Histological observations on the scleractinian coral Porites lutea affected by pink-line syndrome

Abstract

A pink-line syndrome (PLS) was reported in the reef building coral Porites lutea in Kavaratti island of the Lakshadweep archipelago. The affected corals had dead patches colonized by a cyanobacterium Phormidium valderianum and the bordering coral tissue was pink. The histological changes associated with the PLS-affected tissue were examined. Results showed that the zooxanthellae were released from the gastrodermal cells into the coelenteron. Gastrodermal cells undergo necrosis and detachment from the basal membrane. The basic staining of the cytoplasm in the gastrodermal cells bordering the calicoblastic layer suggests accumulation of calcium ions. The ectodermal epithelium and calicoblastic cells showed destruction through ‘apoptosis-like’ processes. Cell swelling and vacuolation were observed in the gastrodermal and ectodermal cells. In this communication how the presence of the cyanobacterium adjacent to the PLS-affected tissue could cause the observed damage is discussed, bring about imbalance and a shift in the coral-zooxanthellae symbiosis.