Zebrafish cannibals

Cannibalism is not uncommon in the animal kingdom. Many different types of organisms occasionally prey on members of their own species – house mice, monarch butterfly larvae, wandering spiders, crows, and many kinds of fish, just to name a few. As Laurel Fox, the author of a scientific paper entitled “Cannibalism in natural populations,” (the source of the examples of cannibalistic species listed above) wrote in 1975, “[c]annibalism is not an aberrant behavior limited to confined or highly stressed populations, but is a normal response to many environmental factors.”

Fox details a number of reasons why cannibalism can occur among animal populations, including a lack of other sources of food, overcrowding, stress, and simple availability. “In many examples initiation and control of cannibalism has not been ascribed to any obvious factor,” Fox writes, “and in these cases cannibalism may be a response primarily to the presence of vulnerable individuals.”

In fact, the behavior is so common among zebrafish (a small, freshwater species popular in both living room fish tanks and research labs) that online guides dedicated to the care of pet zebrafish warn their readers to keep adults separate from eggs and larvae. It’s also common enough that when a group of researchers was searching for a predator of zebrafish larvae to use in a study, they settled on the adult form of the species.

Those researchers recently published the findings of their study – an investigation of the mechanisms behind the ability of larval zebrafish to evade predators – in The Journal of Experimental Biology. Based on earlier experiments, the scientists already knew that the larvae sense their predators by feeling the flow of water that the larger fish push before them as they move rather than by seeing them coming, but in order to successfully avoid being eaten, the larvae need to avoid their predators as well as sense their presence.

This, the researchers found, is exactly what zebrafish larvae do; based on the cues they got from the water flowing around a predator (a dead adult zebrafish that had been preserved with formalin, and was guided through the experimental tank with a robotic arm), the “larvae direct[ed] their escape away from the side of their body exposed to more rapid flow. This suggests that prey fish use a flow reflex that enables predator evasion by generating a directed maneuver at high speed.”

Zebrafish larvae are equipped with the ability to feel a predator coming, and the reflex to swim away from it, even when that predator is an adult zebrafish.

Zebrafish, like many other animals, sometimes exhibit cannibalism. 

(Image by Soulkeeper via Wikimedia Commons)

Lake boundaries

“Historically, lake ecosystems were viewed as isolated ‘habitat islands in a sea of lands’ (Forbes 1887).”

So begins a paper recently published in the journal Oikos by a team of scientists working in northeast Germany. The paper they reference in their opening was written by Stephen Alfred Forbes, a prominent biologist from the 1870s until his death in 1930, and “the founder of the science of ecology in the United States,” according to L. O. Howard, writing for the National Academy of Sciences.

Though Forbes was one of the first scientists to investigate how species interact and influence one another, and emphasized the importance of studying a community rather than a single species in isolation, he did not recognize the ways in which the seemingly distinct habitats of lakes and land are interconnected. In his 1887 paper, he wrote:

“The animals of [a lake] are, as a whole, remarkably isolated – closely related among themselves in all their interests, but so far independent of the land about them that if every terrestrial animal were suddenly annihilated it would doubtless be long before the general multitude of the inhabitants of the lake would feel the effects of this event in any important way. … It forms a little world within itself.”

Recent research suggests that the boundaries of the ‘little world’ of a lake are not impermeable – energy and resources are exchanged between lakes and the terrestrial environments in which they are nestled. Sometimes, as in the study recently described in Oikos, the vector of that exchange is an insect with both aquatic and terrestrial life stages.

The researchers added corn leaves to part of two lakes (each lake was divided in half by a plastic curtain). Corn leaves have a different chemical signature than the other vegetation present in or near the lakes, allowing the researchers to trace the fate of the energy in the leaves as it made its way around the food web, from prey to predator.

In the sides of the lakes to which corn leaves had been added, the scientists found values consistent with the leaves in aquatic insect larvae that ate them, the adult forms of those insects, and the terrestrial spiders that ate the insects. In what they describe as a “boomerang flux,” they demonstrated the transfer of terrestrial carbon (in the form of corn leaves) first to aquatic organisms, and then back to a terrestrial setting.

Referring back to Forbes’ idea of lakes as distinct entities, separate from their surroundings, the scientists write: “a viewpoint of such isolation does not reflect the continued back-and-forth ‘boomerang’ cycling of organic matter (and probably nutrients) across the borders of terrestrial and aquatic systems.”

Forbes’ prescient insights into the importance of the relationships among species introduced the world to the concept of ecology and changed the way future scientists would think about and conduct their studies; with time, those future scientists would realize that the interconnectedness among species extends beyond the bounds of Forbes’ original idea of a lake, and beyond the shores of lakes themselves.

Many insects that spend their immature life stages underwater emerge to live in the terrestrial environment as adults. 

(Original image by Bj.schoenmakers via Wikimedia Commons)

Beavers, planes and parachutes

In 1950, it cost Idaho’s Department of Fish and Game $30 to relocate four beavers via parachute. That included the cost of the boxes to hold the beavers, the parachutes, and flight time, according to a paper published that year in The Journal of Wildlife Management by Idaho Fish and Game Department employee Elmo Heter. The beavers were removed from habitats where they were causing trouble for humans – backing up ponds by building dams, and, in the process, damaging irrigation systems and orchards – and transplanted to places where they would “do much toward improving the habitats of game, fish, and waterfowl and perform important service in watershed conservation.”

Much of Idaho’s wilderness is inaccessible by road, necessitating long journeys on the backs of pack animals for relocated beavers before the plane-and-parachute method was developed. The new method was not only faster and easier for Fish and Game personnel (and, no doubt, the pack animals); it was also safer and less stressful for the beavers, which spent less time in transit and survived the subsequent relocations at a much higher rate.

Heter calls out one beaver in particular who made a direct contribution to the research. “Satisfactory experiments with dummy weights having been completed,” he writes, “one old male beaver, whom we fondly named ‘Geronimo,’ was dropped again and again on the flying field. Each time he scrambled out of the box, someone was on hand to pick him up. Poor fellow! He finally became resigned, and as soon as we approached him, would crawl back into his box ready to go aloft again.” In repayment for his services, Geronimo was released into the wilderness and reportedly established a successful beaver colony.

Beavers are still being transplanted today, and still for the same parallel reasons as in the past – to remove them from places where humans find them a nuisance, and to place them in areas where scientists and managers hope they can do some good.

New research published in the journal Aquatic Conservation: Marine and Freshwater Ecosystems by a team from the University of Helsinki in Finland suggests that beavers may indeed improve habitat for other animals. The scientists studied a series of 28 ponds, half of which were naturally colonized by beavers, and found that beaver activity – primarily flooding due to dam building – resulted in more waterbirds overall, as well as a more diverse waterbird population. The flooded ponds had about seven times as many insects – a prime food source for waterbirds – and were also shallower than the beaver-less ponds, both of which, the authors suspect, made the beaver ponds a more desirable habitat for waterbirds.

Where beavers move in, waterbirds tend to follow – and sometimes the beavers don't need a plane ride and a parachute to get there. 

Beavers fell trees, sometimes of considerable size, and combine them with mud and branches to build their dams.

(Image by D. Gordon E. Robertson via Wikimedia Commons)