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 molle

M. Nishihira and T. Suzuki

Biological Institute, Graduate School of Science, Tohoku University, Aoba-ku, Sendai 980-8578, Japan

Abstract: Six 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 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 distri- bution 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 struc- ture, 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 . 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 communities in seagrass beds at Dravuni Island in 1989, and recorded 3 species of compound ascidians. However, their study did not cover micro- habitat 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 establish- ment 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. ment, leaf blades and sheaths of seagrasses, To estimate size composition of Didemnum epiphytic organisms, and macroalgae growing molle populations on different substrates, on the seagrass body or on the sandy bottom. colonies were collected haphazardly from the Among these microhabitats, compound ascidians seagrasses: Syringodium isoetifolium (at the used seagrass blades and sheaths, or macro- Dravuni seagrass bed) and Halodule uninervis algae with robust thalli growing on the sandy (Forsk.) Ascher. in the seagrass bed and bottom. Sheaths and blades provided the adjacent rock substrates (at Yaokube Island). epiphytic organisms with different light regimes To estimate colony sizes, the wet weights of (see Fig. 7). This is supported by the marked colonies were measured using an electric differences in light intensity and differences in balance after removing the excess water. mobility in terms of the swaying frequency and Ascidians on rock substrates in a coral reef persistence in terms of lifetime of blades and around Sesoko Island (26°38' N, 127°52' E) in sheaths. Life time of blades was estimated at Okinawa were also studied for comparison. 56 days (Aioi and Pollard 1993), but sheaths To quantify the degree of swaying of a stood much longer (pers. observ.). The blade seagrass body, a video was recorded using sways continuously even where there is weak 32 M. Nishihira and T. Suzuki

a. Syringodium isoetifolium water movement; video recording showed that, for example, a 22 cm-long blade sways 7 times per minute with an amplitude measured at blade tip being 34 cm, while sheeths did not

b. Halodule uninervis move. There were several kinds of macroalgae growing as epiphytes on the seagrass blades and sheaths. Among them, Cladosiphon sp. and Dictyota sp. and unidentified filamentous

c. Halophila ova/is were abundant (Fig. 2). These macroalgae seemed to be less important than seagrasses as attachment substrates for com- pound ascidians, since they were thin, soft and /or with much mucus. Of 1,142 ascidian colo- d. Halimeda sp. nies examined in the quantitative samplings, only 160 (14%) were found on these algae. On the sandy bottom, there were several calcare- ous green algae, such as Halimeda sp. and Udotea glaucescens. Some of the Halimeda e. Epiphytic filamentous algae served as important substrates for the ascidian Lissoclinum bistratum (Sluiter).

Ascidian fauna in the seagrass bed Six species of compound ascidians were f. Dictyota sp. found along the transect line (Photograph 1). According to Kott (1980, 1981, 1982), 3 species, Didemnum molle, Lissoclinum bistratum and Trididemnum clinides Kott, have a unicellular symbiotic alga, .Prochloron sp. The other 3 g. Cladosiphon sp. species are non-symbiotic ascidians (Kott 1981). Chlorophyll a of D. molle showed the highest level and L, bistratum the smallest (Koike and Suzuki 1996). These 2 species generally appear green, while T, clinides with an intermediate chlorophyll a content, appears h. Debris brown. In the Dravuni seagrass bed, L. bistratum was the most abundant ascidian in terms of number of colonies, comprising 56.5% of the i. Bottom slope total ascidian colonies, followed by T. clinides (25.6%), D. molle (10.2%), Didemnum sp. cf. albopunctatum Sluiter (6.1%), Didemnum cuculliferum (Sluiter) (1.1%) and Trididemnum discrepans (Sluiter) (0.5%). In an extensive survey of animal communities in the same area in 1989, Mukai & Nojima (pers. comm.) Fig. 2 Distributions of the seagrasses, algal and de- found three species of compound ascidians. bris biomasses (wet weight) in the seagrass bed of Their survey showed that L. bistratum was Dravuni Island, Fiji, a, Syringodium isoetifolium; b, the most abundant ascidian, followed by D. Halodule uninervis; c, Halophila ovalis; d, Halimeda sp., e, epiphytic filamentous algae; f, Dictyota sp., molle. They also found L. voeltzkowi in the g, Cladosiphon sp., h, debris (such as dead seagrass same survey site, but it was not found along leaves). The bottom graph (i) shows a profile of the the transect line in the present survey. transect line in the seagrass bed (depth at low tide). Values in a and b show only the above-ground biomass. Bars represent SD. All the biomass values are averages Ecological distribution of 4 samples. Although the seagrass bed extended over a Compound ascidians in Fijian seagrass bed 33

a. Didemnum molle ascidians gradually decreased, and were very rare at 110 m from the shoreline (5 m depth). Most species showed a maximum density at 40-60 m from the shoreline. No correlation was found between seagrass biomasses (for sam- ples with >20 g wet weight) and pooled ascidian b. Lissociinum bistratum densities (r2 =0.071, n=34, p>0.1; Fig. 4).

c. Trididemnum clinides

d. Didemnum sp. cf. albopunctatum

Fig. 4 Relationship between the abundance of ascidian colonies attached to seagrass (6 species pooled) and e. Didemnum cuculliferum seagrass biomass (2 species pooled) in the Dravuni seagrass bed. Each dot represents a value in a 25 x 25 cm2 area. Samples with a seagrass biomass of X20 g wet weight were used for the analysis.

Thus, the seagrass biomass did not explain f. Trididemnum discrepans the abundance of ascidians as a group. The maximum densities of D, molle, L. bistratum and T. clinides colonies were 980• m2, 11,500•m2, and 3,280•m2, respectively (Fig. 3a, b, c). The distributions of the other three species are also presented (Fig. 3d-f). The horizontal distribution ranges of these six

Fig. 3 Distribution of colonial ascidians along the ascidians greatly overlapped, suggesting that transect line in the seagrass bed of Dravuni Island, differentiation in microhabitat use is not Fiji. Densities are presented separately for those at- established by occupying different places in tached to a seagrass body (solid line) and those at- the seagrass bed or different depths of the tached to benthic algae or debris (dotted line), a, sea bottom. Didemnum molle; b, Lissoclinum bistratum; c, Trididemnum clinides; d, Didemnum sp. cf. albopunctatum; e, Didemnum Distribution of ascidians on the seagrass body cuculliferum; and f, Trididemnum discrepans. Values At the 40 m-point, there were gaps in the are averages of 4 samples, and bars represent SD. seagrass bed providing a range of different seagrass covers within a narrow area. This wide area (Fig. 2), compound ascidians were made it possible to test the effect of seagrass not distributed evenly over the entire seagrass density on the population density and micro- bed, but rather, all species tended to be con- habitat use of ascidians in a limited area. At centrated in one region of the seagrass bed this site, the light intensity at the sediment (Fig. 3). They first appeared at around 30 m surface was low in the stations with dense from the shoreline (about 2 m depth at low seagrass cover. The abundance of seagrass tide) and density increased markedly after shoots colonized by ascidians was not neces- this point. However, as the distance from the sarily proportional to the abundance of shoots shoreline increased, the number of compound (Fig. 5). The plot with a moderately sparse 34 M. Nishihira and T. Suzuki

a. Leaf blades a

b. Sheaths

b

C. Detached from seagrass

Fig. 5 a, Changes in light intensity at the bottom level of the seagrass bed at the 5 stations. Values are relative to those just above the sea surface. b, d. Benthic algae+debris+sediment Densities (bars) of the seagrass shoots, Syringodium isoetifolium, and ratio (%) of the shoots colonized by colonial ascidians to all shoots (®), in the 5 sta- tions at around the 40 m-point along the transect line. The shaded part shows those seagrasses that were colonized by ascidians, and the unshaded parts show those that were not colonized. growth of seagrass (St. 2) had the highest density of ascidians as well as the largest proportion of colonized shoots. This is related Fig. 6 Abundance of colonial ascidians in different to the fact that Lissoclinum bistratum was microhabitats provided by the seagrass Syringodium most abundant in the sparse plots (see Fig. 6). isoetifolium in the 5 stations near the 40 m-point. a, Fig. 6 shows the abundance of colonies for colonies attached to leaf blades; b, colonies on sheaths; each ascidian species in different microhabitats c, colonies detached from the seagrass body during at each station. The most abundant species collection (attachment position could not be speci- L. bistratum, was generally restricted to fied) ; and d, colonies collected on benthic algae, de- stations with low seagrass cover, and was bris or sediment. Numbers on the bar show the density in case of 2000. Different ascidian species are rep- found on both the seagrass body and on resented by different symbols: Dm, Didemnum molle; sediment at the bottom. Few, if any, colonies Lb, Lissoclinum bistratum; Tc, Trididemnum clinides; were found in areas of dense seagrass cover. Da, Didemnum sp. cf. albopunctatum; Dc, Didemnum Many colonies were found detached after the cucullif erum; Td, Trididemnum discrepans. sampling. Trididemnum clinides, the second most abundant species was found in seagrass was more abundant in stations with dense and debris, but not bottom sediments. T. seagrass cover. It was seldom found to attach clinides was found in relatively equal abun- to sheaths. The other three species were rare dance in plots with sheaths, but was found to and never seen on seagrass blades. increase with increasing seagrass density in The vertical distributions of the ascidians plots with blades and debris. Didemnum molle on the seagrass body is depicted in Fig. 7, was found mostly on seagrass blades, and together with the light intensity and amount Compound ascidians in Fijian seagrass bed 35

a b D. molle on Syringodium isoetitoliurr at Dravuni Is. n=160

D. molle on Halodule uninervis at Yaokube Is. n=162

c

D. molleon rock at Yaokube Is. n=177

Fig. 7 a, Vertical distributions of light intensity ex- D. molle on rock and coral skeletons pressed by relative values to those at the water sur- at Sesoko Is., Okinawa face; b, amount of attachment substrates of the sea- n=195 grass body of Syringodium isoetif olium expressed by the total lengths of blades or sheaths in an area of 500 cm2 (5 samples of 100 cm2) ; and c, number of co- lonial ascidians on the seagrass body in an area of 500 cm2. In the bottom graphs, open bars represent colonies on leaf blades, and solid bars represent colo- nies on sheaths.

Fig. 8 Colony size composition of Didemnum molle of attachment substrates in terms of the total populations on stable and unstable substrates. a, length of seagrass blades and sheaths. D. colonies on the blades of Syringodium isoetifolium molle attached almost exclusively to leaf blades at Dravuni Island, Fiji; b, colonies on the blades of (94.8% of the colonies) showing the widest Halodule uninervis at Yaokube Island, Fiji; c, colo- vertical range. L. bistratum had a wide range nies on the rock substrates at Yaokube Island, Fiji; of attachment positions, but was found mostly and d, colonies on the rocks and coral skeletons on a on sheaths (70.0%). T. clinides was found coral reef of Sesoko Island, Okinawa, Japan. Note mostly on sheaths (80.2%), but was occasion- different scales on Y axis. ally also seen on blades. These three species are all symbiont-bearing ascidians. The other sized colonies (Fig. 8). However, the size 3 species, Didemnum sp. cf. albopunctaturn, compositions were different among popula- T. discrepans and D. cucullif erum were less tions on different substrate types (Mann- abundant and symbiont-free, they were found Whitney U test, Halodule vs rock p<0.05, almost exclusively on sheaths. Syringodium vs Halodule and rock, p<0.001), except for the difference between populations Population structure of Didemnum molle on rock substrates in Yaokube and Okinawa On both rocks (stable substrate with great (Mann-Whitney U test, p>0.05). The ascidians temporal persistence) and seagrass blades on rock substrates had colonies of up to 900 unstable substrate with less persistence), mg in wet weight (Fig. 8c, d), whereas maxi- ascidian populations showed a common char- mum colony size on blades of Syringodium acteristic having a great proportion of small- had were only 350 mg (Fig. 8a). The size of 36 M. Nishihira and T. Suzuki

Phoograph. 1 Compound ascidians found on the seagrass, Syringodium isoetifolium in the seagrass bed at Dravuni Island, Fiji. 1, Didemnum molle (Herdman); 2, Didenum cuculliferum (Sluiter); 3, Didemnum sp. cf. albopunctatum Sluiter; 4 , Trididemnum discrepans (Sluiter); 5, Trididemnum clinides Kott; 6, Lissoclinum bistratum (Sluiter). Bars show 5 mm. the colonies on blades of Halodule was between which constitute an important component in those on Syringodium blades and those on rock the Dravuni seagrass bed community. Large (Fig. 8b). Okinawan populations on coral reef differences in mobility in terms of the swaying showed a size structure similar to that of the frequency and illumination regimes developed Fijian rock population (Fig. 8c, d). between the blades and sheaths, which seems These findings suggest either that ascidian to be reflected in the different group of ascidian colonies can grow to larger sizes on stable species using such microhabitats. substrates, or that larger colonies cannot Didemnum molle usually occupied high light maintain themselves on small and unstable habitats by attaching mostly to the seagrass ephemeral substrates. blades. Lissoclinum bistratum also occupied well lit habitats, but tended to be found mostly in places with a thin seagrass cover, attached DISCUSSION to sheaths or the bottom sand surface. Microhabitat partitioning by ascidians Trididemnum clinides were also found in Seagrass beds provides various microhabitats, seagrass, occupying mostly the darker sheaths. which are partitioned into different parts of Each ascidian used slightly different parts of the seagrass body, and the epiphytic commu- the seagrass microhabitats, and this suggests nities that flourish on seagrass bodies. In- a possibility of differences in the light require- habitation on biogenic habitat structures may ment of their symbionts. The remaining 3 be one way by which promotion of multi-species species, D. cuculliferum, Didemnum sp. cf. coexistence in the seagrass bed is achieved albopunctatum, and T. discrepans, all lacking (Nishihira 1993). The blades and sheaths algal symbionts (Kott, 1980), occupied dark provide different microhabitats for various but relatively stable sheaths in dense seagrass organisms, including compound ascidians patches. Compound ascidians in Fijian seagrass bed 37

Only L. bistratum was found regularly grow- patches are more stable than the sandy sedi- ing directly on the sandy bottom. As the ment surface outside the seagrass patches. small gaps in the seagrass bed seem some- On the seagrass itself, the sheath is the most what stable, and colonization by L. bistratum persistent substrate because of its greater may also fix the sand, aggregate colonization persistence and smaller mobility, while blades seems effective for stabilizing sand substrates. are short lied and unstable substrata. The Colonies of this species are flat (see Photo- blade sways continuously and the lifetime of graph 1), and the proportion of the attach- blades has been estimated at 56 days (Aioi ment area to the entire surface of the colony and Pollard 1993), which means that a blade is highest among the 6 species. The other is renewed every two months. Therefore, colo- species had very small attachment areas, nies near the blade tip can be removed from which means they may not be able to stay on the seagrass bed. The growth point of the sand grains for long period. T. clinides had blade is at the base of the blade, and the distal the smallest attachment stalk and was easily portion is old and easily torn off as it ages. detached from the substrate. This may explain Therefore, although occupation of the blade why this symbiont-bearing species was is beneficial in terms of photosynthesis, it is restricted to the stable proximal part of the risky in terms of substrate persistence and seagrass body. stability. Even on such an unstable substrate, Olson (1982) demonstrated that D. molle D. molle still occupies the blades in the seagrass occupying dark places had a greater ratio of bed even though attaching at this position at chlorophyll b to a than those occupying light the distal portion of the blade increases the places. He also showed that the larvae and chance of detachment. How do the ascidians small colonies had a smaller chlorophyll a/ b solve this problem? D. molle can move along ratio than the large mature colonies. Therefore, the substrate and perform colony division it was suspected that the chlorophyll a content (Cowan 1981). This capability seems to be of may be different in ascidian colonies occupying great importance for adaptation to such different microhabitats. In Dravuni seagrass substrates. Methods to overcome the difficul- bed, Suzuki and Nishihira (1994) examined ties associated with unsuitable substrat is the chlorophyll a content of D. molle collected undoubtedly of primary importance for all from different parts of the seagrass body, kinds of sessile animals, because there is but found that it was rather constant always the possibility of them settling on irrespective of attachment positions on the unstable substrates. Since D. molle frequently seagrass. It seems conceivable that the distal attach to short-lived, unstable seagrass portion of the seagrass blade might be more substrate, they might have some unknown suitable for photosynthesis of the algal tactics. Detailed study of their dynamics on symbiont due to its greater light exposure a seagrass substrate is needed. than the proximal part (sheath). Differences in the population structures of As for relative light intensity, the top of the D. molle were detected among the different seagrass meadow showed 58%, the blades layer types of attachment substrates. Blades of 38%, the sheath layer 23%, and the bottom Syringodium, an ephemeral substrate with a substrate surface only 3% of the light inten- blade diameter of only 1.5 mm (the smallest), sity at the sea surface. The actual light inten- supported ascidian colonies of the smallest size sity was 3,500 lux at the top of the seagrass class, whereas the stable long-persisting rock (Pig. 7a), and it was almost comparable to substrate supported some large colonies the intensity at which photosynthetic activity together with the small ones. This seems to is saturated in D. molle (Olson, 1986). The be related to the nature of the substrate, observed microhabitat preference of D. molle especially to its size, lifetime and mobility. may be explained by their tendency to occupy On stable substrates, almost identical colony places more appropriate for photothynthesis size composition was found in different of their algal symbionts. However, the distal geographical regions of Fiji and Okinawa. portion of the blades is unstable and has a Colony size composition may affect larval greater chance of becoming detached. production, and the characteristics of the attachment substrates may be a fundamental Nature of substrate constraint to the growth and population It appears that substrates inside seagrass processes of the ascidians. 38 M. Nishihira and T. Suzuki

water Res 44: 1-17 ACKNOWLEDGMENTS Nishihira M (1992) Biogenic creation of habitat struc- We thank T. Nishikawa for identification of ture and multi-species coexistence. pp86-100 In: Higashi M, Abe T (ed) What is Symbiosphere? the ascidians. We also thank I. Koike, H. Heibon-sha, Tokyo (In Japanese). Mukai, K. Aioi, H. Kunii, S. Nojima and A. Nishihira M (1993) Habitat structure and biodiversity Ijima for helping with field work at Dravuni in the coral reef areas: Ecological process in habi- Island and commenting on the early drafts of tat creation and community development on micro- this ms. The field work was conducted at the atolls of the massive coral Porites. Symbiosphere (Biology International) 29: 26-29 Marine Laboratory of the University of South Olson RR (1982) Ascidian-Prochloron symbiosis: The Pacific. We would like to express our thanks role of larval photoadaptations in mid day larval to the staff of the laboratory and people of release and settlement. Biol Bull 165: 221-240 Dravuni village. Field studies on Okinawan Olson RR (1985) The consequences of short-distance coral reef was assisted by K. Sakai, and we larval dispersal in a sessile marine invertebrate. Ecology 66: 30-39 extend our thanks to the staff of Sesoko Sta- Olson RR (1986) Light-enhanced growth of the ascidian tion, Tropical Biosphere Research Center, Didemnum molle-Prochloron sp. symbiosis. Mar U-niversity of the Ryukyus. We thank W. Biol 93: 437-442 Lau for reading the early draft of the manu- Stoner DS (1992) Vertical distribution of a colonial script. We also thank two anonymous referees ascidian on a coral reef : The roles of larval dis- persal and life history variation. Am Nat 139: for their valuable comments on the manuscript. 802-824 This study was supported partly by a Grant- Stoner AW, Lewis III FP (1985) The influence of quan- in-Aid for Scientific Research on Overseas titative and qualitative aspects of habitat com- Research (Na 03041027) and a grant for Priority plexity in tropical seagrass meadows. J Exp Mar Area ( #319) Projects "Symbiotic Biosphere; Biol Ecol 94: 19-40 Suzuki T, Nishihira M (1994) How does a compound An Ecological Complexity Promoting the ascidian Didemnum molle remain on an unstable Coexistence of Many Species" from the substrate? pp60-75 In Koike I (ed) Developmental Ministry of Education, Science, Sports and Processes and Material Flow in Tropical Seagrass Culture, Japan. It was supported partly by a Beds -Second Phase-. Ocean Research Institute, Grant-in-Aid from Japan foundation. Univ. of Tokyo This is a contribution from Sesoko Station. (Date of acceptance: October 30, 2000)

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