Study author Friedman: 'The fish clades contributing the most fish diversity in today’s oceans are leveraging the water column'

A new Yale study reveals the puzzling dichotomy of fish diversity, with tropical waters being the most species-rich but fish groups in colder climates at higher latitudes generating new species more rapidly. 

The research, conducted by lead author Sarah T. Friedman while a G. Evelyn Hutchinson postdoctoral associate at Yale, and coauthored by Martha Muñoz, an assistant professor of ecology and evolutionary biology at Yale, explains that fish species diversification is triggered by the ability of fish in temperate and polar ecosystems to transition between shallow and deep water.

Published in the journal Nature Communications, the research sheds light on the mechanisms underlying fish species diversification and how climate change may impact this delicate balance. 

Friedman states, "Fishes that make forays into the deep ocean are almost exclusively located in high latitudes, where it's easier to move along the water column. These regions are experiencing the most drastic warming due to climate change, which threatens to disrupt speciation by making it more difficult for fish to change depths." 

To unlock the mystery, the researchers analyzed data on the global species occurrence of 4,067 fish species, studying their geographic range and speciation rate. They found that species-rich lineages in high-latitude regions, including eelpouts, rockfishes, flatfishes, icefishes and snailfishes, transitioned up and down the water column more often than expected. 

Meanwhile, hyper-diverse tropical lineages, such as gobies and wrasses, changed depth less frequently than predicted. The ability of fish clades, evolutionary lineages sharing a common ancestor, to disperse along the depth gradient plays a crucial role in their diversification. 

High-latitude fish clades, adapted to colder waters, can more easily travel into ocean depths where temperatures plummet dramatically. In contrast, tropical fish, accustomed to warm, shallow waters, face significant thermal barriers to transitioning to the deep ocean.

The researchers emphasize that this unique biodiversity engine at higher latitudes is particularly vulnerable to climate change. Due to the uniform water profile in these regions compared to the tropics, the fish inhabiting these areas are finely tuned to their specific environments. Even a small temperature shift can pose a physiological challenge for them, hindering their ability to disperse across the depth column. 

"As the oceans warm, organisms might face steeper barriers to dispersal across the depth column," warns Muñoz. "Over time, I think we'll see a slowdown of this engine of biodiversification." The findings also shed light on the historical pattern of fish diversification. The high biodiversity observed in tropical waters may be a remnant of the deep past when warmer regions were hotbeds of species generation. 

However, over time, most diversification began occurring closer to the Earth's poles. The implications of the study go beyond unraveling the mystery of fish diversity. 

Climate change's potential to disrupt the delicate balance of species diversification raises concerns about the impact on marine ecosystems and the potential loss of biodiversity. The study offers crucial insights for policymakers and conservationists as they seek to protect marine life in the face of a changing climate.

This research was funded by the G. Evelyn Hutchinson Environmental Postdoctoral Fellowship, reflecting Yale's commitment to fostering innovative research collaborations in environmental sciences and promoting academic excellence at the postdoctoral level. 

As climate change continues to reshape the world's ecosystems, understanding the intricacies of biodiversity and species diversification becomes more critical than ever. The groundbreaking research conducted by Yale's team offers valuable knowledge in the quest to preserve the rich tapestry of marine life and safeguard our planet's natural heritage.