The occurrence of destructive grazing fronts is a common phenomenon in sea urchins, but mechanisms governing front formation and dynamics remain poorly understood. We experimentally examined the effect of kelp biomass on the aggregative behavior and movement of a front of green sea urchins Strongylocentrotus droebachiensis at a wave-exposed site on the Atlantic coast of Nova Scotia. We varied kelp (Laminaria digitata and L. longicruris ) abundance in 2 × 2 m plots adjacent to the front in 3 treatments: 50% plant removal, and 100% frond removal and unmanipulated control. In each treatment, we monitored the position of the front and urchin density at the leading edge over 24 d. The mean advance of the front in 24 d (2.27 m) did not differ between treatments, but urchin density was greatest in the control (74.9 urchins 0.25 m -2 ) and lower in whole plant (54.3) and frond (39.4) removal treatments. When urchin density was used as a covariate, front advance was inversely related to kelp biomass and greater in frond and plant removal treatments than in the control. Together, urchin density and kelp biomass explained 75% of variation in front advance. These findings provide the first direct evidence that urchins redistribute themselves along a front to concentrate in patches of greatest food availability. Temporal variation in urchin density at the front was inversely correlated with wave height, and individual grazing rates increased with urchin density, which may explain seasonal variation in front dynamics observed in previous studies.

Green sea urchins Strongylocentrotus droebachiensis along the coast of Nova Scotia, Canada, suffer mass mortalities from infection by the pathogenic amoeba Paramoeba invadens Jones, 1985. It has been speculated that P. invadens could be a form of Neoparamoeba pema quidensis, a species associated with disease in S. droebachiensis and lobsters in the northeast USA. During a disease outbreak in fall 2011, we isolated amoebae from moribund urchins collected from 4 locations along ~200 km of coastline. In laboratory infection trials, we found that timing and rate of morbidity corresponded to that of similar experiments conducted in the early 1980s, when P. invadens was first identified. All isolates had a similar size and morphology to the original description, including an absence of microscales. Sequences of nuclear SSU rDNA show that disease was caused by one 'species' of amoeba across the range sampled. Phylogenetic analyses prove that P. invadens is not conspecific with N. pemaquidensis, but is a distinct species most closely related to N. branchiphila, a suspected pathogen of sea urchins Diadema aff. antillarum in the Canary Islands, Spain. Morphology and closest phylogenetic affinities suggest that P. invadens would be assignable to the genus Neoparamoeba; however, nuclear SSU rDNA trees show that Neoparamoeba and Paramoeba are phylogenetically inseparable. Therefore, we treat Neoparamoeba as a junior synonym of Paramoeba, with P. invadens retaining that name, and N. pemaquidensis and N. aestuarina reverting to their original names (P. pemaquidensis and P. aestuarina), and with new combinations for N. branchiphila Dykova et al., 2005, and N. perurans Young et al., 2007, namely P. branchiphila comb. nov. and P. perurans comb. nov

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