Population size estimate of a reef-flat aggregation of Chromodoris annulata (Opisthobranchia, Chromodoridae)

Abstract

Despite numerous accounts of opisthobranch morphology, life cycle and reproduction, detailed information on field population sizes and movement distances of sea slugs remains scarce. Some recent studies have analysed growth, movement and dispersal in populations of the sea hares Aplysia vaccaria (Winkler, 1955) and A. kurodai (Baba, 1937), and the relation between environmental heterogeneity and spatial distribution in the nudibranch Onchidoris muricata (Muller, 1776), but lacked estimates of population size. Particularly little information is available for species that rarely form mating aggregations and occur at such low densities that quantitative sampling is almost impossible. As a major drawback, mark-recapturebased population size estimates in sea slugs are problematic because individual recognition is often difficult and individual tagging has thus far only been applied to large species of sea hares. Here, we present a short-term capture-recapture-study for a spatially restricted reef-flat aggregation of the tropical IndoPacific nudibranch Chromodoris annulata (Eliot, 1904). This species inhabits various sections of coral reefs, ranging from the shallow subtidal adjacent to mangrove forests up to 15 m depth near the reef crest (e.g. refs 9, 10, records by recreational divers and snorkellers at http://www.seaslugforum.net, personal observations). We surveyed a c. 3 ha (100 300 m) section of the reef-flat at Mangrove Bay (20 km south of El Quseir, Red Sea, Egypt) that covers the full gradient from the rocky and muddy intertidal next to a small mangrove mudflat to the reef crest (depth 1–3 m; Fig. 1). Following initial observations of single C. annulata, we searched the whole area on ten occasions (eight low tides, two high tides) in the eight-day period 5 to 12 May 2003. Most C. annulata were confined to a small area adjacent to the mangrove patch (Fig. 1). We therefore focused our survey effort on this 1435 m (41 m 35 m) core area. Coral rock was only sparsely covered by algae and sponges, but formed many small crevices and holes that served as hides for C. annulata. During low tide the area was partly exposed with a number of submerged small tide pools. At high tides water depths ranged between 30 and 50 cm. Highly variable patterns of purple rings around the rhinophores and orange blotches scattered over the upper mantle (Fig. 2) allowed individual recognition and were sketched for comparison with earlier observations. Sampling locations were recorded on a map in order to calculate minimum movement distances based on straight Euclidian distances between subsequent recordings. This measurement was then divided by the delay between recordings to obtain approximate minimum movement distances per day. We further estimated population size with the modified Lincoln index. This index has been developed for closed populations; i.e. it ignores immigration, emigration, births, and deaths. A closed population appears a realistic approximation for our survey period, since (i) only a single slug was found outside the study area and specimens were concentrated in the centre of the core study area, suggesting low dispersal rates, and (ii) body size distribution of C. annulata (1.5 to 8 cm; mean 4.5 cm) indicated that most animals were adult so that birth did not have a major impact on our population in the course of the study. Given the high recovery rates (see below), mortality was also assumed to be close to zero. For each survey we estimated population size as