Branching dynamics of transplanted colonies of the threatened coral Acropora cervicornis: Morphogenesis, complexity, and modeling

Highlights

• The branch morphogenesis of Acropora cervicornis was examined at two reefs in Puerto Rico.

• Significant spatial differences were found in overall patterns of branch formation.

• Significant differences were also found in colony branching complexity.

• Branch production was accurately predicted with a simple mathematical model.

• Results can be used to guide management and conservation of this threatened coral.

Abstract

Acropora cervicornis is a threatened Caribbean coral that depends on branch fragmentation to proliferate. Understanding the patterns of branch formation is, therefore, essential for the development of management and conservation initiatives. This study describes branch morphogenesis in 100 colony fragments that were transplanted to two reefs in Puerto Rico that differ in light intensity. Four morphometric variables were measured for one year: internode length, branch growth rate, the number of ramifying branches (mother branches; MB), and the number of branches produced (daughter branches; DB). Branching complexity was also evaluated using two indices: the Horton-Strahler bifurcation ratio (R_b) and the Carrillo-Mendoza branching index (CM-BI). A simple discrete model was constructed to estimate the number of harvestable branches over time. No spatial difference was observed when comparing the development of the primary branches, as the mean internode lengths, the mean extension rates, and the mean number of branches produced did not differ statistically between sites. Likewise, internode lengths in secondary branches did not vary significantly between sites. In contrast, the mean branching and growth rates of secondary branches differed statistically between the two study locations. Significant spatial differences were also observed when comparing the total number of MB and the total number of DB but not for the ratio of DB to MB. The CM-BI was more appropriate than the R_b in describing the branching structure of A. cervicornis. The model provided a good fit to the observed branching dynamics; demonstrating its usefulness as a tool for predicting branch productivity of this species. The implications for restoration activities are discussed.

Introduction

Acropora cervicornis is an important species for the functioning of Caribbean coral reef ecosystems. This coral adds a considerable amount of calcium carbonate to the reef framework given its rapid growth and high calcification rates; thereby, contributing significantly to the net accretion of the reef (Gilmore and Hall, 1976, Tunnicliffle, 1983). It is also acknowledged that A. cervicornis plays a vital role in facilitating reef biodiversity because its branching morphology provides suitable habitat and refuge for many reefal species (Precht et al., 2002, Quinn and Kojis, 2006). Unfortunately, since the early 1980s populations of this coral have been declining at an alarming rate over its geographical range (Aronson and Precht, 2001, Miller et al., 2002). By 2003, it was estimated that region-wide abundance was already diminished by as much as 98% (Bruckner, 2003). Today, A. cervicornis is listed as a threatened species under the USA Endangered Species ACT (NMFS, 2006). A variety of complex biological (e.g. diseases), physical (e.g. hurricanes, high seawater temperature), and anthropogenic (e.g. ship groundings) stressors acting alone and synergistically, have prompted its regional collapse. More preoccupying, however, is that even in the absence of a major disturbance, populations of A. cervicornis can suffer a considerable reduction in abundance in a relatively short time (Mercado-Molina et al., 2015a).

Despite the potential of this coral to reproduce both sexually and asexually, several factors including low fertilization rates (Quinn and Kojis, 2006, Vargas-Ángel et al., 2006) and high pre/post settlement mortality of larvae (Tunnicliffe, 1981, Ritson-Williams et al., 2009) have limited the contribution of sexual recruits to local colony abundance. Recent data also suggest that, under current reef conditions, rates of colony fragmentation (asexual reproduction), as well as the survival of loose fragments, are not sufficient to maintain the viability of local populations (Mercado-Molina et al., 2014, Mercado-Molina et al., 2015a, Mercado-Molina et al., 2015b). Consequently, the prospect of the species to naturally rebound to pre-1980s population levels is much in doubt, and human intervention may be imperative.

The potential of individual branches to be established as independent colonies has provided the basis for many conservation strategies aimed at increasing local population abundance. For instance, coral gardening, which is one of the current and most common alternatives employed to propagate the species, consists of collecting individual branches from wild colonies and growing them in nursery units (Bowden-Kerby, 2001, Quinn and Kojis, 2006, Hernández-Delgado et al., 2014). As these nursery reared-fragments (colonies) grow and branch, their newly produced branches are then pruned to be either transplanted to the reef or used to expand/restock existing nursery programs. This branch-based approach has proved to be a significant source of new colonies for restoring depleted populations (Quinn and Kojis, 2006, Hernández-Delgado et al., 2014).

Even though branch fragmentation is the foundation for the propagation of A. cervicornis, either naturally or human-assisted, the dynamics of colony ramification have seldom been investigated. An understanding of the patterns of branch morphogenesis, however, is indispensable to evaluate the intrinsic and extrinsic factors that stimulate/limit branch production and to estimate the number of branches that can be “harvested” for restoration purposes. In this study, 100 colony fragments of A. cervicornis were transplanted to two reefs in Puerto Rico that differ in light intensity. Their branching dynamics were described by measuring 1) sites of branch emergence, 2) the number of branches produced per colony (daughter branches), 3) the number of branches that give rise to a new branch (mother branches), 4) branch growth rate, and 5) branching complexity. In the case of branching complexity, two branching indexes, the Horton–Strahler bifurcation ratio (R_b) and the Carrillo–Mendoza branching index (CM-BI), were compared to determine the most appropriate in describing colony ramification. A third goal was to develop a simple branching model with the purpose of projecting the number of “harvestable” branches into the future. Results of this study will provide valuable information to guide the design of management and conservation plans for this key species.