Bioerosion by pit-forming, temperate-reef sea urchins: History, rates and broader implications

Area of Science:

• Marine Ecology
• Bioerosion Studies
• Coastal Geomorphology

Background:

• Sea urchins are prevalent in rocky temperate reefs globally.
• Their presence in rock cavities suggests they may bore pits, but experimental evidence was lacking.
• Understanding urchin bioerosion is crucial for nearshore system dynamics.

Purpose of the Study:

• To experimentally determine if sea urchins bore pits in rocks.
• To quantify the rates of bioerosion by sea urchins across different rock types.
• To assess the consequences of sea urchin bioerosion on coastal systems.

Main Methods:

• A year-long experiment using purple sea urchins (Strongylocentrotus purpuratus) on four rock types (sandstone, mudstone, granite).
• Field observations of urchin pit size and urchin morphology (height:diameter ratio).
• A short-term experiment to identify the primary bioerosion mechanism (ingestion).

Main Results:

• Sea urchins excavated depressions in sedimentary rocks within months.
• Pit formation rates varied significantly by rock type, from <5 years (sandstone) to >100 years (granite).
• Bioerosion rates were higher when determined by ingestion, reaching up to 11.4 kg m-2 yr-1 on sandstone.
• Urchin size influenced pit size, with deeper pits and taller urchins observed on sedimentary rocks compared to granite.

Conclusions:

• Sea urchins are significant bioeroders, actively excavating pits through ingestion.
• Urchin bioerosion substantially contributes to coastal sediment production, comparable to riverine loads.
• Apex predators like sea otters may indirectly control coastal bioerosion by regulating sea urchin populations.