Distribution, abundance, and microhabitat characterization of small juvenile corals at Palmyra Atoll

Abstract

Juvenile corals are an important component of the population dynamics of corals, but little is known about the ecology and natural history of their early life-history stages. In demographic surveys, small juvenile corals are often grouped with larger and older corals or overlooked entirely due to their small size and cryptic nature. This study describes the distribution, abundance, and microhabitat of small juvenile corals, defined as post-settlement corals ≤5 mm in diameter, at Palmyra Atoll, Central Pacific. A diver-operated pulsating blue light and filter system was used to enhance innate coral fluorescence during daylight to aid in detecting small juvenile corals. Juvenile densities ranged from 0 to 59.5 m -2 and were more than 9 times higher on the fore reef (21.9 ± 0.8 m -2 ) than on the back reef (2.4 ± 0.3 m -2 ). The highest juvenile densities were observed in the middle of the sampled range at 14 m depth on the fore reef. Juvenile corals accounted for >31% of coral colonies in all habitats and depths, which resulted in positively skewed size-frequency distributions. The microhabitat of juvenile corals on coral rubble was best described as a convex surface covered with crustose coralline algae that lacked another coral within a 20 mm radius. This study provides basic ecology and natural history information of small juvenile corals and shows the feasibility of surveying corals ≤5 mm in diameter as a method for monitoring coral populations and assessing environmental changes on a coral reef.