Valter Amaral1,2, Gil Penha-Lopes1, José Paula1 1
Laboratório Marítimo da Guia, Faculdade de Ciências da Universidade de Lisboa, Avenida Nossa Senhora do Cabo 939, 2750-374 Cascais, Portugal
2Marine Biological Association of the UK, Citadel Hill, Plymouth, PL1 2PB, UK
RNA/DNA ratio of crabs as indicator of mangrove habitat
quality
ABSTRACT
Mangrove ecosystems suffer from severe cutting and deterioration, putting at risk their future persistence and that of local communities that depend on them. Pollution, namely wastewater discharge, is a major problem in these ecosystems, especially in peri-urban mangroves. Biochemical indicators of organism physiological condition have been widely used to evaluate habitat quality and early detect impact of stressors, improving the success of management and conservation plans. Grapsid and ocypodid crabs are potential good bioindicators of the quality of mangrove habitats, as they are important and characteristic species of such environments. The potential of RNA/DNA ratio of Perisesarma guttatum and Uca annulipes, as indicators of habitat quality, was studied in a peri-urban and two relatively pristine Mozambican mangroves, in the rainy and dry seasons. RNA/DNA ratio was significantly lower in P. guttatum than in U. annulipes. Such differences were due to higher DNA, rather than lower RNA cellular contents in tissue claw muscle of P. guttatum. Our results indicate that RNA/DNA ratio of U. annulipes may be a useful and good indicator of the quality of mangrove habitats in terms of pollution and seasonal fluctuation. Both species showed seasonal effects on RNA/DNA ratio, but only U. annulipes was significantly affected by pollution. These results suggest that the physiological condition of mangrove crabs was more affected by seasonal fluctuations than by pollution, especially on P. guttatum. We propose the RNA/DNA ratio of U. annulipes as a valid and sensitive bioindicator of quality of mangrove habitats.
KEYWORDS: Biochemical indicators, Environmental conditions, Mozambique, Nucleic acid,
Perisesarma guttatum, Uca annulipes
INTRODUCTION
Mangroves are wetland ecosystems formed by the association of trees and shrubs and various animal taxa that thrive naturally in intertidal zones of brackish and marine coastal habitats (Kathiresan & Qaim 2005, Hogarth 2007). They are limited to zones of low wave energy at tropical and sub-tropical latitudes. Mangroves serve several important ecological functions e.g., they provide recruitment, nursery and feeding areas for many fish and invertebrate species, are a source of nutrients and organic matter to primary and secondary producers, prevent siltation of coral reefs, offer protection and stabilization of shorelines and also act as a sink for pollutants (Abuodha & Kairo 2001, Kathiresan & Qaim 2005, Hogarth 2007). Thus, they constitute coastal resources of economical
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relevance, especially for local communities, as they provide a variety of fishing products and areas, wood supply and other forestry products (Abuodha & Kairo 2001, Kathiresan & Qaim 2005, Hogarth 2007).
This potential for many uses, together with global increase in human populations, climate change and associated sea-level rise have led to severe clearance of mangrove areas for aquaculture, urbanization and costal landfill purposes, and to its deterioration due to direct and indirect effects of pollution and upstream land use (Kathiresan & Qaim 2005, Duke et al. 2007, Hogarth 2007). As mangrove areas become smaller, scattered and degraded, their medium and long-term persistence is at serious risk, jeopardizing essential ecological functions and associated resources, as well as the future of local communities (Alongi 2002, Kathiresan & Qaim 2005, Duke et al. 2007). Consequently, it is now unanimously agreed that there is an urgent need for effective policies, strategies and actions leading to sustainable maintain, protect, and restore these valuable wetland ecosystems (Duke et al. 2007).
Biochemical indicators of organism health and environmental quality of its habitats have become central in such an integrative approach (Dahlhoff 2004, Amaral et al. in press, Amaral et al. in review-a). They are key components of synthetic or metabolic pathways, reflecting metabolic activities directly or indirectly linked to growth, reproduction, survival, and thus physiological condition (Dahlhoff 2004). Biochemical changes usually occur before those in organism growth or reproduction become discernable, which may help early identification of effects of stressors, such as pollution, and thus contribute for preventing the loss of ecosystem functions. RNA/DNA ratio has been especially valuable as a biochemical estimator of organism condition in studies of marine invertebrates and fishes (e.g. Wright & Hetzel 1985, Gilliers et al. 2004, Amaral et al. in press, Amaral et al. in review-a). The principle is that RNA content correlates with new protein synthesis, usually interpreted as beneficial to the organism, reflecting active metabolic rates canalized to growth and reproduction. As DNA content remains relatively constant in an individual, as a function of chromosome number, higher RNA/DNA ratio is expected to arise when environmental conditions are favourable (Dahlhoff 2004).
Because the benthos integrates conditions over time, and the associated fauna is the main link between primary producers and secondary consumers, macrobenthic organisms are considered good indicators of environmental quality of its habitats (Smith et al. 1991, Lohrer et al. 2004, Pagliosa & Barbosa 2006). Crabs constitute an important
macrofauna taxon in terms of abundance, species richness and biomass in mangrove ecosystems (Kathiresan & Qaim 2005, Hogarth 2007). Brachyuran crabs, particularly Grapsidae and Ocypodidae, further enhance ecosystem functioning, as their burrowing and feeding activities substantially improve biotic properties of the sediment, tree colonization, primary and secondary productivity, nutrient cycling and energy flow (e.g. Smith et al. 1991, Skov et al. 2002).
Despite the relevance of such crabs in mangrove ecosystems, and their potential as an evaluation tool of habitat quality, knowledge is lacking on their physiological responses to stressors, namely pollution and seasonal fluctuations. This study was therefore set up to investigate the bioindicator potential of the grapsid Perisesarma guttatum (A. Milne Edwards, 1869) and the ocypodid Uca annulipes (H. Milne Edwards, 1837) to evaluate habitat quality of polluted and non-polluted mangrove areas, in different climatic seasons, as measured through organism physiological condition. Another objective was to provide information on the physiological condition of those species, creating a baseline for future studies. RNA/DNA ratio in crabs of both species was assessed in peri-urban and relatively pristine mangrove systems at Mozambique, in the rainy and dry seasons.