sleep safely. A goby occupies this habitat in exchange for warning of predation. As the goby leaves the burrow to hunt for food, the shrimp uses its antennae to follow the fish.

If the goby is mildly alarmed, the fish undulates its tail as it propels away from the threat, and if the fish spots a definite predator, it quickly turns around and retreats to the burrow. As a result, the fish’s body movements relay specific information about the environment to the shrimp’s antennae as they travel together. Without the aid of its symbiont,  a predator would be able to approach the optically-impaired shrimp more closely. Although, it goes without saying, the shrimp’s second line of defense is not too bad either.

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Temple, S., Hart, N., Marshall, N., & Collin, S. (2010). A spitting image: specializations in archerfish eyes for vision at the interface between air and water Proceedings of the Royal Society B: Biological Sciences DOI: 10.1098/rspb.2010.0345

Lohse, D., Schmitz, B., & Versluis, M. (2001). Snapping shrimp make flashing bubbles Nature, 413 (6855), 477-478 DOI: 10.1038/35097152