Fish and Urchin Grazing Reveal Hidden Shifts Across Reefs

Marine researchers examining eastern Australian coastal waters discovered that while tropical reefs experience intense fish grazing, temperate reef ecosystems rely heavily on bottom-dwelling sea urchins to consume vegetation. The comprehensive study, which spanned 49 sites across 29 degrees of latitude between 2022 and 2025, reveals fundamental differences in how plant consumption operates across thermal zones. The findings were published in Oecologia, shedding light on the structural forces shaping shallow marine habitats.

How Fish and Urchins Shape Coastal Waters

To measure relative plant consumption, the research team deployed 1,340 standardised macroalgal assays using rehydrated sea lettuce, scientifically known as Ulva lactuca. The experiment used two distinct deployment setups to separate the impact of swimming fish from bottom-dwelling grazers. Water-column assays were suspended above the sea floor on 650-millimetre fibreglass stakes so only fish could reach them, while benthic assays lay flat on the benthos to allow consumption by both fish and sea urchins.

Parallel underwater visual surveys involving 67 separate assessments documented local herbivore populations across depths of 2, 5, 8, and 12 metres. The team recorded the abundance and body size of all herbivorous fish and invertebrates across three tropical locations, including Lizard Island, Orpheus Island, and Heron Island, alongside three temperate locations near Sydney and northeastern and southeastern Tasmania.

Surprising Parities and Striking Contrasts

The assays revealed that overall relative consumption levels were remarkably similar between tropical and temperate reefs. However, the underlying mechanisms driving that consumption differed significantly. On tropical reefs, fish and benthic assays were consumed at roughly equal rates, with fish grazing dominating the water column. On temperate reefs, benthic consumption greatly exceeded water-column consumption.

Visual surveys aligned with these assay results, showing a massive shift in dominant animal groups. Herbivorous fish abundance and biomass peaked on tropical reefs, whereas sea urchin abundance was 145-fold higher on temperate reefs, with urchin biomass averaging 141 times greater in cooler waters. The dominant temperate urchin species, Centrostephanus rodgersii, exerts intense grazing pressure that restricts feeding activity to immediate local crevices.

Depth Variations and Macroalgal Cover

Erect macroalgal cover showed a stark 213-fold increase on temperate reefs compared to tropical locations. Tropical sites were almost entirely devoid of erect macroalgae, save for sparse patches of the heavily calcified alga Halimeda, which averaged just 0.3 percent cover. In contrast, temperate reefs averaged 64.1 percent macroalgal cover, dominated by the laminarian kelp Ecklonia radiata, alongside Phyllospora comosa and Sargassum species.

Across temperate depth gradients, macroalgal cover peaked at 79 percent at a depth of 5 metres before declining to 44.6 percent at 12 metres. Temperate herbivory rates also shifted across depth zones, with consumption odds dropping significantly at 5-metre and 12-metre contours compared to shallow 2-metre waters. Conversely, tropical fish herbivory showed no significant variation across the tested depth range.

Implications for Warming Oceans

The authors note that these distinct patterns of constrained versus diffuse herbivory represent a vital baseline for understanding reef tropicalization, a climate-driven process where warm-adapted species migrate toward cooler latitudes. As oceans continue to warm and tropical fish expand into temperate zones, grazing pressure is expected to become less spatially restricted. This shift could accelerate the loss of kelp forests and alter the balance of coastal ecosystems worldwide.

The research was published in Oecologia on October 25, 2026.