Back to index of marine invertebrate studies
THE BIODIVERSITY, ABUNDANCE AND LOCAL DISTRIBUTION PATTERNS OF OPISTHOBRANCH SPECIES
Abstract
Between July and September 2001, volunteers and staff scientists collected quantitative data on the distribution, abundance and substrate preferences of opisthobranchs on the coral reefs off Hoga Island, Kaledupa, Buton. Three reefs – named Sampela, Kaledupa, and Home – were sampled. At each site, fifteen 30 square meter quadrats were laid from the reef edge to 12 meters depth and, at the latter two sites, an additional 15 transects were examined between 12 and 18 meters depth. A total of 88 individuals were found among all 75 transects, comprising a total of 25 species. Habitat information, including coral and sponge cover and the presence of other free-living invertebrate species, was also collected for each transect. In addition, a timed search effort survey of opisthobranch presence/absence and substrate was conducted on 12 reefs throughout the Tukang Besi archipelago.
Introduction
At the present time, there is relatively little
known about the ecology of many marine communities in comparison with the
research that has been conducted in most terrestrial systems. For the coral reef
in particular, an overall understanding of how environmental variation and
biological interactions shape reef communities has been rapidly growing, but
only over the last few decades. Furthermore, there is, of course, more of a
focus on gaining information about the commercially important, the most
conspicuous, and the more numerous species on the reefs.
Opisthobranchs, more commonly known as "sea slugs", do not fall into any of those three categories. Although they are highly colourful and appealing creatures, they are very rare in the landscape of the coral reef. Thus, reef ecologists have largely overlooked sea slugs when examining the abundance, distribution and diversity patterns of coral reef taxa. The primary objectives of this segment of Operation Wallacea’s scientific program were to address those fundamental ecological questions for the Opisthobranchs of the Wakatobi National Marine Park.
The coral reefs near Hoga Island provide a few important examples of the variation inherent in coral reef communities, including differences in sedimentation, slope, water movement, and the percent cover of coral and other sessile invertebrates. Two sites chosen for this project – Home and Kaledupa reefs – are examples of largely undisturbed reefs, allowing for questions about the fundamental habitat parameters of tropical Opisthobranchs. Kaledupa and a third site, Sampela, were ideal for generating information on the effects of sedimentation and the related reduction in coral cover on the abundance, distribution and diversity of this group of coral reef fauna.
Methods
Primary survey Between July and September 2001, three reefs near Hoga Island were examined for the presence of opisthobranchs. Fifteen 30 square meter quadrats were distributed randomly either between the reef edge and 12 meters depth (all reefs) or between 12 meters and 18 meters depth (Kaledupa and Home reefs only). To delineate each quadrat, a measuring tape section 15 meters long was extended at constant depth. One diver from a buddy pair searched either one meter above or one meter below the measuring tape. Each search was conducted for 25 minutes, after which the divers switched sides of the tape and checked that half of the quadrat for five additional minutes.
If an opisthobranch was encountered, the timer was stopped, and the species, size, substrate and activity of the individual was recorded. The presence/absence of an egg case in the immediate vicinity of the opisthobranch was also noted. No samples of either the opisthobranch or the substrate were collected.
The presence and abundance of all other free-living invertebrates, that is, invertebrates that are not primarily sessile on the reef surface or swimming in the water column during their adult life stage, was also noted.
A third diver measured percent hard and soft coral, sponge, sand/rubble, or other cover using a line-intercept method. This method records whichever of the five substrate categories that was primarily in contact with or directly underneath each 10 cm section of the measuring tape.
Secondary survey On 12 separate reefs in the Tukang Besi archipelago, times searches were conducted for the occurrence of opisthobranchs. On each reef, divers were distributed from the reef edge to 18 meters depth. Divers swam along the reef searching for opisthobranchs and opisthobranch egg cases while recording the time spent searching. When an opisthobranch was encountered, timers were stopped and the species, size, substrate and activity of the opisthobranch was recorded.
Results
During the primary survey, a total of 88 individual opisthobranchs were found. Almost all opisthobranchs encountered were in the Order Nudibranchia. There were 25 different species found, however, only two species were found on all three reefs. The primary survey indicates an overall low population abundance of opisthobranchs in this region. However, because very few studies have been conducted on the ecology of tropical opisthobranchs, it is not certain whether this density is typical of many tropical regions. In relation to the density of opisthobranchs, diversity can probably be considered as extremely high.
The data from the secondary survey have not yet been analysed. These data will hopefully give an indication as to whether the abundances and density near Hoga Island are typical of a larger area within the Wakatobi National Marine Park.
Initial results do not indicate a strong relationship between habitat types surveyed and presence/absence or abundance of opisthobranchs. Likewise, there are no apparent differences in opisthobranch distribution among the two depth categories surveyed. However, these are very preliminary assessments only and will be tested statistically prior to publication. Likewise, diversity indices will be applied to determine whether habitat and/or depth influence local opisthobranch diversity.
Reports
The data collected in the primary survey around Hoga Island will be presented in a paper entitled Observations on the diversity and habitat parameters of nudibranchs in the Tukang Besi Islands off Southeast Sulawesi, Indonesia by Johanna Polsenberg, Stanford University by May 2002. The data from the timed surveys will be compiled in a report for the use of managers and scientists working in the Wakatobi Marine National Park.