Numerical answer
1. Counter-illumination solves this problem by matching the intensity of the glow to the residual light filtering down from the surface.
2. A fish swimming in open water two hundred metres down is lit from above by faint sunlight, which casts a dark silhouette visible to any predator hunting from below.
3. Because the calibration must constantly adjust as surface light changes with depth, cloud cover and time of day, the underlying neural control is more sophisticated than the simple glow suggests.
4. That silhouette, rather than the fish's colour or shape, is what predators below primarily use to detect prey against the brighter water above.
2. A fish swimming in open water two hundred metres down is lit from above by faint sunlight, which casts a dark silhouette visible to any predator hunting from below.
3. Because the calibration must constantly adjust as surface light changes with depth, cloud cover and time of day, the underlying neural control is more sophisticated than the simple glow suggests.
4. That silhouette, rather than the fish's colour or shape, is what predators below primarily use to detect prey against the brighter water above.
Show answer and solution
Answer: 2413
Sentence 2 sets up the problem: a silhouette visible to predators below. Sentence 4 elaborates on why that silhouette specifically is the detection cue ('rather than the fish's colour or shape'). Sentence 1 then introduces the solution, counter-illumination, that addresses this exact problem. Sentence 3 closes by adding a further nuance about the sophistication required to sustain that solution. Order: 2 (problem) → 4 (elaboration) → 1 (solution) → 3 (added complexity): 2413.
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