Epibiosis is a facultative association of two organisms: the epibiont and the basibiont (Wahl, 1989). The term epibiont includes organisms that, during the sessile phase of their life cycle, are attached to the surface of a living substratum, while the basibiont lodges and constitutes a support for the epibiont (Threlkeld et al., 1993). Both concepts describe ecological functions (Wahl, 1989). Several crustacean groups such as cladocerans, copepods, cirripedes, isopods, amphipods and decapods include forms that are hosts for macroepibiont invertebrates (Porifera, Cnidaria, Platyhelminthes, Nemertea, Rotifera, Nematoda, Polychaeta, Cirripedia, Decapoda, Gastropoda, Bivalvia, Phoronida, Bryozoa, Ascidiacea and others) (Ross, 1983) and for protozoan microepibionts of the phylum Ciliophora (apostomatids, chonotrichids, suctorians, peritrichs, and heterotrichs) (Corliss, 1979; Small and Lynn, 1985).
The epibiosis involves different aspects, among which: (1) the specificity between epibionts and their crustacean basibionts; (2) the morphological and physiological adaptations of the epibionts; (3) the effects produced by the epibionts on the crustaceans; (4) the possible use of epibionts for the assessment of water quality; (5) the implications of epibionts on cultures of crustaceans and (6) the organization of the epibiont communities. A number of effects are related to epibiosis. These include advantages for the epibiont such as dispersal and geographical expansion, increase of the supply of nutrients and protection against predation (Connell and Keough, 1985; Williams and Moyse, 1988; Abelló et al., 1990; Key et al., 1997). On the other hand, epibiosis can be disadvantageous to the epibiont, creating ontogenetic or behavioural changes of the basibiont. Epibiosis can provide mimetic protection for the basibiont and cleaning. Conversely, epibiosis may have the disadvantage of restricting the mobility of the basibiont, it may affect growth and moulting and the functioning of several organs (eyes, gills, appendages, reproductive systems) and it may cause an increase of the risk of predation. Epibionts and basibionts also may compete for nutrients (Wahl, 1989; Threlkeld et al., 1993; Becker and Wahl, 1996).
Epibiotic associations could represent excellent models to examine diversity patterns among geographical regions on a variety of scales, including whole communities of species in different habitats. Although many crustaceans have been studied for their behaviour, few studies dealt with their associates and virtually nothing is known about the interactions between these epibionts and their basibiont hosts (Utz, 2003). The study of physical and biological factors related to the origin of symbioses, hypersymbioses and predator-prey relationships is a promising field of research (Williams and McDermott, 2004).
Many marine sessile life forms depend on the characteristics of the living substratum to which they are adhered (Gili et al., 1993) and, consequently, structure dynamics, physiology, and ecology of the basibiont may reflect on colonisation patterns of the epibiont species, and on settlement and growth of communities of invertebrates and protists.
Epibiosis is the evolutionary result of the interaction of environmental factors with benthic life forms (Key et al., 1999). It is a dynamic process, and the benefits and disadvantages to the intervening organisms vary depending on environmental conditions (Bush et al., 2001). Epibiosis can appear as a temporal colonisation due to a diminution of basibiont defences (Wahl and Mark, 1999). Epibiosis may modify a number of interactions between the basibiont and biotic and abiotic components of the system. Despite its wide occurrence, epibiosis is still not very well known with respect to its consequences for both basibionts and epibionts. A number of studies of crustacean epibiosis have been performed in freshwater systems, with few focusing on marine and estuarine environments (Carman and Dobbs, 1997). Epibiosis, as one of the closest possible interspecific associations, is a common phenomenon in shallow subtidal communities. Fouling of the basibiont creates a new interface between the basibiont and its environment. Most interactions between a living organism and its biotic and abiotic environment (e.g., predation, mating, defence, mutualism, parasitism, symbiosis, drag) are linked to essential surface features of the organism (Laudien and Wahl, 2004).
Epibiont populations may have relevant functions at the ecosystem level. Epibionts showing high densities may even contribute to energy flow to higher trophic levels. The colonization of a marine hard-bottom community on newly available substrata is governed by pre-settlement (survival and distribution of colonising stages), settlement (composition of coloniser pool, competence of settling stages, substratum preferences) and post-settlement processes (competition, consumption, etc.). When, during recruitment, substratum becomes the limiting factor, dominant competitors may drive competitively inferior species to extinction (Enderlein and Wahl, 2004). Epibiosis is important with regard to biodiversity and conservation. In temperate regions diversity, or species richness, of benthos in soft substrates on the continental shelf and slope may rival that in shallow tropical seas (Brusca and Brusca, 1990).
In tropical mangrove estuaries and related coastal waters in Malaysia and Thailand several crustacean species occur in abundance, constituting a substantial portion of the hyperbenthic fauna (Hanamura et al., in press a, b). These crustaceans also are major dietary resources of mangrove fishes (e.g., Kiso and Mahyam, 2003; Chew et al., 2006; Then et al., 2006), which suggests that they play an important ecological role in the mangrove estuary.
These species showed a generalized distributional pattern in mangrove systems of north-western Peninsular Malaysia, as is indicated in Fig. 1, where adult and subadult Acetes indicus Milne Edwards 1830 and A. japonicus Kishinouye 1905 occur in abundance in the sublittoral zone of the river mouth, while A. sibogae Hansen 1919 tends to be found in increasing numbers in the middle to upper reaches. In contrast, Mesopodopsis orientalis (Tattersall, 1908) has a strong affinity to the littoral zone from coastal to upstream areas. Meanwhile, there is a possibility of the existence of two populations of M. orientalis in the studied areas, i.e., estuarine and coastal populations (Hanamura et al., unpublished data). Consequently, the sampling sites of this mysid are specified for future consideration. Like Mesopodopsis, young juveniles of A. sibogae and Fenneropenaeus merguiensis de Man 1888 occur predominantly along the littoral zone of the estuary swamp (Hanamura et al., in press a, b).
Fig. 1. Distribution of hyperbenthic crustaceans in an estuary system in Peninsular Malaysia (schematic, modified from Hanamura et al., in press b)
Recent ecological studies on the hyperbenthos have revealed that the ciliate-crustacean association is a frequently observed phenomenon in tropical mangrove estuaries and related coastal waters (Hanamura et al., unpublished data). In the present study, the epibiosis on these crustacean species was analyzed, with emphasis on the differences between basibiont species, and between the diverse sampling sites. The epibiosis was also considered in terms of preference of epibiotic species and with respect to the structure composition of the epibiont community. The purpose of this study is (1) to analyze if the epibiotic communities on the diverse basibiont species and localities present particular characteristics related to the species found, and (2) their distribution on the anatomical units of the basibiont. We will propose explanations for the different patterns of colonization. We hypothesize that dominant mangrove crustacean species present a characteristic epibiosis specific to each species and different from populations living on open sea environments.