Abstract
Reef-building corals show a marked decrease in total species richness from the tropics to high latitude regions. Several hypotheses have been proposed to account for this pattern in the context of abiotic and biotic factors, including temperature thresholds, light limitation, aragonite saturation, nutrient or sediment loads, larval dispersal constraints, competition with macro-algae or other invertebrates, and availability of suitable settlement cues or micro-algal symbionts. Surprisingly, there is a paucity of data supporting several of these hypotheses. Given the immense pressures faced by corals in the Anthropocene, it is critical to understand the factors limiting their distribution in order to predict potential range expansions and the role that high latitude reefs can play as refuges from climate change. This review examines these factors and outlines critical research areas to address knowledge gaps in our understanding of light/temperature interactions, coral-Symbiodiniaceae associations, settlement cues, and competition in high latitude reefs.
Highlights
A decline in the species richness of tropical marine organisms at increasing distances from the centre of biodiversity in the Indo-Australian Archipelago is one of the most prominent patterns in biogeography [1,2]
Recent field investigations have documented highly diverse and thriving coral reefs in regions characterized by very high turbidity and low light conditions, such as the nearshore environments of the Great Barrier Reef (GBR) [84], the Bonaparte Archipelago in Western Australia [85], and South Atlantic reefs along the coast of Brazil [86]
Wicks et al [20] observed that Symbiodiniaceae ITS2 types that were usually distinct in coral species with horizontal versus maternal symbiont acquisition were shared at high latitude reefs on Eastern Australia (Lord Howe Island), which suggests that the environmental diversity of beneficial Symbiodiniaceae types may be limited
Summary
A decline in the species richness of tropical marine organisms at increasing distances from the centre of biodiversity in the Indo-Australian Archipelago is one of the most prominent patterns in biogeography [1,2]. More recent work has demonstrated the importance of allelo-chemicals and herbivory in mediating coral-algal interactions (e.g., [11,12]) and these factors might be involved in limiting coral latitudinal distributions. In addition to these established hypotheses, latitudinal limits of tropical corals might be influenced by the availability of suitable settlement cues, such as specific species of crustose coralline algae that are required to induce larval settlement in many tropical coral species [13,14]. Diversity 2021, 13, 632 expansion of coral species to show that experimental support for much of the conventional wisdom is lacking (e.g., competition with macro-algae; dispersal) and outline promising areas for future research
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