Introduction: The Surprising Diversity of Planktivorous Reef Fishes
When swimming along coral reefs, you’ll often notice schools of sleek, torpedo-shaped fishes darting through the currents, feeding on microscopic plankton. These plankton-feeding reef fishes—known as planktivores—were long assumed to share a set of specialized morphological traits: forked tails, large eyes, streamlined bodies, and extendable jaws for suction-feeding.
However, a groundbreaking new study published in Reviews in Fish Biology and Fisheries (2025) [DOI: 10.1007/s11160-025-09940-z] reveals a far more complex reality. Contrary to long-held assumptions, planktivorous fishes do not conform to a single body shape. Instead, they exhibit the broadest range of body forms among all reef fish feeding groups.
Convergent Evolution: Do Similar Challenges Lead to Similar Traits?
In evolutionary biology, convergent evolution describes how unrelated species can develop similar traits when adapting to similar challenges. Classic examples include the torpedo-shaped bodies of dolphins, sharks, and tunas—adapted for fast swimming.
Researchers sought to explore whether a similar convergence occurred among plankton-feeding reef fishes, which all share the challenge of capturing tiny plankton from the water column.
Study Methodology: Measuring Morphology Across Species
To investigate, scientists analyzed nearly 300 species of reef fishes across 12 globally distributed families—including surgeonfishes, damselfishes, wrasses, and snappers. Researchers measured 15 key traits related to feeding, swimming, and vision, such as:
- Jaw length
- Tail shape
- Eye size
- Pupil dimensions
These morphological traits were then compared using phylogenetic evolutionary trees to assess patterns of convergence and divergence.
Key Finding: Planktivores Show the Most Morphological Variation
Surprisingly, the study found that planktivorous fishes do not converge on a single “ideal” body type. Instead, they display the highest morphological diversity compared to other feeding groups on coral reefs.
While some, like the streamlined schooling fusiliers, fit the classic plankton-feeder profile, many others break the mold:
- Tiny gobies (approx. 3 cm long) cling to coral branches, ambushing plankton with a sit-and-wait strategy.
- Deep-bodied damselfishes venture short distances from corals to feed, defying the streamlined stereotype.
This striking variation highlights the wide adaptability of plankton-feeding fishes.
Why So Much Diversity? Behavior and Habitat Hold the Clues
The study attributes this morphological diversity to the wide range of ecological behaviors and habitats among planktivores. Their body forms are shaped not just by diet, but by:
- Time of feeding (day vs. night)
- Habitat depth (shallow vs. deep reefs)
- Feeding strategy (active vs. passive)
- Prey type and size preferences
These variations demand different body shapes, explaining the wide spectrum observed.
Even fishes not typically classified as planktivores—such as yellowmask surgeonfishes—can opportunistically switch to plankton feeding when conditions are right. This suggests a built-in flexibility rooted in their life cycle, as most reef fishes begin life as plankton-eating larvae in the open ocean.
Rethinking Reef Fish Evolution and Ecology
This research challenges the outdated assumption that planktivorous reef fishes evolve toward a uniform body plan. Instead, the findings suggest that plankton-feeding is a flexible and accessible strategy, not limited to one body type or lineage.
Implications for Marine Biology:
- The label “planktivore” may oversimplify complex ecological behaviors.
- Evolutionary pathways to plankton-feeding are more diverse than previously thought.
- Morphological adaptations are heavily influenced by environmental context and behavioral flexibility.
Conclusion: Many Ways to Be a Planktivore
This study reshapes our understanding of reef fish diversity. Rather than converging on a single solution, planktivorous fishes exemplify evolutionary versatility—demonstrating that there is no one-size-fits-all approach to surviving and thriving on coral reefs.
In essence, there are many ways to be a planktivore—each uniquely adapted to a specific niche within the reef ecosystem.
Citation:
Reviews in Fish Biology and Fisheries (2025). “Amazing diversity of shapes found among plankton-feeding fishes.” DOI: 10.1007/s11160-025-09940-z