Abundance, population structure and microhabitat use of compound ascidians in a Fijian seagrass bed, with special reference to Didemnum molle

Abstract

Six species of compound ascidians found in a Fijian seagrass bed dominated by Syringodium isoetifolium were divided into two groups, each occupying somewhat different microhabitats provided by the seagrass. Didemnum molle and Lissoclinum bistratum, both with algal symbiont Prochloron sp., were abundant in high light microhabitats. D. molle was mostly attach to seagrass blades (maximum colony density: 980m-2), while L. bistratum occurred both on seagrass and sediment surfaces in places with sparse seagrass cover (maximum colony density: 11, 500m-2). Trididemnum clinides also had the symbiont Prochloron sp., but it mostly occupied dark microhabitats such as the sheaths of the seagrass. The other 3 species, Didemnum cuculiferum, D. sp. cf. albopunctatum and Trididemnum discrepans, lacked algal symbionts and were rare, all occupying dark places such as seagrass sheaths in areas with dense seagrass cover. Sympatric ascidians, thus, co-exist in seagrass beds and show a different microhabitat use. Ascidians were not distributed evenly over the area of the seagrass bed, but were concentrated in an area between 30 and 84m from the shore, independent of the distribution of seagrass biomass. In dense seagrass patches, light intensities varied greatly between the top and the basal part of the seagrass, and persistence and stability of seagrass as an attachment substrate were also different between leaf blades and sheaths. Populations of D. molle on the seagrasses included many smaller colonies. There were no colonies as large as those in the population on the more stable nearby rock substrates. The small size of the seagrass blades (1.5mm in diameter), their short lifetime (1.5mo) and their lower persistence and stability as an attachment substrate may explain the small size of the colonies on the seagrass.