Mar Biol (2010) 157:2503–2509 DOI 10.1007/s00227-010-1514-5
ORIGINAL PAPER
Variability in the spatial association patterns of sponge assemblages in response to environmental heterogeneity James J. Bell · Jade Berman · Timothy Jones · Leanne J. Hepburn
Received: 28 September 2009 / Accepted: 5 July 2010 / Published online: 14 July 2010 Springer-Verlag 2010
Abstract Previous work on tropical sponge assemblages has provided strong evidence that sponges coexist on coral reefs through a diversity of positive and negative associations; however, the majority of this work has focused on Caribbean coral reefs. Here, we investigate the intra-phyletic spatial associations between the 20 most abundant sponge species at two sites experiencing diVerent environmental regimes in the Wakatobi National Marine Park, Indonesia. We used a Monte Carlo simulation approach to compare the number of spatial associations between each species pair to that expected if species distribution patterns were non-associative (i.e. random). We found that sponges were predominately randomly distributed at the high coral cover site, whereas most sponges were negatively associated with other sponges at the sedimented, low coral cover site. We also found diVerences between distribution patterns for speciWc species at the two sites; a number of species that showed a random distribution pattern at the high coral cover site had negative association patterns at the low coral cover site. Our research supports recent ecological studies suggesting that interactions between species are unlikely to be homogenously distributed, as we found that
Communicated by F. Bulleri. J. J. Bell (&) · J. Berman · T. Jones Centre for Marine Environmental and Economic Research, School of Biological Sciences, Victoria University of Wellington, Po Box 600, Wellington, New Zealand e-mail:
[email protected] L. J. Hepburn Coral Reef Research Unit, Department of Biological Sciences, University of Essex, Wivenhoe Park, Colchester CO4 3SQ, UK
some sponge species interactions diVered depending on the environmental regimes in which they were found; this suggests that species interactions may be spatially variable. Finally, our results contrast with studies from elsewhere, as the sponge assemblages at these two sites in the Wakatobi appear to be dominated by negative associations and random distribution patterns rather than widespread competition.
Introduction Sponges are an important component of reef ecosystems across the world, which have a number of important ecological roles, and are therefore likely to be important in maintaining ecosystem functioning (WulV 2001; Diaz and Rützler 2001; Rützler 2004; Bell 2008). As sponges often occupy a signiWcant amount of primary coral reef substratum (e.g. Diaz et al. 1990; Schmahl 1990; WulV 2001; Bell and Smith 2004), they frequently encounter other reef organisms. There has been much discussion regarding sponge spatial interactions and associations on coral reefs, and especially those relationships with corals (see review by WulV 2006). Space is generally limiting in ‘healthy’ coral reef environments, which might be expected to lead to either competition or cooperation over evolutionary time scales between the dominant sessile encrusting organisms that share similar habitat requirements. Previous examinations of intra-phyletic sponge spatial interactions and associations have identiWed a diversity of interactions including both cooperative and competitive associations (Rützler 1970, 2004; WulV 1997, 2005, 2006, 2007, 2008; Ávila and Carballo 2009). However, these observations are predominately from research conducted in the Caribbean and PaciWc and whether this is the case for sponge assemblages
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elsewhere, including the Indo-PaciWc, where sponge diversity is considerably higher, remains unknown. The Indo-PaciWc region contains the highest concentration of marine biodiversity in the world, including sponges (Briggs 1987; Hooper and Lévi 1994; van Soest 1994), and this higher biodiversity may mean important structuring processes may be diVerent compared to other tropical regions where diversity is lower. Although research on sponge ecology in the Indo-PaciWc lags considerably behind that in the Caribbean (see review by Rützler 2004), there have been a number of recent ecological studies on the Indonesian sponge fauna particularly focused on the Spermonde Archipelago (Becking et al. 2006; Cleary et al. 2005; de Voogd et al. 2006; de Voogd 2007; de Voogd et al. 2005, 2006; de Voogd and Cleary 2007) and the Wakatobi National Marine Park (WNMP) (Bell and Smith 2004; Bell 2007a, b). These studies have particularly focused on sponge spatial distribution patterns, monitoring, ecological functions, bioactive compounds and the potential for sponge aquaculture. Furthermore, de Voogd et al. (2004) investigated the spatial interactions between four sponge species with other sponges and also scleractinian corals, reporting a high proportion (85%) of tissue necrosis when sponges interacted with corals, but a lower level of necrosis in sponge intra-phyletic interactions (25%). Observations of sponge–sponge interactions based on single interval sampling have the potential to misinterpret the potential outcomes of spatial interactions (see Aerts and van Soest 1997; Aerts 1998, 2000; WulV 2006); however, there is the potential for identifying associations between species from instantaneous observations. Such assessments for the entire assemblage allow an estimation of the predominant nature of sponge associations. Based on the assumption that if associations between sponge species living in the same microhabitat (i.e. they have the same microhabitat requirements) are random, then the number of associations between any two sponge species should be proportional to the relative abundance of those species (i.e. a rare species should meet a common species less often, compared with two common species), we examined spatial associations between sponges on two tropical Indo-PaciWc coral reefs in the Wakatobi National Marine Park (WNMP) in Indonesia. The aim of this study was to use spatial association data to determine whether the dominant components of the sponge fauna on two Indo-PaciWc coral reefs are positively or negatively associated with other sponge species or are randomly distributed. SpeciWc objectives were to (1) quantify the abundance (area occupied) of the 20 most abundant species at two sites in the WMNP; and (2) determine the number of intra-phyletic spatial associations between these 20 species and compare the number of associations for species pairs with that expected if association/
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distribution patterns are random using a Monte Carlo simulation approach.
Materials and methods Study sites We examined the intra-phyletic sponge associations at two sites (Hoga and Sampela; separated by approximately 1 km) on the fringing reefs of Hoga Island in the vicinity of Kaladupa in SE Sulawesi, Indonesia (see Bell and Smith 2004 for exact site locations and maps). Sampela is considered to be a degraded and light-limited reef system, which experiences heavy rates of Wne sedimentation. This site has been subject to heavy levels of Wshing and coral extraction (mining) in the past. Sampela and Hoga also have diVerent gross sediment deposition rates with 7.54 § 0.76 g d wt m¡2 d¡1 being recorded at Hoga and 20.16 § 1.76 g d wt m¡2 d¡1 at Sampela (Crabbe and Smith 2002). The Xow rates at the two sites are similar at approximately 10–20 cm s¡1 (authors unpublished data). These sites diVer signiWcantly in their overall community composition and diversity, with lower values of coral cover at Sampela (currently 5–10%) compared with Hoga (currently 30–40%), which has been declining over the past 5–6 years. Sponges are abundant at both sites, although there are major diVerences in assemblage composition, richness and overall sponge abundance (see Bell and Smith 2004). Characterisation of sponge spatial associations and abundance Sponge–sponge associations were recorded in 120 haphazardly placed quadrats (0.5 £ 0.5 m £ 120 = 30 m2 in total) placed on the reef slope between 10 and 15 m at each site. Since environmental variability is known to inXuence sponge assemblages at the two study sites (see Bell and Smith 2004), interactions were only examined on vertical cliV surfaces. Spatial interactions can be classiWed in a number of diVerent ways including overgrowth, loss (overgrown) or tied/stand-oV (no apparent winner or loser) (e.g. see Aerts and van Soest 1997; Aerts 1998, 2000; Bell and Barnes 2003; de Voogd et al. 2004). However, we were interested in the total number of spatial associations between sponges, rather than trying to distinguish the outcomes of the associations/interactions. Species that were associated with more other species than expected based on their abundance were considered to be positively associated with other sponges. Species that interacted with fewer species than expected were considered to be negatively associated with other sponge species. For the purpose of this study, sponges were considered to be in an association
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when one sponge was in contact with another (90% of all sponges at the sites). The abundance of each sponge was estimated as the overall percentage cover in 60 haphazardly placed quadrats at the same sites using a divided quadrat (15 m2 in total at each site). Simulation of the expected distribution of associations The number of observed associations was compared to that which would be expected based on the relative abundance, and random distribution of sponges, using a Monte Carlo simulation approach similar to that used by Turon et al. (1996). The relative abundances of each sponge species (area occupied) were calculated for the 20 species with the highest number of observed associations and used to determine the number of associations between species pairs if interactions were random. The relative abundances were then ordered in decreasing order, and cumulative abundances were calculated. A random number was then generated between 0 and 1, which was used to choose the Wrst species in an association. For example, for Wve species, we might have relative abundances of 0.4, 0.3, 0.1, 0.1 and 0.1, and cumulative abundances of 0.4, 0.7, 0.8, 0.9 and 1. If the random number falls within the interval 0–0.4, then species 1 is chosen, if it falls within 0.4–0.7, species 2 is chosen, etc. This same technique was applied to determine the second species in the association by generating a second random number. However, in the second run, the relative abundances were adjusted to account for the fact that the Wrst sponge chosen could not be involved in an association with itself. For example, if species 1 was chosen as the Wrst species in an association and had a total relative abundance of n, then in the second round its relative abundance would be adjusted to n ¡ 1. The cumulative abundances were similarly adjusted. To generate the total number of observed associations, this process was repeated the same number of times equal to the number of associations observed at each site (Hoga n = 681, Sampela n = 850). From each set of simulated associations, the number of associations between any two species (interspeciWc and intraspeciWc) was recorded as well as the total number of associations each species was involved in. This whole process was repeated 1,000 times to generate a distribution of expected values for each association type (every possible association between any two species) and for the total number of associations each species was involved in. This represents the distribution of possible values if associations were due to the random distribution of sponges. The number of observed associations
2505
was then compared to the distribution of simulated associations. Where the observed number of associations fell between the lower 2.5th percentile and the upper 2.5th percentile of the generated distribution, the number of observed associations was deemed to be not signiWcantly diVerent from a random distribution at the 95% signiWcance level. However, if the number of observed associations was below the lower 2.5th percentile, then the observed associations were signiWcantly less than expected (negatively associated), and if greater than the upper 2.5th percentile, the observed associations were signiWcantly more than expected (positively associated). The simulation was conducted by R version 2.9.0.
Results The interactions between the diVerent sponge species are shown in Tables 1 and 2. The top 20 interacting species were involved in a total of 681 and 850 intra-phyletic associations at Hoga and Sampela, respectively (Tables 1, 2), which represented 85 and 91% of all sponge–sponge associations. At Hoga, the 20 most abundant sponge species in the total 15 m2 area (representing >90% of all sponges present) covered a total area of 18.49% (Fig. 1). At Sampela, the 20 most abundant species (which showed some degree of overlap with the species found at Hoga) covered a total of 31.4% of the 15 m2 sampling area. Despite the two sites having similar overall sponge richness (see Bell and Smith 2004), the sponge assemblage is more even at Hoga, while Sampela is primarily dominated by three species (Fig. 1). Simulation results are shown in Table 3, which shows the number of observed associations compared to the distribution of the simulated associations. Observed values above or below the upper or lower 2.5th percentile of the simulated distribution indicate positive or negative associations, respectively; observed values within these upper and lower percentiles show random distribution patterns. At Hoga, of the 20 sponges involved in the most interactions, the majority were randomly distributed (11 species), with three species being positively associated with other species (Pseudoceratina sp., Sycon sp. and Clathrina sp.), and six species were negatively associated with other species. In contrast, at Sampela, most species were negatively associated with other species (12 species), while only two were positively associated with other species (Callyspongia sp. and Lamellodysidea herbacea) and six were randomly distributed. Nine of the 20 most abundant species were shared between the two sites including Pseudoceratina sp., Lamellodysidea herbacea, Prosuberites sp., Haliclona sp. 2, Chalinula milnei, Niphates sp. 1, Clathria mima, Agelas sp. and Clathrina sp. Of these species, Lamellodysidea herbacea,
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Table 1 The reported spatial associations between the 20 species with the most associations within a 30-m2 sampling area at Hoga 1 2 Pseudoceratina sp. (1) Lamellodysidea herbacea (2) Sycon sp. (3) Prosuberites sp. (4) Chelonaplysilla sp. (5) Suberitidae sp. 1 (6) Haliclona sp. 2 (7) Chalinula milnei (8)
0 59 16
3
4 5
6
7
33 1 19 21 14
8
9
10
11
12
13
14
15
16
17
18
19
20
Total interactions
1
36
12
13
15
8
14
5
5
15
7
3
3
283
103 1
0
3 16
0
5
9
6
3
0
3
4
1
8
6
3
0
187
3 0
0
0
0
0
0
4
0
0
3
0
0
0
1
0
0
0
146
0 26
3
0 21
0
8
6
3
0
1
0
5
0
0
2
0
77
0
0 15
0
0
0
0
0
1
0
0
0
0
2
0
0
63
2
3
6
0
8
0
6
2
2
0
3
2
2
0
63
0 0
9
0
0
2
0
2
0
2
0
0
0
0
0
60
0
0
2
1
0
3
1
5
5
2
6
0
2
62
0
2
0
1
0
0
0
0
0
0
0
0
50
0
0
2
0
1
2
0
0
0
0
0
42
0
0
0
0
0
0
0
0
0
2
38
0
2
2
0
0
0
2
5
0
35
0
3
3
4
0
0
0
0
37
2
0
0
0
0
0
0
29
0
0
0
0
0
0
23
0
0
0
1
0
21
Pericharax sp. (9) Dendrilla sp. (10) Haliclona sp. 3 (11) Niphates sp. W (12) Chalinula nematifera (13) Clathrina sp. (14) Pseudohalichondria sp. (15) Dysidea sp. (16) Clathrina sp. (17) Agelas sp. (18) Clathria mima (19) Placospongia melobesioides (20)
0
0
0
0
29
0
1
0
26
0
17
0
7
0
Species are ordered in the table in increasing order of the total number of associations they were involved in. The total number of associations each species was involved in is also shown
Haliclona sp. 2 and Chalinula milnei are of particular interest. Haliclona sp. 2 and Chalinula milnei were randomly distributed at Hoga, but negatively associated with other sponges at Sampela, while Lamellodysidea herbacea was randomly distributed at Hoga, but positively associated with other sponges at Sampela. In contrast, Pseudoceratina sp. was positively associated with other sponges at Hoga and randomly distributed at Sampela.
Discussion Our results show that the distribution patterns and associations are variable between sponge species; and for some species, these patterns appear to be dependent on environmental characteristics. At Hoga, the majority of sponge species were randomly distributed, while at Sampela most sponges were negatively associated with other sponge species, with the exception of Lamellodysidea herbacea that was strongly positively associated with other sponges. Our results from these two sites demonstrate that it is diYcult to generalise patterns of intra-phyletic associations between coral reef sponges in the Indo-PaciWc and that associations may vary for individual species in relation to environmental heterogeneity. This diversity of interactions is consistent
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with that reported for other sponge assemblages (Rützler 1970, 2004; WulV 1997, 2005, 2006, 2007, 2008; Ávila and Carballo 2009). However, unlike many studies in the Caribbean, our results do not suggest widespread cooperation between sponges at these two sites. There has been recent renewed interest in the way that species coexist and the interactions between species, since such interactions are unlikely to be homogenously distributed (Bascompte 2009). The heterogeneity in species interactions is thought to be important for stability and species-coexistence (Bascompte and Jordano 2007) and our research emphasises this point, as interactions between some sponge species diVered depending on the environmental regimes in which they were found; this suggests that species interactions may be spatially variable. The Sampela and Hoga sites diVer in a number of respects including environmental regimes, sponge abundance and coral cover (Bell and Smith 2004). The sponge species composition at the two sites also diVers considerably, which may account for the overall diVerences in spatial associations between sites. Most of the associations between sponge species at the Hoga site were random, with only a few sponge species being positively or negatively associated with other sponge species. At Sampela, the majority of interactions were negative associations with
Mar Biol (2010) 157:2503–2509
2507
Table 2 The reported spatial associations between the 20 species with the most associations within a 30-m2 sampling area at Sampela 1 Lamellodysidea herbacea (1) Prosuberites sp. (2) Clathria sp. (3) Liosina sp. (4) Callyspongia sp. (5) Chalinula milnei (6) Axinyssa sp. (7)
2
3
4
5
6
7
8
9
10 11 12 13 14 15 16 17 18 19 20 Total associations
301 203 22 30 43 29 25 23 13 11 3 24
4
0
11 1
2
3
2
5
2
733
2
0
8
0
0
2
8
0 0
0
0
0 0
0
0
0
2
0
249
0
6
5
0
1
1
4
0 5
3
2
0 3
0
5
1
0
5
65
0
0
0
0
3
1
0 0
0
2
0 0
0
0
0
0
0
42
0
0
2
1
0
0 1
1
0
0 2
0
0
1
0
0
64
0
0
0
0
0 0
0
0
0 0
0
0
0
0
0
29
0
0
0
0 0
4
0
0 0
0
0
0
0
0
32
0
0
0 0
0
0
0 0
0
2
0
0
0
32
0
0 0
0
4
0 0
0
0
0
0
0
30
0 0
0
0
0 0
0
0
0
0
0
11
0
2
0
0 0
0
2
0
0
0
13
0
0
0 0
0
2
0
0
0
16
0
0 0
0
0
0
0
0
8
0 0
0
0
0
0
0
11
0
0
0
0
0
0
6
0
0
0
0
0
2
Pachastrellidae sp. 1 (8) Niphates sp. 1 (9) Aaptos sp. (10) Jaspis splendens (11) Lissodendoryx sp. (12) Niphates sp. 2 (13) Clathria mima (14) Pseudoceratina sp. (15) Placinolopha sp. (16) Unknown species (sample lost) (17) Haliclona sp. 1 (18) Agelas sp. (19) Haliclona sp. 2 (20)
0
0
0
0
14
0
0
0
4
0
0
7
0
7
Species are ordered in the table in increasing order of the total number of associations they were involved in. The total number of associations each species was involved in is also shown
other sponges, with the exception of Lamellodysidea herbacea. Of the 20 most abundant species, nine species were shared between the sites (Pseudoceratina sp., Lamellodysidea herbacea, Prosuberites sp., Haliclona sp. 2, Chalinula milnei, Niphates sp. 1, Clathria mima, Agelas sp. and Clathrina sp). Interestingly, the nature of the associations for four of these species diVered between the sites. This suggests that although assemblage composition diVerences may account for some of the overall diVerences between sites, some species association patterns are dependent on habitat type. Haliclona sp. 2 and Chalinula milnei were all involved in fewer associations than predicted based on their relative abundance at Sampela, while at Hoga they were randomly distributed, and Lamellodysidea herbacea was involved in more interactions than expected at Sampela and randomly distributed at Hoga. In contrast, Pseudoceratina sp. was positively associated with other sponges at Hoga and randomly distributed at Sampela. Without a good understanding of the ecology of each of these species, it is diYcult to explain these species-speciWc results; currently, this information is unavailable, but will be a focus of future research. At Sampela, there is considerably more free space compared to Hoga, since coral cover is much reduced through anthropogenic disturbance (5–10% compared with 40–
45%). It is possible that in areas where space is not so limiting, associations with other sponge species are less important for survival, resulting in negative association patterns. WulV (1997) described a mutually beneWcial relationship between three common Caribbean species; although the species diVer from each other in their susceptibility to predation, smothering by sediment, breakage by storm waves, pulverisation by storm waves, toppling by storm waves, fragment mortality and response to pathogens, the spongespeciWc growth and survival rates increased when they were tightly adhered to each other. More recently, WulV (2008) showed that one species of these previously examined species exploits these mutualisms as it beneWts more from the relationship with other species than the species it adheres to. These studies further demonstrate that more research is required to elucidate the exact nature of the associations identiWed in the present study before the distribution patterns can be fully understood. It is important to note that most of the species found at Sampela were relatively rare, which has the potential to inXuence the results. However, Lamellodysidea herbacea is of considerable interest since it was abundant at both sites and was randomly distributed at Hoga, but was positively associated with other sponge species at Sampela. Our preliminary observations of this species suggest it is aggressive in its
123
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Mar Biol (2010) 157:2503–2509
3.0
300
Percentage cover
2.5
Interactions
250
2.0
200
1.5
150
1.0
100
0.5
50
0.0
0
Number of interactions
Area
Hoga
Sampela
Species
Observed data
Species
Observed data
Pseudoceratina sp. (1)
1+
Lamellodysidea herbacea (1)
1+
Lamellodysidea herbacea (2)
0.174
Prosuberites sp. (2)
0*
Sycon sp. (3)
1+
Clathria sp. (3)
0*
Prosuberites sp. (4)
0*
Liosina sp. (4)
0*
Chelonaplysilla sp. (5)
0.114
Callyspongia sp. (5)
0.999+
Suberitidae sp. 1 (6)
0.959
Chalinula milnei (6)
0*
Haliclona sp. 2 (7)
0.633
Axinyssa sp. (7)
0.006*
Chalinula milnei (8)
0.084
Pachastrellidae sp. 1 (8)
0*
Pericharax sp. (9)
0.255
Niphates sp. 1 (9)
0.751
Dendrilla sp. (10)
0*
Aaptos sp. (10)
0.0895
Haliclona sp. 3 (11)
0*
Jaspis splendens (11)
0*
Niphates sp. 1 (12)
0.392
Lissodendoryx sp. (12)
0*
0.088
Niphates sp. 2 (13)
0*
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
Chalinula nematifera (13)
Species
Clathrina sp. (14)
0.093
Clathria mima (14)
0*
Pseudohalichondria sp. (15)
0.852
Pseudoceratina sp. (15)
0.058
Dysidea sp. (16)
0*
Placinolopha sp. (16)
0*
Clathrina sp. (17)
1+
Unknown species (17)
0.676
Agelas sp. (18)
0.268
Haliclona sp. 1 (18)
0.244
Clathria mima (19)
0*
Agelas sp. (19)
0.06
Placospongia melobesioides (20)
0*
Haliclona sp. 2 (20)
0.002*
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
Species Sampela
1200 Area Interactions
10
1000
8
800
6
600
4
400
2
200
0
Number of interactions
12
Percentage cover
Table 3 The signiWcance of Monte Carlo simulations (1,000 iterations) used to generate a distribution of expected values (if distributions are random) for each sponge species pairing based on the relative abundance of each species (see text for further detail)
0
Fig. 1 Sponge abundance and interactions. The abundance (area occupied) of the 20 most abundant species (left axis) at two sites in the Wakatobi Marine National Park, and the total number of intra-phyletic associations (right axis) each species is involved in. Species numbers correspond to species in Tables 1 and 2. Note the diVerent scales on the graph axes
interactions with other sponges and corals (causing tissue necrosis), and it warrants further focus as it may be a species that other species negatively associate with or it may change the distribution patterns of other species by overgrowth or necrosis from this species. Future work will focus on the speciWc nature of those relationships that deviate from being random since interaction types are thought to be important in meditating sponge assemblage structure. There is considerable research from the Caribbean, Mediterranean and PaciWc suggesting that sponge interactions are mediated through cooperative rather than competitive interactions (Rützler 1970; Sarà 1970), with only a few reports of elimination of one sponge species by another through competition (WulV 2005). Although research from the Caribbean (see above) supports increased survival through cooperation for selected species, the extent of cooperation at the assemblage level has not been assessed to our knowledge; and therefore, this may be an exceptional situation rather than a general feature of Caribbean sponge assemblages. Earlier research by Sarà (1970) and Rützler (1970) included a large number of species for sponge assemblages in the Mediterranean. Neither
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The number of observed associations was compared to the distribution of simulated associations. If the number of observed associations was below the lower 2.5th percentile of the distribution of simulated results, then the observed associations were signiWcantly less than expected (negatively associated), and if greater than the upper 2.5th percentile, the observed associations were signiWcantly more than expected (positively associated). * and + indicate those species that were signiWcantly negatively or positively associated with other sponges, respectively
of these authors found the elimination of one sponge species by another through spatial competition and overgrowth. We did Wnd examples of sponges overgrowing or growing on other sponge species with no apparent negative eVects on the underlying sponge, which is consistent with other studies, and potentially represents cooperation for these species. Through histological examination, Rützler (1970) found no obvious negative impacts to sponges when one sponge overgrew another in the Adriatic from which he concluded that overgrown sponges were morphologically suited to being overgrown in that they were able to maintain access to the water column, concluding that sponges where cooperating rather than competing, which may facilitate high sponge richness in space-limited environments.
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However, in our study, there were relatively few examples of sponges being positively associated with other sponges, with only 5 of the possible 40 species exhibiting associative distribution patterns across the two sites (Clathrina sp. Sycon sp. and Pseudoceratina sp. at Hoga and Lamellodysidea herbacea Callyspongia sp. at Sampela), with 17 species showing negative association patterns (6 at Hoga and 11 at Sampela) and the remaining 18 species showing random distribution patterns. Our results, therefore, contrast with studies from elsewhere, as the sponge assemblages at these two sites in the Wakatobi appear to be dominated by negative associations and random distribution patterns rather than widespread competition, which contrasts with results from other coral reef systems. Acknowledgments We are indebted to the staV of the Hoga Island research station, whose help has been invaluable in allowing us to conduct research in Indonesia. Dr. Bell would also like to express his gratitude to Operation Wallacea for Wnancial and logistic support for his research since 2003, which has allowed access to research facilities in the WNMP. Finally, Dr. Bell is grateful to the PADI Foundation for providing Wnancial assistance for this speciWc project.
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