The evolution of novel traits is a central theme in understanding how organisms adapt to their environments, and sea robins (family Triglidae) provide a fascinating case study. Recent research, led by Nicholas Bellano of Harvard University, reveals how these marine fish have developed unique sensory organs—papillae—on their leg-like pectoral fin extensions, enabling a specialized digging behavior to locate prey buried in the substrate. This innovative adaptation represents a major evolutionary leap, combining morphology, behavior, and sensory biology into a single framework.
Sea robins are bottom-dwelling fish known for their unusual appearance and behaviors. As Nicholas Bellano aptly remarked, "As you said, it’s super weird. It’s as if you asked a child to draw an animal that’s made up of a bunch of other animals. This is a fish that has these wing-like fins, hence the name sea robin. It has sometimes armor in certain species." The "legs" of these fish, derived from separated fin rays, initially served locomotory purposes.
The study showed that sensory papillae in sea robins develop several weeks after hatching. Initially, larval sea robins are pelagic and lack legs. Over the first five weeks of development, their legs form, and papillae begin to emerge. Notably, the onset of digging behavior—a critical adaptation for predation—coincides with the formation of papillae. To further understand the role of papillae, researchers crossed Prionotus carolinus (a species that digs and has papillae) with Prionotus evolans (a non-digging species lacking papillae). The offspring of these hybrids displayed legs with papillae, mirroring the digging parent.
The team expanded their investigation across the diverse phylogenetic tree of sea robins. Surprisingly, leg papillae were found in only a small clade closely related to P. carolinus. Other species displayed fin ray-like legs with no evidence of papillae, suggesting that the ancestral condition was limited to locomotory functions. This pattern indicates that sensory papillae represent a recent innovation, unique to certain lineages of sea robins.
The evolution of sensory papillae demonstrates how novel traits can enable organisms to exploit new ecological niches. Initially, fin ray-like legs likely evolved for substrate manipulation and locomotion. Over time, these structures gained sensory capabilities, culminating in the development of papillae. This adaptation allowed sea robins to localize and excavate buried prey, providing a significant advantage in their benthic habitats.
By leveraging the biodiversity of this group, scientists aim to understand how developmental programs produce novel sensory traits and drive ecological diversification. The sea robin's leg papillae stand as a striking example of evolutionary innovation, offering valuable insights into the processes that shape the natural world.
Sources: Nature, Science Friday
