Larval Settlement and Surfaces: Implications in Development of Antifouling Strategies

Abstract

Marine biofouling is a natural process that imposes technical operational problems and economic losses on marine-related activities. Marine biofouling communities are complex, diverse, highly dynamic ecosystems consisting of a range of organisms. Larvae of these organisms spend a part of their lives in the planktonic stage before settling on a surface. Passive transport and deposition of larvae were considered responsible for the observed spatial variation in settlement pattern, whereas breeding season and larval survival have been associated with temporal fluctuations. Hydrodynamic conditions influence the transport and deposition of larvae near the surface boundary layer, while dissolved environmental stimuli have been associated with the induction of settlement and metamorphic behaviour. Over the last two decades, chemical cues and physiological processing of the cue hasbeen the subject of study. Sufficient information has been obtained on the settlement mechanisms, the nature of chemical substances, involvement of chemosensory receptors and signal transduction pathways downstream. Knowledge on the settlement mechanism is imperative for developing a suitable control strategy. At present, more is known about chemosensory reception and downstream processing of the sensory cue than the location of these receptors. The need to control biofouling on underwater surfaces has given rise to many different technologies. Conventional antifouling strategy employs the use of biocidal surface coatings. The rationale behind these coatings is to kill everything. Historically, different solutions for control of fouling have been employed. It was not until the development of cold-plastic antifouling paints (copper oxide and tributyltin oxide or fluoride) in the later part of the twentieth century that a truly long-lasting protection was achieved. Unfortunately, the accumulation of slow-degrading organotin moieties in the water column has resulted in sub-lethal effects on non-target organisms, which led to its progressive abandonment. Insights into the larval sensory recognition of physical cues and adhesion resulted in the development of foul release coatings based on low surface energy phenomenon. Another alternative approach for control of biofoulingand inhibition of larval settlement lies in inhibiting the neurophysiological processes involved in larval settlement. This has been experimented upon using natural bioactive molecules and synthetic analogues, which bind to specific receptors inhibiting larval settlement. Several pharmacological compounds, natural products and synthetic analogues that inhibit the metabolic processes underlying settlement have been identified through laboratory bioassays. However, realization of these compounds into commercial coatings is yet to happen. The reasons may be attributed to reproducibility of laboratory results in ecologically realistic field experiments. The need for suitable bioassays and knowledge of the broadspectrum activity of these compounds is obvious.