Population density, biomass and bioerosion represent three related, but very different lenses of the role’s parrotfish play in coral reefs. The assumption that number reflects process can be misleading, particularly when number and biomass are decoupled in their distribution, and when the process in question is linked to species identity, and scales allometrically 28,29. In Lakshadweep, the distribution of parrotfish across its reefs was influenced by wave exposure regimes and the way species responded to these regimes was mediated by individual size and body shape. As an assemblage, the deep locations had a greater number of large parrotfish individuals, and individuals of key excavating species (like Chlorurus strongylocephalus and Chlorurus enneacanthus) that accounted for the bulk of parrotfish bioerosion on the reef. In exposed reefs, subject to the highest year-round wave energy, a greater number of small individuals as well as overall higher parrotfish density were found than at sheltered locations. This differential filtering of species and sizes results in considerable heterogeneity in the spatial distribution of bioerosional processes on the reef. This finding is in contrast to other studies which found greater excavator abundance in more exposed conditions 30.
Lakshadweep’s reefs are uniquely placed to explore the consequences of wave exposure on community composition and ecosystem processes. The turbulent west and the relatively calm east create strongly contrasting conditions that influence benthic composition and post-disturbance recovery, structural stability regimes, fish assemblages and behaviour 31–33. It is important to note that the wave exposure contrasts between east and west do not persist year-round. From October to April, western shallow reefs can be as calm as the east. However, for the 6 months of the summer monsoon, the direction of the south-westerly winds represent an environmental forcing large enough to influence the distribution of resident species. This is especially important in the case of species like parrotfish, which hold and patrol territories 34,35. Wave energy also attenuates with depth and deeper reefs likely represent low wave energy conditions independent of aspect. The shallow leeward reefs of the east lie at the midpoint of this wave exposure gradient. While admittedly coarse, this gradient helps make sense of the distribution of parrotfish in Lakshadweep reefs, showing that exposure is a strong environmental filter for parrotfish communities, driving patterns in species density and biomass, and contributing to an unequal distribution of bioerosional processes on the reef. The high exposure western shallow reefs had lower parrotfish biomass and erosion compared to reefs in low or medium exposures, while the bulk of parrotfish bioerosion was on lower exposure, deeper locations (Fig 1c). These distributional patterns are mediated by size and species-specific differences in their ability to cope with exposure regimes.
Parrotfish biomass was much higher in lower wave energy regimes (Fig 1b). Despite not having as many large parrotfish individuals as deep reefs, sheltered shallow reefs had high densities of small parrotfish from several different species and so their sheer number offset their lower per capita biomass compared to the deeper reefs. This decoupling between density and biomass, mediated through size is important to factor in when considering the overall role that a species plays within an ecosystem.
Swimming through the viscous medium of sea water has a large energetic cost – a cost that varies across the reefscape, with the tide and with the season. In negotiating currents and waves, fish can either be streamlined and small, or larger and invest in robust swimming architecture and musculature36. We found that body size was an important factor in determining parrotfish distributions in Lakshadweep. Large parrotfish individuals were found in much lower numbers at high wave exposure sites. Even though greater musculature in larger parrotfish may allow them to be stronger swimmers, the cost of increased drag appears to outweigh the benefit of increased swimming abilities. In contrast, smaller individuals did better in accessing shallower waters with high wave exposure. Smaller size classes of shoaling Scarus psittacus and, to a lesser extent, Chlorurus sordidus, which were the dominant parrotfish in terms of numbers (over 65% of mean parrotfish density) were found disproportionately in shallower waters; in fact, S. psittacus was a distinct indicator of shallow reefs. Compared with larger parrotfish species, these species have a small cross-sectional area, and likely experience much less hydrodynamic drag in turbulent conditions 37.
Body shape and morphology also play an important role in determining a fish’s ability to navigate turbulent conditions. Compared with acanthurids, the other dominant herbivore group in most coral reefs, scarids are more fusiform in shape, which helps them in accessing wave-exposed fronts 19,25. However, scarids do show intra-guild variation in body depth ratio. Parrotfish BDR seemed to be an important factor in determining how abundant smaller individuals were in conditions of varying wave exposure. Most species with a high BDR were completely absent from regions of high wave exposure. For these species, it is possible that the half-yearly monsoon is strong enough to either limit recruitment or reduce post-settlement survival of these species on shallow western sites resulting in compositional differences between locations. However, species with a low BDR tended to do well in even high exposure sites. More streamlined species like Scarus psittacus, S. scaber and Chlorurus sordidus, with a lower body depth, were likely able to navigate high exposure regimes while escaping competition from deeper bodied and larger parrotfish individuals. The one exception to this trend was S. prasiognathos, a relatively deep-bodied species, that was found across exposure regimes, regardless of size class (Fig. 2). However, terminal phase individuals are often much deeper bodied than the initial phase in this species, and the smaller individuals were most often in the initial phase; dimorphic differences in BDR may potentially explain this pattern. Smaller females of S. prasiognathos also tended to travel in tight shoals in shallow reefs, potentially reducing individual drag and allowing them to access shallow exposed sites.
Taken together, these patterns indicate that some deep-bodied species may be able to compensate for a less efficient shape with increased muscle power as they grow in size. Therefore, the strength of the environmental filter can vary ontologically, as individuals age and potentially change their shape as well as their ability to manoeuvre strong physical gradients. Body size is a universally powerful proxy of species life history 38. It acts as an indicator of species age, and in sexually dimorphic species, of its sex 39,40. These could be indicative of different ontological or physiological states which could drive differences in diet, grouping and other behaviours 41,42. In addition, size is often a good indicator of an individual’s ability to compete with its conspecifics 43,44 or navigate harsh environmental conditions. Another key factor that could cause spatial separation in size classes between individuals of the same species is the distribution of resources across the reef. Without evaluating resource distributions between deep and shallow reefs, it is difficult to unequivocally know if this was a factor in determining parrotfish distribution. Smaller individuals may also be competitively excluded from deeper waters by their larger conspecifics and congeners. However, algal cover is typically higher in shallower, flushed reefs 45,46, lending support to the idea that environmental filters could be limiting larger individuals from these locations.
The consequences of this size separation are even more stark when considering carbonate removal by parrotfish. The bioerosion potential of parrotfish is strongly linked to their size. The extent of carbonate removal by a parrotfish of a given species may grow disproportionately as they increase in size 47. Many scraping species of parrotfish contribute very little to total carbonate removal, while large excavators are voracious consumers of coral and other carbonate material 48,49. Trends in bioerosion track biomass differences across most reefs in Lakshadweep. However, despite having a biomass similar to deeper locations, eastern shallow sites have relatively low bioerosion. As discussed above, these locations were dominated by smaller individuals that contributed significantly to total biomass but very little to bioerosion.
Not all species contribute equally to bioerosion in Lakshadweep. Chlorurus strongylocephalus was the key bioeroder in the reef – contributing over 65% to the total carbonate removal by parrotfish in the reefs (Fig 3). This was despite the fact that in terms of biomass it represented only 22% of the assemblage, and only 3% of the total density. Yet, it was a ubiquitous species, found in low numbers in most reefs that we sampled and significantly influenced reef accretion rates across the islands. Previous research has shown that large excavators can contribute much higher rates of bioerosion than scrapers of the same size. In Maldivian reefs, C. strongylocephalus was responsible for roughly 130 times the bioerosion function compared to a scraping species of similar size, Scarus rubroviolaceus47. In low carbonate-producing reefs like Kavaratti, this means that C. strongylocephalus distribution and behaviour can tip reefs from being net accreting to net eroding. On the flip side, given the importance of excavating parrotfish to beach dynamics, this species may be critical for island growth and stability. Few other species also contribute to this function including Scarus rubroviolaceus and Chlorurus sordidus. Many of the large individuals of these carbonate removing species inhabited deeper waters, likely because of the energetic costs of swimming in more exposed conditions. In locations with large populations of Bolbometopon muricatum (Green humphead parrot fish, a large fish weighing roughly 75kg), this one species can be overwhelmingly important in carbonate removal, moving over large home ranges transporting material over several kilometres 50. Although present, B. muricatum is rare in Lakshadweep reefs and we did not observe any during our surveys. In contrast, Chlorurus strongylocephalus is much more common, and likely to have much smaller home ranges as we know from home range studies on its close congener Chlorurus microrhinos 34. While the total quantity of per capita carbonate removal may not compare to B. muricatum, it’s much more limited home range means that its overall impact may be more concentrated and more predictable in space.
Our findings contrast with reports from the Great barrier reef or the Maldives, where greater excavator abundance was found in more exposed outer shelf reef habitats30,51. However, other studies from Palau and Lakshadweep have shown that wave exposure can significantly limit the function of herbivores (including parrotfish) on reefs19,25. It may be difficult to completely resolve what drives these geographical differences in parrotfish distributional patterns. Given the greater strength of the 5-month long southwest monsoon in the northern Indian Ocean, the exposure contrast in Lakshadweep is likely considerably stronger than further south along the archipelagic ridge52. It is possible that in less turbulent conditions, large-bodied parrotfish may seek out the more productive, well aerated environments of shallow exposed reefs. Beyond a threshold however, the costs of navigating these conditions may outweigh any potential resource benefits. In a related behavioural study, we observed that large bodied individuals of key parrotfish species significantly reduced their foraging in high wave exposure conditions (publication in review), indicating why large bodied parrotfish such as C. strongylocephalus may avoid high exposure conditions in Lakshadweep reefs. It is important to note that our findings pertaining to parrotfish body depth are exploratory, and further studies are required before strong inferences can be made.
Understanding the variation in functional roles of parrotfish due to various environmental filters is key to understanding the bioerosional capacity of coral reefs, which in turn is a crucial component of reef health. Previous research on the effects of wave exposure on fish has focused largely on the body shape and other measures of swimming performances 19,23,25,53. Here we show that apart from its underlying traits, exposure mediated body size plays an essential role in shaping the distribution of a species in space, which means that the distribution of functions could also vary as individuals grow. For mobile species that are important mediators of ecosystem processes, a complex set of abiotic and biotic factors could together determine how these functions vary across the reefscape. This depends on how communities are assembled in relation to environmental gradients and how individuals within these communities dynamically respond to their proximate conditions in space and time. Our results highlight that these responses can disproportionately influence the strength of ecosystem processes across the reef.