Branching dynamics of transplanted colonies of the threatened coral Acropora cervicornis: Morphogenesis, complexity, and modeling
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 (Rb) 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.
Section snippets
Study sites
The branching dynamics of A. cervicornis transplants were investigated at Punta Soldado (PSOL) and Tamarindo (TAM), two reefs located in the Island of Culebra, approximately 27 km off the east coast of Puerto Rico. The study areas, which are currently part of a reef restoration program run by the local NGO Sociedad Ambiente Marino (SAM), are characterized by low topographic relief, depths varying between 3 and 4 m, low wave energy, and relatively good water quality. Spatial difference in light
Colony development
The branching patterns exhibited by the primary branch of transplanted fragments were very similar between sites. No significant differences were observed when comparing the mean branch extension rates, the number of daughter branches emerging from the main branch or when comparing internodes length. During the 365 days that the transplants were followed, principal branches grew at 13 cm per year (PSOL: 13.13 ± 0.96; TAM: 13.40 ± 1.61: mean ± SE) while producing about 5 new branches (PSOL: 5.29 ± 0.28;
Discussion
Studies directed at understanding or describing the patterns of branch formation in A. cervicornis are scarce; which is surprising given the dependence of this coral on branch fragmentation to propagate itself. This study provides the most comprehensive baseline information describing such process, and therefore it provides relevant information concerning the productivity of A. cervicornis nurseries and their potential contribution to restoration activities.
It was found that: 1) new branches
Acknowledgements
This project was partially funded by Puerto Rico Center for Environmental Neuroscience (NSF grant HRD#1137725, Graduate Fellowship to A.E. Mercado-Molina), University of Puerto Rico Sea Grant College Program (SEED money, PD-294 to A.E. Mercado-Molina), institutional funds of the UPR-RP, UPR Sea Grant (NOAA award NA10OAR41700062, project R-92-1-10 to A.M. Sabat), the Center for Applied Tropical Ecology and Conservation (NSF grant HRD #0734826, Research assistantship to A.E. Mercado-Molina), and
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