Coral Reefs (2007) 26:113–126 DOI 10.1007/s00338-006-0184-8 REPORT The structuring role of microhabitat type in coral degradation zones: a case study with marine nematodes from Kenya and Zanzibar M. Raes · M. De Troch · S. G. M. Ndaro · A. Muthumbi · K. Guilini · A. Vanreusel Received: 31 July 2006 / Accepted: 20 November 2006 / Published online: 7 February 2007 © Springer-Verlag 2007 Abstract Nematode genus assemblages were identi- three-dimensional structure of coral fragments was Wed from four locations in coral degradation zones found. DiVerences between nematode assemblages in (CDZs) along the African east coast: Watamu and the coralline sediment and on coral fragments were Tiwi Beach (Kenya) and Matemwe and Makunduchi attributed to the exposed nature of the latter habitat, (Zanzibar). Three microhabitat types were distin- its large surface area and its microbial or algal cover. guished: coralline sediment, coral gravel and coral frag- DiVerences in available food sources were reXected in ments. Nematode community composition was nematode trophic composition. comparable to that of other studies dealing with the same habitat. The presence of a common genus pool in Keywords Coral degradation zones · Nematodes · CDZs was reXected in the considerable similarities Microhabitats · Spatial turnover · Indian Ocean between samples. The addition of coral fragments as a habitat for nematodes resulted in an increased impor- tance of taxa typical for coarse sediments and large Introduction substrata. Local and regional turnover were of the same order of magnitude. The structuring eVect of Numerous studies have investigated the major factors microhabitat type clearly overrode the eVect on a local that structure nematode community composition in and regional scale. DiVerences in sediment characteris- coral reef associated sediments (Alongi 1986; Boucher tics were more important in structuring the nematode and Gourbault 1990; Gourbault and Renaud-Mornant assemblages than diVerences between the coralline 1990; Tietjen 1991; Ólafsson et al. 1995; Boucher 1997; sediment and coral fragments. No eVect related to the Ndaro and Ólafsson 1999; Netto et al. 1999; Kotta and Boucher 2001; de Jesús-Navarrete 2003). These eVorts have produced a list of variables that are, at least Communicated by Ecology Editor P.J. Mumby. potentially, determinant: (1) mean sediment grain size, (2) sediment clay-silt content, (3) sediment sorting, (4) M. Raes (&) · M. De Troch · K. Guilini · A. Vanreusel Marine Biology Section, Biology Department, sediment oxygen content, (5) position of redox poten- Ghent University, Sterre complex - S8, tial discontinuity (RPD) layer in the sediment, (6) Krijgslaan 281, 9000 Gent, Belgium organic content of the sediment, (7) extent of bioturba- e-mail: [email protected] tion by macrobenthos, (8) macroXoral biomass, (9) S. G. M. Ndaro water depth, (10) longitude, (11) latitude and (12) sam- Department of Aquatic Environment and Conservation, pling scale. The vast majority of these factors is related University of Dar Es Salaam, P.O. Box 35064, to sediment characteristics. Dar Es Salaam, Tanzania Coral reef associated sediments can be found in all X A. Muthumbi functional zones of the reef, e.g. the reef crest, reef at, Department of Zoology, University of Nairobi, outer reef, reef pools, reef lagoon, etc. Most of the P.O. Box 30197, Nairobi, Kenya studies mentioned above have however focused on 123 114 Coral Reefs (2007) 26:113–126 lagoons, which separate the reef platform from the structure compares to the changes due to turnover on a adjacent land mass. In this zone, dead coral material local and regional scale. The key questions here are: coming from the main reef or from scattered coral (1) do CDZs harbour a typical nematode community? thickets in the lagoon itself (e.g. from patch reefs), is (2) how strong is turnover in taxonomic composition progressively degraded into smaller pieces (coral operating at local and regional scales? And (3) is gravel) until it becomes coralline sediment. Because of microhabitat structure an additional source for varia- the dynamic character of this functional zone, the term tion in nematode community composition? Coral Degradation Zone (CDZ) will be used through- out the text when referring to the lagoon between the reef and the coast. As a result of this gradual process of Materials and methods degradation, the CDZ sediment will obviously be mixed with coral fragments of diVerent sizes and Sampling sites, procedure and microhabitats shapes. The presence of coral degradation products (dead coral fragments, coral gravel) on and in the sedi- Meiofauna samples were collected in the CDZ of the ment may have a profound eVect on nematode assem- fringing reefs stretching along the south coast of Kenya blage structure on a local scale. So far, these coral and along the east coast of Zanzibar Island (Unguja, degradation products have not yet been considered as Tanzania) (Fig. 1). In Kenya, samples were taken on an ecologically valuable habitat in tropical CDZs. two locations: Watamu, the northernmost location The present study contributes to the knowledge of (03°23ЈS, 40°00ЈE; 27th February 2002), and Tiwi nematode assemblages in CDZs of the Indian Ocean, a Beach, more to the south (4°14ЈS, 39°36ЈE; 22nd–23rd subject investigated in only a limited number of studies February 2002). In Zanzibar, samples were taken in [Thomassin et al. 1976 (Madagascar); Ólafsson et al. Matemwe, located in the north of the island (5°52ЈS, 1995 (Zanzibar); Ndaro and Ólafsson 1999 (Zanzibar)]. 39°21ЈE; 24th August 2004 and 31st August 2004), and Here, three distinct microhabitat types (i.e. coralline Makunduchi (6°28ЈS, 39°32ЈE and 6°25ЈS, 39°34ЈE; sediment, coral gravel and coral fragments) are distin- 15th August 2004 and 25th August 2004), located in the guished. This unique approach makes it possible to south of the island. Distance between both Kenyan investigate how changes in community composition locations is 104 km, between Tiwi Beach and Matemwe resulting from the structuring role of microhabitat 183.5 km and between both Zanzibar locations 70 km. Fig. 1 Map of the study area; 38˚ 0' E 39˚ 0' E 40˚ 0' E 41˚ 0' E 42˚ 0' E location of sampling sites is indicated. The northernmost island is Pemba, the southern- Watamu N most island is Unguja (Zanzibar Island) m -500 4˚ 0' S 0m Tiwi Beach -100 m 200 5˚ 0' S - m 0m 0 0 0 0 20 - -3 -100 m Matemwe 6˚ 0' S Makunduchi km 0 50 7˚ 0' S 123 Coral Reefs (2007) 26:113–126 115 On each location, coralline sediment (six samples in sample 200 nematodes (or all nematodes when less both Kenya locations; two samples in both Zanzibar than 200 individuals were present in the examined locations) and coral fragments (two samples in Wat- sample) were randomly picked out. They were subse- amu, three samples in Tiwi Beach and six samples in quently mounted onto slides using the formalin-etha- both Zanzibar locations) were collected. Coral gravel nol-glycerol technique of Seinhorst (1959) and Vincx was collected in Watamu, Matemwe and Makunduchi (1996), and identiWed up to the genus level, using (two samples in all three locations), but not in Tiwi Lorenzen (1994), Warwick et al. (1998), the Desmo- Beach due to the absence of this microhabitat in the doridae key in NeMys (Deprez et al. 2004), and origi- sampled area. Coral gravel can easily be distinguished nal descriptions. The trophic composition of the from coralline sediment because small pieces of coral nematode community was analysed according to the can still be recognised in this microhabitat, whereas classiWcation of Wieser (1953). this is no longer true for the sediment (Fig. 2). The dead coral fragments were either compact or branched, Statistical analyses unaVected or eroded. In Zanzibar, diVerent coral mor- photypes were collected, which were identiWed as Fun- The PRIMER5 software (Plymouth Marine Labora- gia, Goniastrea, Pocillopora, Tubipora and Porites/ tory; Clarke and Gorley 2001) was used to calculate Stylophora. Porites and Sylophora were considered as Bray-Curtis (dis)similarities between all samples. Sam- the same morphotype in this study, based on their ples were grouped together in three ways, concor- robustness and branching morphology. All sampling dantly with three spatial scales: (1) the diVerent regions was carried out under water. Sediment samples were (Kenya/Zanzibar: regional scale), (2) the diVerent sam- taken with a Perspex sediment core (10 cm²), coral pling locations (Watamu/Tiwi Beach/Makunduchi/ gravel was gently scooped out with a spoon or sedi- Matemwe: local scale) and (3) the diVerent habitats ment core and coral fragments were taken out manu- (coralline sediment/coral gravel/coral fragments: ally. Coral gravel and coral fragments were put directly microhabitat scale). The obtained similarity matrix was into Wrm plastic bags or buckets. used to produce non-metric multidimensional scaling two-dimensional plots (MDS). The stress value gives a Laboratory analyses measure for goodness-of-Wt of the MDS ordination: a low-stress value (<0.2) indicates a good ordination with Meiofauna extraction from coralline sediment and no real prospect for a misleading interpretation coral gravel was done by decantation with Wltered (Clarke 1993). One-way, two-way crossed and two-way seawater over a 1 mm and a 32 m sieve and subse- nested Analysis of Similarities (ANOSIM) were car- quent centrifugation. Coral fragments were rinsed oV ried out to test for signiWcant diVerences in the nema- thoroughly, also with Wltered seawater, over the same tode community structure between diVerent groups sieves. The material collected on the 32 m sieve was that were delimited beforehand. Similarity of Percent- then subjected to density gradient centrifugation, ages (SIMPER) was used to investigate which genera using Ludox (a colloidal silica polymer; speciWc grav- were responsible for these diVerences. Due to diVer- ity 1.18) as a Xotation medium (Heip et al.