A field colonization experiment on small-scale distributions of algal symbiont-bearing larger foraminifera on reef rubble

A field colonization experiment using artificial substrate was conducted to examine small-scale distributions of algal symbiont-bearing larger foraminifera on reef rubble. The artificial substrate called "artificial rubble" (AR) consisted of two 10 X 10 X 3 cm fireproof bricks which were stacked and tied with thick fishing lines. Five replicates of the AR were placed at each of five experimental sites in coral reef-associated environments off Ishigaki Island (Okinawa, Japan) in mid-April, 1999. Ten weeks later, the fishing lines were carefully cut underwater using a knife, and the upper and lower sections of the AR were recovered separately. Comparisons of live larger foraminiferal abundances on natural reef rubble versus AR showed that mean densities of amphisteginids, calcarinids and nummulitids on AR were generally an order of magnitude lower than those on natural reef rubble, which ranged from <10(2) to >10(3) individuals per 100 cm(2) of bottom. On the other hand, mean densities of peneroplids and soritids on AR were comparable with those on natural reef rubble, suggesting that these taxa are the most efficient colonizers, at least during the experimental period. Analysis of foraminiferal abundances on the upper versus lower sections of AR revealed that live larger foraminifera were more common on the upper than lower section in shallow-water environments (<6 m depth), whereas they were more common on the lower section in reef slope environments; the difference was statistically significant only at a moat (shallow lagoon) site (alpha = 0.05). Peneroplids, soritids and reef flat-dwelling calcarinids were always more common on the upper than lower section. Particularly at the moat site, Peneroplis antillarum, P planatus, Sorites orbiculus and Neorotalia calcar were significantly more abundant on the upper section (alpha = 0.05). In contrast, amphisteginids, nummulitids and reef slope-dwelling calcarinids were consistently more common on the lower section, although their abundances were patchy and highly variable. Individuals of Amphistegina radiata and Heterostegina depressa were uncommon on upper sections (<20% of those encountered), and that difference was significant for A. radiata at a 30-m-deep reef slope site (alpha = 0.05). The prevalence of each taxon of larger foraminifera on either the upper or lower section of AR is possibly related to positive phototaxis, adaptation to high light, mechanisms for attachment, cryptic behavior, and epiphytization on experimental substrates.