Cycle of Predation: Muskie

1. Muskie head-on profile example

2. Muskie wait and attack video

As an apex predator, the muskie (Esox masquinongy) has evolved specific adaptations that optimize its efficiency at every stage in the cycle of predation:

Stage 1: Search or Wait?
Muskies predominantly wait patiently in dense aquatic vegetation or near submerged structures, using these areas as cover. Their exceptional sensory systems, including sharp eyesight and a highly sensitive lateral line, enable them to detect even the slightest movements or vibrations in the water. This strategy allows them to conserve energy while remaining nearly invisible to prey, leveraging their natural camouflage to blend seamlessly into their surroundings. In the second linked video, you can observe the muskie’s ability to hover in place with its paired fins and remain poised for a strike, showcasing its remarkable control.

Stage 2: Pursue, Ambush, or Lure?
Muskies excel at ambush predation, remaining motionless and hidden within dense aquatic vegetation or near submerged structures until prey approaches. Their strategy hinges on stealth and surprise. When they strike, their streamlined, elongated bodies and powerful caudal fin allow for explosive bursts of speed. Their dorsal fin set far back on their body, acts like the rear positioning of a drag racer, ideally placed for acceleration and speed. The muskie’s large, forked tail is the equivalent of the engine and big wheels, propelling it with immense power.

Additionally, when viewed head-on (as seen in the first linked video), the muskie’s sagittiform shape, combined with the vermiculation patterns on its body, disrupts its outline and makes it less noticeable to prey. This contrasts the more conspicuous, laterally compressed oval shape of other piscivores, like bass.

Stage 3: Attack and Capture
The muskie’s attack is swift and precise. Its large mouth and sharp, backward-pointing teeth are adapted to grasp slippery prey securely. This dentition prevents the prey from wriggling free.

Stage 4: Handling
After capturing its prey, the muskie manipulates it into a head-first position for swallowing. This reduces resistance as the prey slides down the esophagus. Handling time is brief, reducing vulnerability to predators (of which big muskies have few) or potential prey escapes. 

Risks and Rewards
The muskie’s ambush strategy has distinct trade-offs. On the one hand, it conserves energy by eliminating the need for prolonged chases or cruising in search of food. On the other hand, its success depends on prey availability and precise timing. Missed strikes can result in wasted effort, but successful ambushes yield high-energy prey. 

Buoyancy

Explain how fishes release gas from the gas bladder and the potential consequences when they cannot do so quickly enough to compensate for sudden changes in depth. 

Fish regulate buoyancy using their swim bladder by absorbing gas into their bloodstream via rete mirabile and releasing it through specialized structures like the oval window or the pneumatic duct, depending on the species. Physostomous fish can release gas through a connection (pneumatic duct) between the swim bladder and the esophagus, allowing them to adjust to depth changes rapidly. In contrast, physoclistous fish rely on slow gas exchange through the bloodstream (oval window), making them more vulnerable to rapid pressure changes.

When a fish is pulled to the surface too quickly, it cannot release gas from its swim bladder fast enough to compensate for the decreasing pressure, leading to barotrauma. The decrease in pressure causes the swim bladder to increase in volume and overinflate, often forcing internal organs out of the mouth, bulging the eyes, or causing hemorrhaging. If not properly vented or returned to depth using descending devices, affected fish may be unable to swim back down.

Reflection on Dahlke et al. (2020)

The groups of fishes the most and least susceptible to climate change.

The most climate-susceptible groups include polar and cold-water species, such as Antarctic icefishes (Notothenioidei), with narrow thermal tolerance ranges and low adaptability to warming waters. Fish with highly specific spawning habitat and timing requirements, such as salmonids (trout, salmon) and many reef fish, are also highly vulnerable because they synchronize spawning with seasonal plankton blooms or require precise temperatures for egg development, and climate change may disrupt these conditions. Additionally, marine species with fixed spawning locations, such as Atlantic cod, are at risk because they may be unable to shift their reproduction sites in response to warming. Large-bodied fish with high metabolic demands, including tunas and groupers, are particularly susceptible since warming exacerbates oxygen limitations, reducing survival.

In contrast, the least vulnerable fish include temperate and tropical freshwater species like killifishes, which have wide temperature tolerances and thrive in variable environments. Generalist species with flexible spawning strategies, such as tilapia and catfish, are more resilient as they can reproduce across various temperatures and habitats. Similarly, highly migratory pelagic fish, such as some sharks and mackerel, have lower susceptibility because they can shift their ranges to avoid extreme temperatures.

The authors determined these patterns by analyzing upper and lower temperature limits across 694 species. They found that spawning adults and embryos had narrower thermal tolerance ranges than larvae and nonreproductive adults. They further supported their conclusions with the oxygen-limitation hypothesis, showing that aerobic capacity changes with life stage, affecting thermal limits. Climate projections (SSP scenarios) revealed that by 2100, up to 60% of fish species may exceed their reproductive thermal limits under high-emission scenarios (SSP 5–8.5).

This study demonstrated that polar and stenothermal species had the smallest safety margins. In contrast, eurythermal freshwater species were least affected, emphasizing the importance of considering reproductive thermal limits, not just adult tolerance, when assessing climate risk.

Dahlke, F. T., Wohlrab, S., Butzin, M., & Pörtner, H. O. (2020). Thermal bottlenecks in the life cycle define climate vulnerability of fish. Science, 369(6499), 65-70.

Four-Eyes

The four-eyed fish (Anableps anableps) has evolved an extraordinary adaptation: split pupils that allow it to see both above and below the water simultaneously. 

Despite its name, this species doesn’t have four eyes; each of its two eyes is divided into upper and lower sections, enabling it to function as though it has double vision. This adaptation is crucial for these fish, which inhabit brackish and freshwater environments along the coastlines of Central and South America. As it swims near the surface, the upper half of its eye is adapted for aerial vision, detecting predators like birds. In contrast, the lower half specializes in underwater vision, helping it locate prey and navigate its surroundings. The split pupil accounts for the different refractive properties of air and water, ensuring that both halves focus images accurately. This remarkable visual system allows the four-eyed fish to simultaneously scan two environments, increasing its chances of survival in a predator-rich habitat.

Sivak, J. G. (1976). Optics of the eye of the “four-eyed fish” (Anableps anableps). Vision Research, 16(5), 531-IN6. https://doi.org/10.1016/0042-6989(76)90035-3