Pathological effects of cyanobacteria on sea fans in southeast Florida

- Sea fans covered by benthic filamentous cyanobacteria.
- A sea fan naturally affected by cyanobacteria is described by histopathology.
- An aggressive host cellular response to surface colonization by cyanobacteria.
- Dramatic amoebocyte response to the presence of surface cyanobacteria.
- Host tissues were severely necrotic at the advanced stage of cyanobacterial overgrowth.

In early August 2008, observations by divers indicated that sea fans, particularly Gorgonia ventalina, Gorgonia flabellum, and Iciligorgia schrammi, were being covered by benthic filamentous cyanobacteria. From August 2008 through January 2009 and again in April 2009, tissue samples from a targeted G. ventalina colony affected by cyanobacteria and from a nearby, apparently healthy (without cyanobacteria) control colony, were collected monthly for histopathological examination. The primary cellular response of the sea fan to overgrowth by cyanobacteria was an increase in the number of acidophilic amoebocytes (with their granular contents dispersed) that were scattered throughout the coenenchyme tissue. Necrosis of scleroblasts and zooxanthellae and infiltration of degranulated amoebocytes were observed in the sea fan surface tissues at sites overgrown with cyanobacteria. Fungal hyphae in the axial skeleton were qualitatively more prominent in cyanobacteria-affected sea fans than in controls.