Sneaky Predators

By Arina Favilla, SRC intern

“Everything you see exists together in a delicate balance, ” Mufasa wisely tells Simba in The Lion King right before a pouncing lesson. This is true of any ecosystem on the planet—the sun provides energy for plants to grow, plants are grazed on by herbivores, who are eaten by consumers, who are prey to other predators. Any prey-predator imbalance can have cascading effects on the entire ecosystem, particularly when invasive predators are especially sneaky predators, beating Simba in the element of surprise.

The element of surprise is difficult to accomplish in the aquatic environment because there are several cues (smell, sight, vibrations) that warn prey of a nearby predator and illicit a fast-start response, allowing them to get as far away as quickly as possible. It is debated whether this fast-start response is an autonomic response, similar to a knee-jerk reflex, or whether an individual can optimize their escape response in accordance to the threat.

Image of the red lionfish (Pterois volitans) displaying its characteristic fins and venomous spines. (From Wikimedia Commons)

Image of the red lionfish (Pterois volitans) displaying its characteristic fins and venomous spines. (From Wikimedia Commons)

McCormick and Allan (2016) investigated the red lionfish’s (Pterois volitans) success as a predator by determining the response of prey. The red lionfish, native to the Pacific Ocean, is a threatening invasive species in the Caribbean because of their success as predators easily devouring 8-10% of their body weight each day. They quickly decimate reef fish populations and destroy the delicate balance of a reef ecosystem. Moreover, recent research suggests lionfish are successful, sneaky predators by avoiding associative learning, a survival mechanism that allows prey to associate cues with dangerous predators leading to effective fast-start responses and successful escapes.

The study compared the response of whitetail damselfish to two predators, the red lionfish and the common rockcod, as well as a non-predator fish, the three-lined butterflyfish. First, the damselfish were conditioned to associate potential risk with the sight and odor of the two predator species coupled with chemical alarm cues. Previous studies have shown tropical fish species, including damselfish, can quickly learn to associate cues of a predator as a threat. Damselfish were then exposed to olfactory cues (seawater from the predator or non-predator tank) and/or visual cues (predator or non-predator tank placed adjacent to the damselfish tank) before being startled by a stimulus (release of a metal weight at the water’s surface) to provoke the fast-start response.

Comparison of the different aspects of the damselfish’s fast-start response when forewarned through chemical (white), visual (light grey), or a combination of cues (dark grey) of either one of two predators (red lionfish or rockcod), a non-predator (butterflyfish), or controls. The optimal fast-start response would have a short response latency time, high average response speed and maximum speed, and large distance travelled. Damselfish exposed to controls had the lowest response while those exposed to the rockcod had the highest response. Both the butterflyfish and lionfish elicited similar intermediate responses. (McCormick and Allan 2016)

Comparison of the different aspects of the damselfish’s fast-start response when forewarned through chemical (white), visual (light grey), or a combination of cues (dark grey) of either one of two predators (red lionfish or rockcod), a non-predator (butterflyfish), or controls. The optimal fast-start response would have a short response latency time, high average response speed and maximum speed, and large distance travelled. Damselfish exposed to controls had the lowest response while those exposed to the rockcod had the highest response. Both the butterflyfish and lionfish elicited similar intermediate responses. (McCormick and Allan 2016)

McCormick and Allan (2016) found that the damselfish had greater fast-start responses when forewarned about the predatory rockcod through olfactory or visual cues, but showed similar ineffective fast-start responses—slow to react and slower speeds—when exposed to the cues for the lionfish as well as the non-predator butterflyfish and controls (Figure 2). In other words, the damselfish misidentify the lionfish as a non-predator, reducing its chance of escape if attacked. These results suggest that lionfish are capable of circumventing associative learning, leading to higher success rates in attacking prey. The findings of this study begin to explain the success of lionfish as predators, but further studies are required to better understand the mechanisms lionfish use to avoid forewarning of prey.

Works cited

McCormick, M. I. and B. J. M. Allan. 2016. Lionfish misidentification circumvents an optimized escape response by prey. Conservation Physiology 4:1–9.

Early Life History Predator-Prey Interactions and Habitat Use of the American Eel and American Conger

By Alison Enchelmaier, RJD Graduate Student

Declines in the American eel, Anguilla rostrata, have raised interest in studying the species’ early life history. Potential causes could include overfishing, increased predation, and habitat loss; but determining the cause is difficult due to the American eel’s complex life history. One potential factor, predation, is important to consider as the refuge value of estuarine nursery habitats are being reevaluated (Musumeci et al., 2014).Another factor is habitat competition. American eels arrive in North Atlantic estuaries in their early juvenile stage called glass eels, from the winter to spring. This overlaps with another species called Conger oceanicus. Commonly called the American conger, this species arrives in estuaries in the spring to summer. Considering both species have similar life histories, overlapping arrival to estuaries, and the American conger is a piscavore it is possible that both species compete for habitat space and C. oceanicus may prey on A. rostrata (Musumeci et al., 2014).

Figure 1 (1)

Early juvenile stage of eel growth called glass eels. Photo credit: http://www.caryinstitute.org/students/hudson-data-jam-competition/data-jam-data-sets/eels-hudson-river-tributaries-nysdec

Musumeci et al. (2014) examined the habitat use of both American congers and American eels by placing them in bowls containing sand and PVC pipe shelter. Each eel was observed to see how often they buried themselves in the sand, rested on top of the sand, or sheltered in the PVC pipe. American eels spent nearly even time sheltered (39%), buried (30%), and on top of the sand (31%). Alternatively, American congers divided their time lying on top of the sand (52%) and sheltering (48%). No congers buried themselves in the sand (Musumeci et al., 2014). From these observations it appears that there is some overlap in both species habitat use, though only American eels bury themselves.

To examine predator-prey relationships, Musumeci et al. (2014) placed two eels of varying sizes together for 24 hours.  Predator-prey pairs were one American conger and one American eel, two American congers, or two American eels. American congers successfully preyed on American eels in 45% of the trials. American congers did not seem to have a size preference, as they preyed on American eels of similar and smaller sizes.  American eels did not prey on American congers in any of the trials. When two congers were paired together cannibalism occurred in 16% of the trials, usually when the eels were different sizes. Only 1% (one instance) of American eel trials resulted in cannibalism.

Figure 2 (1)

Predator-prey interaction between an American conger (larger, predator) and American eel (smaller, prey). Photo credit: Musumeci et al., 2014)

Interactions between American congers and American eels are likely as they have been collected together for several decades. Both species arrival to estuaries and habitat preferences overlap, which means that they may compete for habitat space and demonstrate predator-prey interactions (Musumeci et al., 2014). Based on predation trials, it appears that American eels are potential prey for American congers, which has not yet been observed in nature. American conger cannibalism was frequent enough to indicate that it may occur in nature, though it has not yet been observed in field. The lack of American eel cannibalism contradicts culture system observations of cannibalism between similarly sized juveniles (Musumeci et al., 2014).American conger predation on American eels may affect the American eel’s survival and habitat use. This new information shows that predation and habitat space competition could play a part in the American eel’s decline.

 

Reference: Musumeci, V.L., Able, K.W., Sullivan, M.C., & Smith, J.M. (2014). Estuarine predator—prey interactions in the early life history of two eels (Anguilla rostrata and Conger oceanicus). Environmental Biology of Fishes. First published online: 1 November 2013.

 

Predator identity and its indirect effects on fishing

By Laura Louon,
Marine conservation student

Few would be surprised by the fact that fishing causes a reduction in the population of the targeted fish. That is a direct effect of fishing. But nothing in the ocean happens in a vacuum; if you decrease the number of individuals of one species, you are bound to see an effect on at least one other species, if not the entirety of the ecological community. When developing holistic management and conservation plans, it is therefore imperative that managers also consider the indirect effects of decreasing the population of a species in an ecosystem as to make the correct decisions. But how do you measure, and hence predict, these indirect effects?

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