Growth responses of mixotrophic giant clams on nearshore turbid coral reefs

Abstract

Increasing evidence suggests that nearshore turbid coral reefs may mitigate bleaching of reef building calcifiers and play a critical role in the future of marine biodiversity in coastal areas. However, biomineralization processes on turbid reefs are relatively understudied compared to clear water counterparts and most published work focuses on corals. Here, we investigate how the mixotrophic giant clam Tridacna squamosa, a bivalve with ecological, cultural and economic significance, grows across a mosaic of less turbid to turbid reefs in the Coral Triangle. We construct growth chronologies from live and dead collected shells by measuring daily growth increments with petrography and scanning electron microscopy (SEM) to gain insight into growth rate on daily, seasonal and annual scales. We find annual growth is not significantly different across a turbidity gradient when scaled to ontogeny, while seasonal growth highly varies. Kd(490) (a measurement positively correlated with turbidity) and chlorophyll-a are likely important factors driving seasonal growth on a turbid reef near a river, compared to sea surface temperature (SST), cloud cover and rainfall on a less turbid reef. On a daily scale, we investigate increment microstructure and spectral characteristics of chronologies, finding a relationship between tidal range and daily increments. Overall, our results indicate that light-enhanced calcification is likely most important in the less turbid reef, compared to heterotrophic feeding in the turbid reef. The trophic plasticity of T. squamosa may allow for its sustained growth in marginal conditions, supporting evidence that these habitats serve as important conservation hotspots for diverse reef building taxa.

Giant clams modify crystallographic and geochemical pathways of shell formation in response to turbidity

“…We find that shells formed in the high turbid reef situated near a river source have a different shell microstructure in the inner shell layer, consisting of paired daily growth increments with higher crystallographic co-orientation and lower Mg/Ca and Sr/Ca, compared to lower crystallographic coorientation and higher Mg/Ca and Sr/Ca in the low turbid reef. Key environmental differences between the two reef localities are Kd(490), total suspended solids and chlorophyll-a, which are elevated in the high turbid reef (20). In a previous study (20), we hypothesized that elevated concentrations of these parameters year-round may suggest T. squamosa can utilize different relative contributions of feeding type to total organismal energy requirements (20).…”

“…Key environmental differences between the two reef localities are Kd(490), total suspended solids and chlorophyll-a, which are elevated in the high turbid reef (20). In a previous study (20), we hypothesized that elevated concentrations of these parameters year-round may suggest T. squamosa can utilize different relative contributions of feeding type to total organismal energy requirements (20). These environmentally driven changes to physiological machinery may imprint on its biomineralization pathways and thence architecture and geochemistry of shells.…”

“…Two regular morphologies of daily growth increments that intersected lamellae were defined: 1. CCL-P in high turbid Triangle reef, composed of frequent irregular or regular simple prisms coupled with a thinner layer of smaller, oblique crystallites (20,30), varying in length and masking the underlying microstructure (Fig. 2d, e); 2.…”

“…So far, work exploring differences in the balance between autotrophy and heterotrophy has shown that the fluted giant clam Tridacna squamosa can sustain similar rates of growth in high turbid and low turbid reefs, likely due to compensatory heterotrophic feeding under turbidity (19,20). Relative contributions of autotrophy versus heterotrophy are particularly dynamic for T. squamosa from turbid environments with diverse food resources compared to other species of giant clam (20), in similarity to corals with high mixotrophic capacity (21,22). Yet, to understand if its biomineralization changes in response to turbidity, more information is needed, especially in relation to the properties of shell biominerals that are important for health and survival of the organism.…”

Heterotrophy in marine animal forests in an era of climate change

“…Other shallow MAF builders, such as bivalves (e.g. giant clams), are also symbiotic mixotrophs and follow similar feeding strategies as symbiotic corals (Mills et al ., 2023). Finally, sponges dominate reef diversity and biomass in some areas, and are increasingly recognised as key MAF engineers that efficiently capture, retain, and transfer energy and nutrients within the reef (Richter et al ., 2001; de Goeij et al ., 2008).…”