Reproductive cycle and ecology of the tropical ascidian Halocynthia spinosa in the Red Sea

Coral-reef community structure has generally changed following declines in coverage by reef-building organisms. Ascidians as fast-growing, filter-feeding organisms, are of concern due to their invasiveness potential, but few studies have examined their life-history traits in tropical environments. The present study examined the distribution, settlement patterns, and reproductive cycle of the solitary, hermaphroditic ascidian Halocynthia spinosa in Eilat, Red Sea (29°30′N, 34°55′E), from August 2013 to August 2015. This species has a native distribution in the western Indian Ocean, with a continuous presence along the southern and eastern coasts of Africa. In Eilat, the field survey found that H. spinosa was prevalent in shallow depths, on cryptic, shaded substrates. Histological analysis of spermatogenesis and oogenesis, and a determination of the gonad index in monthly samples revealed a seasonal reproductive cycle that was significantly correlated with seawater temperature, associated with photoperiod and food availability. All measures of reproduction showed the highest activity in the summer (June–August), and a short resting period in the winter months (December–March). H. spinosa individuals were rarely observed on artificial structures. Thus, the introduction of H. spinosa into new regions as part of a fouling community is relatively unlikely. These findings contribute to understanding ecological interactions of non-reef-building organisms such as ascidians, and add to the knowledge of ascidian reproductive patterns.