Invertebrates (pdf)

	Natural mortality of hard corals during summer 2003
	Rediscovery of a rare skipper--White-banded Awl (Hasora taminatus)
	A photo record of the coral reef mantis shrimp, Pseudosquilla ciliata in Hong Kong

Natural mortality of hard corals during summer 2003

by Andy Cornish

Natural mortality in local coral communities was observed during July and August 2003 and attributed to 3 possible causes, Drupella, low tides and typhoon Imbudo.

Several hundred Drupella cf. rugosa snails were observed feeding in dense concentrations on four large (around 70 cm in height) Platygyra cf. sinensis colonies at Sharp Island on 10 July (Fig. 1). All four colonies were within 5 m of each other and at < 3 m depth. When the site was revisited on 17 July the majority of each colony had been completely stripped of live tissue and another colony nearby of the same species was being attacked. The largest quantitative survey of the phenomenon in Hong Kong (Morton and Blackmore, 2000) noted that while Acropora pruinosa seems to be the favourite prey of Drupella rugosa, other species may be attacked when the Acropora is scarce, such as at Sharp Island. That report concluded that such outbreaks posed "little threat" to coral communities in Hong Kong, in part because the Drupella normally only eat the connective tissue between polyps, allowing subsequent recovery. In the case reported in 2003, however, all tissue was removed with the loss of several decades-old colonies.

Partial coral mortality was apparent at Coral Beach, within the Hoi Ha Wan Marine Park, when the coral community there was visited on the 17 July. The top 3 or so centimeters of all of the shallowest corals, mostly Platygyra species, were bleached white and covered in various gastropods. It seems likely that the upper surfaces had been exposed to the air and sun for extended periods on the 14 and 15 July when there were extremely low tides (0.04 m and 0.06 m, respectively at Tai Po Kau, Hong Kong Observatory pers. comm.) and that the dead tissue was subsequently being eaten by the gastropods. Such mortality limits the vertical growth of corals on shallow reefs resulting in "micro-atolls" of massive corals, where growth is only possible horizontally leading to colonies with a large dead eroded center and a ring of living tissue on the sides.

Typhoon Imbudo hit Hong Kong on the 24 July, causing damage to many large coral colonies in the Hoi Ha Wan and Ping Chau Marine Parks. At Moon Island in Hoi Ha, a number of large, internally bio-eroded Platygyra colonies in the shallowest waters (< 2 m depth) toppled over. Also, at least six large black coral colonies (Antipathes sp.) snapped near the base, or the holdfast came free, and they were transported from depths of > 7 m into shallower waters. This species is bush-like with numerous branches which must create significant drag in the water, making them vulnerable to strong surges. Coral Beach (Hoi Ha) is dominated by Pavona decussata and less damage was evident although some individual Pavona plates and a large stand measuring around 2 x 1 x1 m had pulled out of the sand and fallen over. At Ping Chau the primary damage was caused by various species of massive coral including Platygyra cf. sinensis separating at the base from the siltstone bedrock and toppling down a siltstone "step." There was also one small patch at < 2 m depth where loose siltstone slabs had been moved considerably by the waves, smashing corals in their path. Although greater than the damage caused by the direct hit of Typhoon Victor in 1997, I estimate that < 5% of all coral colonies were affected in both parks and even those that toppled should still be able to grow on those living surfaces not now smothered by sand etc.

Morton, B. & Blackmore, G. (2000). The impacts of an outbreak of corallivorous gastropods, Drupella rugosa and Cronia margariticola (Muricidae), on Hong Kong’s scleractinian corals. Final report to The Government of the Hong Kong Special Administrative Region, China 30 April 2000. Swire Institute of Marine Science, The University of Hong Kong. pp 48.