A prolonged unusual red tide (about 10 months) recently occurred in the Persian Gulf. It started in August 2008 near the Strait of Hormuz and remained till May 2009 at Iranian side. Following this event, corals experienced severe settlement by fouling organisms, specifically serpulid worms, which settled not only on dead corals but also started to overgrow live polyps (Fig. 1d–e). This was followed by an increase in sedimentation, asphyxiation, and catastrophic death of corals in the described area.

Fig. 1
figure 1

a The coral community before the red tide (photo: M.S. Ranjbar), b Corals covered by serpulids three months after red tide, c Dead corals covered by algae six months after red tide, d–e Serpulid worms overgrowing live coral tissue, and f Corals smothered by overgrowth

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The affected coral assemblage, predominantly consisting of Porites species (Fig. 1a), is situated in the southeast of Qeshm Island (26° 55.689′ N, 56° 16.032′ E), in the Strait of Hormuz. It has an area of about 12 ha and is located 100–150 m from the shoreline at 2–8 m depth. The serpulid settlement increased dramatically after the red tide: three months after the beginning of the red tide, levels of infestation had reached 47 ± 9.3 per 25 cm2 in some colonies (Fig. 1b, d–e). The serpulids covered, smothered and killed more than 90% of the Porites colonies in early stages (Fig. 1f); later, the same scenario happened to other species, with Goniopora as an exception. Subsequently, the dead colonies became covered by green filamentous algae (Fig. 1c). Similar observations with different levels of mortality were made at nearby islands (Larak, Hengam).

Previous studies suggest that the density of coral associates may indicate the health of coral communities (Risk et al. 2001; Scaps and Denis 2008). As most of these associates are filter feeders, a higher nutrient concentration will increase their numbers, followed by an increase of coral stress and a decrease of their ability to resist the settlement and overgrowth of harmful epibionts (Holmes et al. 2000).