This study was conducted to assess the recruitment rate of coral spats and other invertebrates near to the coral reef ecosystem of Gulf of Mannar. The reef region examined was at Kurusadai Reef Complex (Gulf of Mannar) for this assessment. There were two sets of invertebrate settlement tripods deployed and exposed at a depth of 2m. Seventy two settlement tiles measuring 20x20 cm in two different angles were fixed in these tripods in the benthic reef ecosystem. In order to assess the impact of Sea surface temperature on the invertebrate settlement, Onset Hobo Pendant® Temperature logger was also attached with the tripods. The sediment traps in duplicate were also erected in the study site to assess the sedimentation rate of the reef ecosystem. About 80% of recruited coral spats were observed on the tiles which were exposed at 60° angle. Principal component analysis also supported that the most influencing species were coral spats followed by barnacles and bivalves in the 60° angle exposed settlement tiles. It was also observed that the most influencing place of coral spats’ recruitment was on the inner tiles. The mean sedimentation rate observed was 14.6 ± 4.8 mg.cm−2.d−1. The lower density of coral spats observed on 90° and 60° angle exposed outer tiles might be due to the observed sedimentation rate and it is believed that coral spats preferred the shaded environment initially for further perpetuation in the environment. Moreover, Bray-Curtis cluster analysis supported that the coral spats found on the inner tiles having 80% similarity in this assessment. The coral spats found in this assessment were species from Pocilloporidae and Poritidae.
Coral recruitment is a vital process in the benthic ecosystem as it stabilizes the reef ecosystem against natural (Sawall et al., 2010) and anthropogenic disturbances (Wilson, 2010). The study of the recruitment process is critical in identifying resilience of coral reef ecosystem from various threats and disturbances (Babcock et al., 2003; Davies et al., 2013; Lukoschek et al., 2013; Martinez and Abelson, 2013; Bauman et al., 2015; Stubler et al., 2016). The settlement of coral larvae (microscopic planulae) on the benthic substrate can develop into a reef ecosystem under suitable environmental conditions (Crabbe et al., 2003). It also permits further improvement of diversity in corals and associated organisms on the benthic reef ecosystem. The most influencing factors on the further development of settled larvae are light, turbidity and sedimentation (Meesters et al., 1998).
The reefs of Gulf of Mannar occur on the southeast coast of India, located in a chain of 21 Islands along a 140 km stretch between Rameswaram and Tuticorin on the southeast coast of India (Marimuthu et al., 2010). The Kurusadai Reef Complex (KRC) is situated very near to Rameswaram Island which includes fringing and patch reefs in and around Kurusadai, Pullivasal and Poomarichan Islands. This reef complex is part of the Gulf of Mannar Marine Biosphere Reserve due to its unique environmental conditions with abundant coral reefs, seagrass, and mangroves which support spawning and feeding of finfish and shellfish. About 48 species of Scleractinian corals were identified from Kurusadai and Manoli Reef Complexes (Marimuthu et al., 2010) and the dominant species observed were Acropora muricata, Acropora cytherea and Pocillopora damicornis. KRC has already faced two catastrophic bleaching events, one in 1998 and the other in 2002 (Arthur, 2000; Kumaraguru et al., 2003) but has recovered slowly. Hence, the assessment of coral recruitment pattern is urgently needed in order to assess the capability of the reef ecosystem to recover from such natural and anthropogenic threats.
Material and methods
Two sets of invertebrate settlement tripods (Figure 1) were deployed at Kurusadai Reef Complex, Gulf of Mannar (N 09° 14’44.04 and E 79° 13’08.88”) on the southeast coast of India between December 2015 and March 2016. They were erected at a depth of 2m in the reef flat region. In order to study the recruitment rate of various invertebrates along with coral spats, ceramic tiles (measuring 20cm x 20cm) were connected with the tripod at two different angles (60° and 90°). In the beginning, 36 tiles were attached at each tripod and 12 tiles each were retrieved at monthly intervals. Tiles exposed at 60° angle were facing the marine environment in two ways- on the outer and inner side (Figure 1a). The rough surface of the ceramic tiles was allowed to be exposed in this system. In order to monitor the sea surface temperature during the study period, Onset Hobo Pendant® Temp 64K (UA-002-64) logger was also attached to the tripod near the recruitment tiles. Sediment traps (English et al., 1997) measuring 11.5cm in height and 7.5cm in diameter were also erected near this system to estimate the sedimentation rate of the ecosystem (Figure 1). The entire system was exposed for three months and retrieved on March 12, 2016, in order to assess the intensity of coral spats along with other invertebrate spats available in the KRC. All spats observed in this study were examined and photographed under the stereomicroscope (NIKON SMZ25).
In order to interpret the coral spat data within the benthic reef ecosystem, sessile benthic communities were also studied in this reef complex at 6 sites using the line intercept transect method (English et al., 1997). A 50-m long flexible underwater tape was laid on the reefs roughly parallel to the shore with replicates in the entire reef complex. Video transects were also taken for further analysis and the benthos coming under the transition points were recorded using international codes (English et al., 1997; Kumaraguru et al., 2003; Al-Sofyani et al., 2014; Ravindran et al., 2014; Sawall et al., 2014; Marimuthu et al. 2016). The raw data were sorted and assessed using AIMS Reef Monitoring Data Entry System V1.6 Data Entry Program -Long term reef monitoring project, Australian Institute of Marine Sciences (ARMDES, 1996). Principal component analysis (PCA) was used to identify the most influencing invertebrates of the study site and Bray-Curtis cluster analysis was carried out using PRIMER 7 Version 7.0.5 (Clarke and Gorley, 2015).
Results and discussion
The coral spats recorded from the settlement tiles were species of Pocilloporidae and Poritidae (Figure 1). The settled juvenile corals (Scleractinian coral recruits) from the settlement plate were identified based on the skeletal morphology. The recruits were examined under a dissection microscope at 40x magnification, and the identified recruits belonged to 2 families. The morphology of the skeleton of juvenile characters was cross-checked with the description given by Babcock et al. (2003) as follows.
Spat of Poritidae – The corallite (Figure 1e) had grown by an extension of the basal plate (approx. 400µm dia) beyond the epitheca. The primary septa, distinguishing this species, extended to the perimeter of the new boundary of the basal plate. The juvenile corallum had 10 to 12 corallites.
Spat of Pocilloporidae – The spat was identified based on the pattern of skeleton formation, including the structure of the septa, columella and corallite wall (Baird and Babcock, 2000). The diameter of the primary corallite (Figure 1f) of the spats was calculated as 400µm.
Spats of Pocilloporidae – It was observed that the corallite wall formed through the growth and fusion of lateral outgrowth of the basal ridges. It was also observed that first cycle into the center of the basal plate among 3-5 cycles of basal ridges (Figure 1h). The diameter of the basal disc (Figure 1g) and the primary corallite (Figure 1h) of the spats were calculated as more than 2500 µm and 600µm respectively.
After three months exposure, coral spats were observed having settled on 25% of the tiles. This indicated the current status of the coral spat found in the study sites during the exposure period. Different coral spats of various sizes were observed based on the exposure time as presented in Figure 1. The overall status of settlement rate of other invertebrates was presented in Figure 2. Figure 2a and 2b show the overall status of mean coral spat abundance (n = 4), and other invertebrates settled on the tiles. Apart from coral spats, other invertebrates observed were tube polychaetes, bivalves, barnacles, ascidians, gastropods, amphipods and bryozoans. Tube polychaetes (41.75 ± 30.53 nos. tile−1) and ascidians (11 ± 6.58nos. tile−1) were observed as dominant among other invertebrates. Of the two different angles of exposure tested in this study, almost 80% of coral spats recruited on the tiles which were exposed at 60° angle. Moreover, all coral spats recruited on the tiles were on the inner tiles. It is believed that the lower recruitment rate of coral spats on the outer tiles is due to the rate and effect of sedimentation (14.6 ± 4.8 mg.cm−2.d−1). Therefore, the occurrence of coral spats was observed more on the inner tiles. Principal component analysis also supports the finding that the contribution of coral spats was found more on the inner tiles than the outer (Figure 3). In this assessment, there was a strong variability (PC1: 53.1% variance) between the two different types of exposure (60° and 90° angle) and the most influencing species were corals followed by barnacles and bivalves. It was also observed that less variability (PC2: 28.2% variance) existed between inner and outer tiles of 60° angle exposure and the most influencing variable observed was coral spats. Tiles exposed at 60° angle showed considerably higher spat recruitment than the tiles exposed at 90° angle- which indicates substrate preference of the coral spats (Fig. 2). Moreover, the density and availability of coral spats on the tiles showed that there was greater similarity observed on similarly exposed tiles such as the inner tiles (Fig. 3). Bray-curtis clustering revealed 80% similarity was observed between inner tiles exposed at 60° angle. It was noted that the intensity of coral spats was greater on the inner tiles- which was a shaded environment with less impact from sedimentation. Among the coral spats observed in this study, the occurrence of Pocilloporidae was greatest, followed by Poritidae. Spats from Acroporidae were absent in this study.
In the benthic environment, the dominant species observed were Acropora muricata (Linnaeus, 1758), A. hyacinthus, Acropora spp., Montipora digitata (Dana, 1846), Echinopora lamellosa (Esper, 1795) and Porites sp. The other species observed were Galaxea fascicularis (Linnaeus, 1767), Favites abdita (Ellis & Solander, 1786), Porites lobata (Dana, 1846), Hydnophora sp., Pavona sp., Dipsastraea favus (Forskal, 1775), Dipsastraea pallida (Dana, 1846)”, Goniastrea sp., Pocillopora damicornis (Linnaeus, 1758) and Siderastrea sp. PCA (Figure 4) shows that there was significant variability (PC1: 47.8% variance) between Pullivasal-middle and the rest of the selected study sites. In this, Porites spp. was observed as the most influencing coral species at the site of Kurusadai Island. Hence, the observed coral spats correlated with the availability of coral diversity in the benthic ecosystem from the biophysical assessment. During the study period, the sea surface temperature (SST) was observed to range between 26.48 and 31.17 °C. Though, the high SST was observed once in the month of December 2015, the peak average was observed in the month of March 2016 (Figure 5). Moreover, the gradual increasing trend in SST was observed between December 2015 and March 2016. However, there was no symptom of coral bleaching seen in the KRC during the study period.
The knowledge of recruitment pattern is vital for effective management of marine ecosystem with natural disturbances (Babcock et al., 2003). The status of corals spat diversity and the influencing factors on their recruitment pattern have been reported worldwide viz., Virgin Islands (Rogers et al., 1984), Solitary Islands Marine Reserve, Eastern Australia (Harriott and Banks, 1995), Great Barrier Reef (Hughes et al., 1999; Maida et al., 2001; Baird et al., 2012), Indo-Pacific reefs (Babcock et al., 2003), Florida keys (Moulding, 2005), Mombasa Marine National Park and Reserve, Kenya (Mangubhai et al., 2007), Sulu Sea (Garcia and Alino, 2008), Red Sea (Martinez and Abelson, 2013), Gulf of Mexico (Davies et al., 2013), Northern Taiwan (Ho and Dai, 2014), Spermonde Archipelago (Sawall et al., 2013) and Equatorial reef systems (Bauman et al., 2015). Coral spat recruitment at KRC was much less than in the pristine Great Barrier Reef, but more similar to the rate in the Red Sea (Table 1). Comparison of the coral recruitment rate of KRC with other studies from different geographic regions (Abelson et al., 2005) is useful in estimating the present health status of selected coral reef sites. Even though the recruitment rate observed was much less compared to other reefs, the observed results will be helpful as a baseline data for conservation specialists and administrators in the context of coral reef recruitment.
This study is the first of its kind from the Indian coastal region as Scleractinian coral species identification at the spat level and their recruitment rate are positive indicators for coral reef health. Authorities involved in conservation and protection of coastal zones of India should take note of the settlement time and settlement rate of coral spats towards effectively conserving the reef ecosystem from reef fishing during this critical period. Results and findings of this and similar studies may act as a reference point for future policy or status of the health of the marine environment.
The authors gratefully acknowledge the PCCF (Wildlife) and Chief Wildlife Warden, Gulf of Mannar Marine National Park, Tamil Nadu State Forest Department for their permission and support to carry out this work. We are grateful to the reviewers’ critical comments, which improved the manuscript substantially.
N. Marimuthu http://orcid.org/0000-0002-7427-3111