The risky nightlife of undersized sea urchins

Context

Longspined sea urchins (Centrostephanus rodgersii) form extensive urchin barrens in south-eastern Australia, threatening biodiversity and lucrative fishery stocks. Although large urchins are readily visible on reefs, small or ‘undersized’ urchins have often been considered non-emergent, cryptic, and largely inaccessible to predators, meaning smaller predators are considered not to contribute to top–down urchin control.

Aims

Here, we aim to investigate variation in nocturnal movement across urchin size classes and discuss the associated ecological implications.

Methods

Using timelapse footage we measured timing of movement, distance covered, and displacement of different sized sea urchins in various habitats.

Key results

Small urchins emerge from cryptic habitats and are active overnight on open reef areas. At dusk, smaller urchins emerge later than larger urchins, whereas at dawn, movement of all size classes of urchins decline at a similar rate.

Conclusions

The nocturnal emergence and movement of small urchins on open reef spaces makes them accessible to nocturnal predators, such as the southern rock lobster (Jasus edwardsii).

Implications

This time–space overlap of predator and prey implies that rock lobsters (including small lobsters) may be inflicting higher predatory pressure than previously considered on undersized sea urchins.