Fish swimming overhead can make jawfish quite nervousWhen we think of compatibility problems between marine fish, what usually comes to mind is one specimen behaving outright aggressively toward another with the underlying motive of establishing territorial dominance or acquiring a meal. But sometimes incompatibility among fish has less to do with overt aggression or hungriness than, shall we say, differing “piscinalities.” Here are a few examples to illustrate my point: Is that a predator overhead? In some cases, pelagic fish can keep bottom-dwelling tankmates in a perpetual state of anxiety despite having no interest whatsoever in their doings. I’ve observed this in jawfish that share a tank with larger, open-swimming species that present no real threat to them, such as tangs and plankton-feeding triggers (e.g., Xanthichthys spp.). In this circumstance, a jawfish will tend to remain perpetually concealed in its burrow—with maybe just its eyes and mouth visible—rather than emerge and hover over it. And who can blame the jawfish? For burrowing species, danger usually comes from above, and they have to decided in a fraction of a second—based on the suspect’s body shape and movement—whether something passing overhead poses a threat or not
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