Is global ocean sprawl a cause of jellyfish blooms?

© The Ecological Society of America www.frontiersinecology.org M explanations have been suggested for possible drivers of the apparent increase of jellyfish blooms in many coastal waters around the globe, including the depletion of predators and competitors of jellyfish by overfishing, accidental translocations, eutrophication of coastal waters, changes in freshwater flows, human modification of coastal geomorphology, and climate change (Mills 2001; Purcell et al. 2007; Purcell 2012). Most of these explanations focus on factors that affect the performance of the pelagic phase of jellyfish; however, the life history of many jellyfish species includes an often unnoticed benthic stage – in the form of polyps – from which jellyfish are produced (Boero et al. 2008). A major obstacle in identifying the causes underlying these jellyfish blooms lies in the difficulty of locating the habitat of the polyps from which the most problematic coastal jellyfish blooms develop (Mills 2001; Boero et al. 2008). In most coastal jellyfish species with bipartite life histories, embryos develop into free-swimming planula larvae, which settle on hard substrates and metamorphose into sessile polyps (Boero et al. 2008). These polyps eventually develop into juvenile medusae, such as in box jellyfish or cubozoans, or segment asexually (ie strobilate) to release juveniles named ephyrae, as is the case in the true jellyfish or scyphozoans (Arai 1997). Jellyfish polyps are very small (a few millimeters in length; Figure 1), are inconspicuous, and typically inhabit shaded environments, often suspended underneath horizontal surfaces of rocks and shells (Pitt 2000; Holst and Jarms 2007). The small size of the polyps and the difficulty of sampling them complicates locating and measuring polyp colonies in the vast coastal, marine, and estuarine environments. Here, we examine the hypothesis that the growing proliferation of artificial structures associated with shipping, aquaculture, and other coastal industries, as well as the number and size of shoreline stabilizing structures, provides habitat for jellyfish polyps; such structures may therefore play an important role in increasing the numbers of jellyfish blooms. Support for this hypothesis is derived from observations and experimental evidence demonstrating that jellyfish larvae settle in abundance on artificial structures in coastal waters, forming dense concentrations of polyps. CONCEPTS AND QUESTIONS