Of Compound Ascidians in a Fijian Seagrass Bed, with Special Reference to Didemnum Molle

Galaxea, JCRS, 2: 29-38 (2000) RS Jc Japanese Coral Reef soci~rr Abundance, population structure and microhabitat use of compound ascidians in a Fijian seagrass bed, with special reference to Didemnum molle M. Nishihira and T. Suzuki Biological Institute, Graduate School of Science, Tohoku University, Aoba-ku, Sendai 980-8578, Japan 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: 980 m-2), while L. bistratum occurred both on seagrass and sediment surfaces in places with sparse seagrass cover (maximum colony density: 11,500 m-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 84 m 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 (1.5 mm in diameter), their short lifetime (1.5 mo) and their lower persistence and stability as an attachment substrate may explain the small size of the colonies on the seagrass. Key words: Compound ascidians, Didemnum molle, Microhabitat use, Population structure, Seagrass bed, Species diversity algal symbiont Prochloron sp. (Kott 1980, 1981, INTRODUCTION 1982). Didemnum molle Herdman, one of such Substrate persistence is important for sessile symbiotic ascidians, usually occupies a hard animals. Like other attached organisms, substratum (Kott 1981, Olson 1982, Stoner 1992; compound ascidians need an attachment Nishihira and Suzuki 1994, Koike and Suzuki substrate for survival and growth, access to 1996). This ascidian generally forms large and occupation of an appropriate substrate is aggregations, and usually colonizes hard one of their fundamental ecological require- substrates such as dead coral skeletons and ments. This may partly explain why compound rocks. Olson (1982, 1985, 1986) conducted ascidians have been found mostly on hard studies on populations of D. molle existing on substrates such as rocks, except for some stable hard rock substrates at Heron Island, species colonizing ephemeral or temporary Great Barrier Reef. He demonstrated an substrates such as the bodies of macrophytes. adaptive significance of the timing of larval Some tropical compound ascidians have an release, larval behavior and susceptibility of 30 M. Nishihira and T. Suzuki younger colonies to strong light. He also suggested that characteristics of the composi- tion of chlorophyll pigments were related to the growth stages and micro-distribution of the colonies. However, there have been no other detailed ecological studies of D. molle on its habitat use. Studies of D. molle populations on unsta- ble ephemeral substrates have been especially lacking. Mukai and Nojima (pers. comm.) conducted a survey of animal communities in seagrass beds at Dravuni Island in 1989, and recorded 3 species of compound ascidians. However, their study did not cover microhabitat use of the compound ascidians. Thus, microhabitat use and population structure of these ascidians are areas which require further study. In seagrass beds, the plant structure provides organisms with various microhabitats, and plays a key role in sublittoral sandy habitat as a structuring agent for microhabitat use (see Mukai 1990). Once the sandy substrate is covered by a dense growth of seagrasses, it offers a variety of microhabitats that are open to invasion and population establishment of various organisms. In this way, seagrass beds promote the establishment of phytal animal communities by providing organisms with a variety of attachment substrates, even though they may not be permanent. Therefore, in a dense seagrass Fig. 1 Map showing the study site in the seagrass bed, we can expect diversified biogenic bed at Dravuni Island (a), and the location of microhabitats (Hall and Bell 1988, Stoner and Dravuni and Yaokube Islands (b), in the Great Lewis 1985). This, in turn, may result in Astrolabe Reef, Fiji. mufti-species coexistence, and increase the species richness compared to situation in protected from oceanic waves. The west coast adjacent, unvegetated sandy bottoms. of the island is on the leeward side, and the The objective of the present study was to seagrass bed we studied was, therefore, clarify species diversity, habitat partitioning usually relatively calm. and the characteristics of population struc- The seagrass bed was an almost pure stand ture of compound scidians in a seagrass bed. of Syringodium isoetifolium (Aschers.) (Aioi and Pollard,1993), covering an area of almost 9 ha. The depth of the bed ranged from 3 to MATERIALS AND METHODS 10 m. In the center of the seagrass bed, a 167 Field studies were conducted from October m-long transect line was set perpendicular to to November, 1991 in a seagrass bed along the the shoreline. The bottom profile of the north-western coast of Dravuni Island (18°48'S, seagrass bed was surveyed along this census 178° 36' E), within the Great Astrolabe Reef, Fiji line by depth measurements using a line scale (Fig. 1). The Great Astrolabe Reef is an at intervals of 5 m on a calm day. oceanic ribbon reef with lagoon depths For quantitative sampling, four quadrats extending to 40 m. It encircles several islands were set at 15 sampling points with intervals of various sizes, including Dravuni Island and of about 10 to 20 m along the line. The sizes Yaokube Island which are located in the of the quadrats varied depending on the northern region. Therefore, these islands are seagrass cover: 25 x 25 cm2 in regions with Compound ascidians in Fijian seagrass bed 31 sparse seagrass cover and 10 x 10 cm2 in an underwater video camera (Model CCD-TR regions with a dense cover. All the seagrasses 705, Sony) packed in a water-proof housing. growing inside the quadrat were collected The waving frequencies and bending degrees (the above-ground parts of the plants were of the blades and sheaths were counted and clipped with scissors) and kept in a plastic measured on a video monitor. bag. Following this, the detached ascidians, small seagrasses such as Halophila ovalis (R. RESULTS Br.) Hook, and filamentous algae inside the quadrat were collected and stored in a sepa- Microhabitats in the seagrass bed rate bag in the field. SCUBA was used for The seagrass flora present in the waters all the samplings and observations. off Fiji is simple with only four species (den Samples were taken to a laboratory on the Hartog, 1970; Mukai, 1993), of which three shore and number of shoots was recorded, and can be found in the seagrass bed of Dravuni above-ground lengths of all the blades and Island. The Dravuni seagrass bed is an almost sheaths of the seagrasses were recorded to pure stand of Syringodium isoetif olium (Aioi the nearest millimeter for each shoot. The & Pollard, 1993), with sporadic growths of species name and attachment positions of the Halophila ovalis and Halodule uninervis. The ascidians on the seagrass body were recorded seagrass beds cover an area of 30 to 197 m for each colony. The seagrass, algae and from the shoreline, and depth ranges of 2 to epiphytes were weighed separately with an 6 m from the low water mark (Fig. 2). In electric balance (Model FX-300, A & D Co. the central part of the seagrass bed (the Ltd.) after removing excess water using a region between 40 and 125 m points along the paper towel. census line), the seagrass cover is dense with In a region supporting a high-density ascidian interspersed gaps of sparse cover. population (40 m from the beach), five Generally seagrass cover and biomass were stations with different seagrass covers were high in areas where Syringodium was abun- selected within a small area to cover the range. dant, whereas Halodule occurred in either shal- The light intensity at each station was meas- low or deep areas. Halophila was very sparse ured on the bottom substrate below the except in offshore regions of the seagrass seagrass canopy and at just above the sea bed, especially where Syringodium cover was surface, and its effect on the abundance and not dense. The density of Syringodium varied micro-distribution of ascidians was investi- largely with a maximum of approximately gated. All measurements were conducted in 6,000 shoots • m'2, and the biomass (wet weight) a short period on the same day by a Minolta also varied with a maximum wet weight of 3.6 illuminometer (Model T-1M, Minolta). At each kg•m2. station, all seagrasses in two quadrats (10 x In the seagrass bed, there were several 10 cm2) were clipped at their bases with scis- distinct types of microhabitats for epibenthic sors and collected in separate plastic bags. sessile organisms, including sediment on the For each shoot, attachment of ascidians was sandy bottom, debris accumulated on the sedi- studied.

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