MARINE ECOLOGY PROGRESS SERIES Published June 5 Mar Ecol Prog Ser Are vertical distribution patterns of scleractinian corals maintained by pre- or post-settlement processes? A case study of three contrasting species Craig ~undyll~",Russ ~abcock~ 'Australian Institute of Marine Science, PMB 3. Townsville MC, Queensland 4810, Australia 'Zoology Department, University of Queensland, Brisbane, Queensland 4072, Australia 3~eighMarine Laboratory. University of Auckland, PO Box 349, Warkworth. New Zealand ABSTRACT. Vertical zonation of hard corals in tropical coral reef Systems is a weil documented and predictable pattern in the organisation of tropical coral communities. However, the mechanisms that underly vertical zonation of hard corals are poorly understood. Two alternative conceptual models for the maintenance of vertical zonation are considered; depth-dependent settlement of larvae, or indis- criminant settlement of larvae with distnbutions determined by differential post-settlement mortality with depth. These alternative models were evaluated by undertaking field translocation experiments using 10 d old juveniles of 3 species with contrasting adult distnbutions: Goniastrea aspera (Verrill- restncted to shallow reef flat habitats; Oxypora lacera (Verrili)-restncted to low light, lower slope habitats; and Platygyra daedalea (Ehs and Solander) -a species with a broad depth range. Laboratory- raised larvae of each species were settled onto terracotta settlement plates and the plates translocated to 3 depths in the field: reef flat (0 m), mid slope (5 m), lower slope (10 m). Survival of the laboratory- settled juveniles was monitored at 3, 6 and 12 mo after settlement. For each of the 3 species, juveniles survived equaliy well at all 3 depths over the 12 mo study penod. The pattern of survivorship of G. aspera and P. daedalea among depths, however, was dependent on plate surface (top, bottom). Given the absence of depth-dependent survival of juveniles from the 2 zone-specific species G. aspera and 0. lacera, we reject the hypothesis that adult distribution patterns of these 2 species is determined by indiscriminant settlement of larvae followed by differential early post-settlement mortality with depth. KEY WORDS: Settlement. Early post-settlement mortality . Vertical zonation . Corals INTRODUCTION of corals (Glynn 1976, Sheppard 1982, Huston 1985, Chadwick 1991), although the widespread importance On tropical coral reefs, vertical zonation represents a of biological controls for depth distributions of corals major pattern in the organisation of scleractinian coral on tropical reefs has yet to be demonstrated. Previous communities. Differences among species of corals in attempts to unravel the processes underlying vertical tolerances to several physical factors (wave energy, zonation of tropical corals have focused primarily on currents, desiccation, light, temperature and sedimen- factors affecting mature, coral populations (reviewed tation) are thought to determine the upper and/or by Sheppard 1982),and with few exceptions (e.g. Bab- lower limits of species depth distnbutions (Done 1982, cock & Mundy 1996, Mundy & Babcock 1998) have not Sheppard 1982, Sebens & Done 1992). Biological fac- examined the role of early life-history processes in tors (competition and predation) have also been sug- maintaining vertical zonation. gested as important controls on the depth distributions Results from several descnptive studies of coral dis- tributions indicate that the depth distribution of corals 'Present address School of Zoology, University of Tasmania, may be Set dunng the early life-history stages of corals GPO Box 252-05. Hobart, Tasmania 7001, Australia. (e.g. settlement, early post-settlement). Bak & Engel E-mail: [email protected] (1979) and Rogers et al. (1984) found that the depth 0 Inter-Research 2000 Resale of full article not permitted 110 Mar Ecol Prog Ser 198: 109-119,2000 distribution of juvenile corals of most species rnirrors the range inhabited by adult corals. This was quanti- that of the adult corals, at least in terms of presencel fied by placing 10 d old corals of 3 species (Goniastrea absence. Moreover, depth-specific recruitment of coral aspera, Platygyra daedalea, Oxypora lacera) with con- species has been observed during recovery of coral trasting depth ranges at 3 depths in the field and mon- communities from intense Acanthaster planci preda- itoring their survival over a 12 mo period. Due to the tion (Colgan 1987). This suggests verticai zonation difficulty in locating recently settled juveniles (0 to 3 may be set and maintained through 2 alternative mo) in the field and the poor taxonomic resolution mechanisms; widespread and random settlement of achievable for those juvenile corals that are found, the planulae followed by differential early post-settlement field experiments were conducted using laboratory- mortaiity with depth, or depth-dependent settlement. raised larvae of the target species. However, few studies on zonation of sessile marine invertebrates have been able to demonstrate a causal link between larval behaviour in the water column METHODS prior to settlement, depth-dependent settlement and the depth distribution of adult populations in zone-spe- Study species and site description. The study was cific species (e.g. Grosberg 1982). Differences in carried out on the windward slope of the fringing reef photo-responses of lamae of the polychaete Spirorbis at Orpheus Island, centrai Great Barrier Reef (18'35' S, boreaiis from populations at different depths have also 146"29'E), approximately 15 km offshore from the been correlated with the vertical distribution of the 2 Queensland coast. The study site was located on North populations, and the differences between the popuia- East Reef (NER) near the outer edge of the channel tions appear to have a genetic basis (Doyle 1974). between Orpheus and Pelorus Islands. NER is occa- The fact that coral planulae are primitive lamae with sionally exposed to high sedimentation from turbid limited mobility and no specialised sensory Organs coastal waters flushed through the channel by tidal (Harrison & Wailace 1990) suggests dispersal and set- currents, with an average visibility of approximately 8 tlement of coral planulae is a stochastic, passive pro- to 10 m. The study site slopes gently from the reef flat cess. This impiies an irnportant role for post-settlement to a depth of 10 m, where it drops more steeply to the processes. However, coral larvae of some species have channel floor. Between 8 and 10 m a fine layer of sedi- been shown to prefer a particular orientation at settle- ment Covers the natural substratum, with little coral ment in response to Light (Morse et al. 1988, Babcock growth below 10 m. & Mundy 1996). Additionally, competent planulae of Three species of scleractinian corals with contrasting some zone-specific coral species are able to recognise adult depth distributions were used in this study: Goni- shallow and deep reef light regimes, and that light astrea aspera, Platygyra daedalea, and Oxypora lac- intensity and spectral quality significantly affect settle- era. G. aspera is a small- to medium-sized massive ment density in these species (Mundy & Babcock 1998). coral and is abundant on the reef flat (Babcock 1984, This indicates that settlement preferences of lamae of 1991) but rare or absent on the reef slope. P. daedalea some zone-specific coral species may be adaptive. is a massive brain coral and is common over a wide Available data on patterns of post-settlement mortal- range of depths, including the reef flat and lower slope ity and settlement of tropical corals with depth are also (Miller 1994). 0. lacera forms large thin laminar plates inconclusive. Patterns of early post-settlement survival (Veron & Pichon 1980) and is abundant on the lower of the zone-specific coral species Platygyra sinensis slope at a depth of 10 m, and rare or absent in shal- and Oxypora lacera could not explain the depth distri- lower depths at this reef. G. aspera and 0.lacera were bution patterns of adult corals of the 2 study species chosen because they have non-overlapping depth (Babcock & Mundy 1996). However, the patterns of ranges, and are dominant species within their respec- settlement density of these species in smaü field enclo- tive zones. P. daedalea was chosen as a reference spe- sures at deep and shallow sites, while not contradic- cies, as the depth range of P. daedalea overlaps the tory, do not Support the alternative model that vertical depth range of G. aspera and 0.lacera, and is cornmon zonation of corals is determined by depth-dependent across all depths. All 3 species are broadcast spawning settlement of larvae. hermaphrodites with sirnilar larval duration and rnini- Here we seek evidence for 1 of 2 alternative concep- mum period to settlement competency (4 to 5 d, Bab- tual models for the maintenance of vertical zonation: cock 1985, Babcock & Heyward 1986). Differencec in depth-dependent settlement of larvae, or indiscrimi- adult depth distnbutions were considered unlikely to nant settlement of larvae with distributions deterniined be a result of varying reproductive ecology. by differential post-settlement mortality with depth. Species distribution. To confirm the vertical distnb- Specifically, we test whether recently settled juveniles ution of the study species at NER, quantitative surveys of zone-specific species could survive at depths outside of Goniastrea aspera, Platygyra daedalea, and Oxy- Mundy & Babcock: Vertical