Aquatic predators are limited to eating prey that’ll fit into their mouth whole. But growing ever bigger and bigger isn’t the solution for prey fish, so they found another way. A common anti-predator defense is having spines in their fins, and fish sporting these fin spines have successfully – over evolutionary time – managed to expand their body dimension to become harder to swallow. Fish with spines on their backs and bellies evolve towards deeper (or taller) bodies, while those with spines on their sides evolve towards wider bodies, according to new findings published in Proceedings of the Royal Society B this week.
Predation pressure is known to drive changes in the defensive structures and body shape of fish. Being deeper (taller) bodied protects fish from predators with a limited gape. For example, moonfish and seabats are almost deeper and wider than they are long. Additionally, several fish lineages have evolved spines within one or more of their fins: dorsal, pectoral, pelvic, or anal (see diagram). In the wild, spine length increases with predator abundance, and fish exposed to predation cues in the lab have been known to increase their body depth.
Samantha Price of UC Davis and colleagues hypothesized that lengthening spines and increasing body dimensions evolve synergistically. Together, these traits can further enlarge body dimensions that hungry predators must overcome. "We therefore expect that the orientation of the spines will predict which body dimension increases in the presence of predators," they write.
To investigate, the team studied 347 fish families spanning a huge diversity of shapes and sizes. They examined evolutionary relationships and, using museum specimens, took measurements of an adult of one species from each of the families. Consistent with their predictions, fish with spines extending vertically (dorsal and anal fins) have deeper bodies, while fish with spines on the horizontal plane (pectoral fins) have wider bodies. Fish without spines could expand in either dimension.
Evolving these traits together further increased the minimum predator mouth size necessary to eat them.
Image in the text: Lateral view diagram of a generalized acanthomorph fish illustrating the position of fins, spines and several linear morphometric measurements (top). Dorsal view diagram of a generalized siluriform fish illustrating the position of the fins, spines and body width measurements (bottom). S.A. Price et al., Proc. R. Soc. B 2015