Mark J.A. Vermeij

Abstract

Reef-building corals are ecosystem engineers that compete with other benthic organisms for space and resources. Corals harvest energy through their surface by photosynthesis and heterotrophic feeding, and they divert part of this energy to defend their outer colony perimeter against competitors. Here, we hypothesized that corals with a larger space-filling surface and smaller perimeters increase energy gain while reducing the exposure to competitors. This predicted an association between these two geometric properties of corals and the competitive outcome against other benthic organisms. To test the prediction, fifty coral colonies from the Caribbean island of Curaçao were rendered using digital 3D and 2D reconstructions. The surface areas, perimeters, box-counting dimensions (as a proxy of surface and perimeter space-filling), and other geometric properties were extracted and analyzed with respect to the percentage of the perimeter losing or winning against competitors based on the coral tissue apparent growth or damage. The increase in surface space-filling dimension was the only significant single indicator of coral winning outcomes, but the combination of surface space-filling dimension with perimeter length increased the statistical prediction of coral competition outcomes. Corals with larger surface space-filling dimensions (Ds > 2) and smaller perimeters displayed more winning outcomes, confirming the initial hypothesis. We propose that the space-filling property of coral surfaces complemented with other proxies of coral competitiveness, such as life history traits, will provide a more accurate quantitative characterization of coral competition outcomes on coral reefs. This framework also applies to other organisms or ecological systems that rely on complex surfaces to obtain energy for competition.

Abstract. In contrast with a general decline of Caribbean reef corals, a previously rare sun coral is
increasing in abundance within shallow coral communities on Curacao. This azooxanthellate scleractinian
was identified as Cladopsammia manuelensis, which has an amphi-Atlantic distribution. Over the last decade,
C. manuelensis has increased abundance along the leeward coast of Curacao (southern Caribbean)
between depths of 4 and 30 m. This species was initially not noticed because it resembles the invasive coral
Tubastraea coccinea, which was introduced to Curacao from the Indo-Pacific around 1940. However, in contrast
to T. coccinea, C. manuelensis was previously only present on deeper reef sections (>70 m) of Caribbean
reefs. Our observations illustrate how the sudden increase in abundance of a previously unnoticed, apparently
cryptogenic species could result from natural dynamics on present-day reefs, but also could easily be
mistaken for an invasive species. The finding that deep reef sections can harbor species capable of colonizing
shallower reef zones highlights the importance of thorough inventories of reef communities across
large depth ranges, which can help us to discriminate between range increases of native species and the
arrival of invasives.

Key words: bathymetric distribution; Cladopsammia; coral reefs; cryptogenic; deep water; Dendrophylliidae; invasive;
native; Rhizopsammia; Tubastraea;.