Competition induces allelopathy but suppresses growth and anti-herbivore defence in a chemically rich seaweed

Many seaweeds and terrestrial plants induce chemical defences in response to herbivory, but whether they induce chemical defences against competitors (allelopathy) remains poorly understood. We evaluated whether two tropical seaweeds induce allelopathy in response to competition with a reef-building coral. We also assessed the effects of competition on seaweed growth and seaweed chemical defence against herbivores. Following 8 days of competition with the coral Porites cylindrica, the chemically rich seaweed Galaxaura filamentosa induced increased allelochemicals and became nearly twice as damaging to the coral. However, it also experienced significantly reduced growth and increased palatability to herbivores (because of reduced chemical defences). Under the same conditions, the seaweed Sargassum polycystum did not induce allelopathy and did not experience a change in growth or palatability. This is the first demonstration of induced allelopathy in a seaweed, or of competitors reducing seaweed chemical defences against herbivores. Our results suggest that the chemical ecology of coral–seaweed–herbivore interactions can be complex and nuanced, highlighting the need to incorporate greater ecological complexity into the study of chemical defence.     

Scarid Biomass on the Northern Great Barrier Reef: The Influence of Exposure, Depth and Substrata

Here I investigate whether patterns of scarid biomass across the continental shelf of the northern Great Barrier Reef can be explained by species associating with particular characteristics of the reef environment. Despite the widely documented tendency of scarids to graze and browse over exposed calcareous reef surfaces, scarid biomass was not significantly correlated with the availability of feeding substrata for any species investigated. Indeed positive correlations between biomass and substrata variables were rare for the 18 species investigated, indicating that biomass in these taxa was not strongly reliant on the availability of preferred substrata quantified at the spatial scale of sites (1620m²). Rather, species specific biomass was commonly highly variable between sites, suggesting that local aggregation of scarids commonly occurs at this scale. Such spatial patchiness potentially reflects the effects of spatially variable recruitment, fishes associating with unmeasured habitat characteristics or aggregating prior to spawning. Despite variability in the biomass of individual species between replicate sites within exposure regimes, exposure was generally a far more reliable predictor of biomass than the other variables quantified. This study provides little evidence to indicate that adult scarids have strict habitat requirements, rather they appear to be habitat generalists whose biomass is strongly influenced by exposure but weakly related to the cover of particular reef substratum.     

Competitors as accomplices: seaweed competitors hide corals from predatory sea stars

Indirect biotic effects arising from multispecies interactions can alter the structure and function of ecological communities—often in surprising ways that can vary in direction and magnitude. On Pacific coral reefs, predation by the crown-of-thorns sea star, Acanthaster planci, is associated with broad-scale losses of coral cover and increases of macroalgal cover. Macroalgal blooms increase coral–macroalgal competition and can generate further coral decline. However, using a combination of manipulative field experiments and observations, we demonstrate that macroalgae, such as Sargassum polycystum, produce associational refuges for corals and dramatically reduce their consumption by Acanthaster. Thus, as Acanthaster densities increase, macroalgae can become coral mutualists, despite being competitors that significantly suppress coral growth. Field feeding experiments revealed that the protective effects of macroalgae were strong enough to cause Acanthaster to consume low-preference corals instead of high-preference corals surrounded by macroalgae. This highlights the context-dependent nature of coral–algal interactions when consumers are common. Macroalgal creation of associational refuges from Acanthaster predation may have important implications for the structure, function and resilience of reef communities subject to an increasing number of biotic disturbances.     

Mobility of Stichodactyla Gigantea Sea Anemones and Implications for Resident False Clown Anemonefish, Amphiprion Ocellaris

For reef fishes that do not move between habitat patches following settlement, habitat selection is expected during settlement. Although false clown anemonefish, Amphiprion ocellaris, are sedentary following settlement, they are not especially discriminating during settlement, and are commonly found occupying anemones at which no apparent nest site exists. In this study I report on mobility of Stichodactyla gigantea sea anemones, including anemones with resident false clowns. I argue that anemone mobility can help explain why settling false clowns are not more discriminating: although the per annum probability of an anemone moving is low, the probability of that anemone moving over the course of a resident's life is considerably higher. Therefore, an anemone's current microhabitat may not be a good predictor of its microhabitat and suitability as a host in the future.     

Effects of substrata, resident conspecifics and damselfish on the settlement and recruitment of the stoplight parrotfish, Sparisoma viride

Recruitment plays an important role in the population dynamics of marine organisms and is often quantified as a surrogate for settlement. When quantified, recruitment includes settlement plus a period of time in the benthic habitat. Therefore, it is essential to determine whether post-settlement processes alter patterns established at settlement. I conducted a series of experiments on 2.0 m² patch reefs to examine the importance of pre- and post-settlement processes to the distribution and abundance of recruits of the stoplight parrotfish, Sparisoma viride, on the Tague Bay reef, St. Croix, USVI. Recruitment was higher to the coral Porites porites than to another common coral Montastrea annularis, but there was no evidence of microhabitat choice at settlement. This result, in conjunction with the examination of the size classes of recruits present on P. porites and M. annularis patch reefs in a separate experiment suggested that differences in recruitment were established after settlement. Stoplights settled in higher numbers to patch reefs that contained conspecific residents, and persistence was higher at higher recruit density. Although resident damselfish directed significant amounts of agonistic behavior towards newly stoplight recruits, damselfish presence had no effect on settlement. However, damselfish presence did reduce stoplight recruitment. These results demonstrate that both pre- and post-settlement processes influence the recruitment of stoplight parrotfish. More importantly, these results indicate that benthic processes can alter recruitment patterns from initial settlement patterns, and indicate that workers should be careful in using recruitment as a proxy for settlement.     

大型海藻在珊瑚礁退化过程中的作用

随着全球范围珊瑚礁的退化,大型海藻在珊瑚礁区的覆盖度呈增多的趋势。大型海藻的大量生长,妨碍了珊瑚的生长、繁殖、恢复等过程。概括起来,大型海藻对珊瑚生长、繁殖及恢复过程所产生的不利影响主要包括:(1)大型海藻通过与珊瑚竞争空间和光照而影响珊瑚生长;(2)大型海藻与珊瑚直接接触时,通过摩擦作用及释放化感物质而影响珊瑚生长;(3)大型海藻的大量生长打破了珊瑚与海藻的竞争平衡,珊瑚为应对大型海藻的入侵而把用于生长和繁殖的能量转移到组织修复与防御上,进而造成珊瑚繁殖能量的减少;(4)大型海藻通过影响珊瑚幼虫的附着及附着后的存活率,而阻碍珊瑚群落的发展;(5)海藻还能通过富集沉积物、释放病原体及扰乱珊瑚共生微生物的生长等而间接影响珊瑚生长。明确的竞争机制有利于研究海藻与珊瑚的相互作用过程。在总结前人对海藻与珊瑚的竞争机制研究的基础上,把两者的竞争机制划分成物理机制、化学机制、微生物机制三大类,物理机制是研究得比较透彻的竞争机制,而化学机制与微生物机制则需要更深入的研究,是当前研究的热点。目前,我国对珊瑚礁中底栖海藻与珊瑚的相互作用研究甚少;鉴于此,对底栖海藻功能群的划分类型以及三大类型底栖海藻对珊瑚的作用特点做了简要介绍,并对珊瑚礁退化的现状和退化珊瑚礁区内海藻的表现做了概述。在此基础上,再综述国外关于大型海藻对珊瑚的影响研究进展,指出我国应该加强对南海珊瑚礁区大型海藻的种类分布及丰富度等的调查,评价大型海藻对南海珊瑚礁的影响现状;并结合生理学、分子生物学技术和生态学研究手段,在细胞与分子水平上探索海藻对珊瑚的影响机制,以期为珊瑚礁生态系统的保护提供参考。     

Reef fish assemblages in north-western Sri Lanka: distribution patterns and influences of fishing practises

The distribution and abundance of reef fishes in relation to habitat structure were studied within Bar Reef Marine Sanctuary (BRMS) and on an adjacent reef, disturbed by destructive fishing techniques, in north-western Sri Lanka, by visually censusing 135 species groups using fifty metre belt-transects. Two types of continental shelf patch-reefs are found in the study area: coral reefs and sandstone reefs, which are divided into distinct habitats, four for the coral reef (shallow reef flat, shallow patch reef, deep reef flat and Porites domes) and two for the sandstone reef (structured sandstone-reef and flat sandstone-reef). Fish assemblages varied in structure between reef types and among habitats within reef types. Functional aspects of habitat structure and composition, such as available food and shelter, seemed to be important factors influencing distribution patterns. The strongest separation in the organisation of fish assemblages in BRMS was between reef types: 19% of all species were confined to the coral-reef patches while 22% were restricted to the sandstone reef patches and 59% were represented on both reef types. In terms of distribution among habitats, 21% of all species were restricted to one habitat while only 1.5% were present in all. The highest density of fish was in the coral reef habitats while highest species diversity was found in the most structurally complex habitat: the structured sandstone-reef. This habitat also had the highest proportion of species with restricted distribution. Planktivores were the most abundant trophic group in BRMS, and the species composition of the group varied among habitats. The comparison of the disturbed reef with BRMS suggested that habitat alteration caused by destructive fishing methods has strongly influenced the fish community. Within the fished area the structure of the fish assemblages was more heterogeneous, fish abundance was lower by an order of magnitude and species numbers were lower than in BRMS.     

珊瑚礁生态修复研究进展

珊瑚礁生态系统有着很高的生物多样性和重要的生态功能。20世纪80年代以后全球范围内珊瑚礁的大面积退化引起了人们广泛的关注。简述了世界珊瑚礁资源现状,破坏原因,生态修复方法以及我国的珊瑚礁资源现状和修复策略等。国际上通用的生态修复策略主要是根据珊瑚的两种繁殖方式进行的,此外再配合人为的适度干扰,增加珊瑚的成活率。方法主要有：珊瑚移植、Gardening、人工渔礁、底质稳固、幼体附着等以及对相关利益者的宣传,海岸带的保护等。我国珊瑚礁退化严重,但是由于缺乏相关的科技资料报道和技术支持,缺乏系统的研究,使得珊瑚礁的生态修复成绩甚微,今后应在该领域开展更多的工作。