Successful coral recruitment is critical to the maintenance of reef communities. In Hong Kong, southern China, sea surface temperatures drop to 14–16°C in winter, creating a marginal environment for coral growth. Such low temperatures appear not to be critical for coral recruit survival. However, occasionally mass recruitment of invertebrates such as the barnacle Amphibalanus amphitrite occurs in winter and this is a serious threat to coral recruit survival. The barnacles have a fast growth rate and eventually cover 85–95% of open space on settlement plates (Fig. 1a, b, c), other open rock surfaces, in cracks and on damaged surfaces of coral colonies (Fig. 1d, e). The barnacles completely overwhelm the coral recruits that are simply overgrown (Fig. 1c). This mass recruitment of barnacles is not an annual event, but only happens under conditions of a prolonged (2 months) and colder than normal winters (<16°C) such as occurred this year (2010). A comparable outbreak of barnacle recruitment happened in 2000, under similar prolonged and relatively cold winter conditions. Such a periodic mass recruitment of barnacles likely wipes out successful coral recruitment in that particular year and reflects the additional difficulty with which coral recruits face in successfully establishing themselves in a marginal environment.

Fig. 1
figure 1

a Coral recruit (Poritidae) on settlement plate covered with sediment (photo taken 22 Dec 2009); b Same coral recruit with sediment removed. Note the recruitment of barnacles around the coral recruit; c Same coral recruit (arrow) overgrown by barnacles on 8 Jan 2010; d and e Massive recruitment of barnacles (arrows) occupying open and/or damaged space on the surface of coral colonies. Photos taken at around 2 m below chart datum

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