Histological and ultrastructural features of Aplysina cauliformis affected by Aplysina red band syndrome

Abstract

AbstractAplysina red band syndrome (ARBS) affects shallow–water branching Caribbean sponges of the genus Aplysina. Signs of ARBS consist of a red band that advances from an initial circular lesion in both directions along the branch of the sponge, leaving behind spongin fibers devoid of living tissue. The red band is dominated by the filamentous cyanobacterium Leptolyngbya sp., which is responsible for the red coloration, although its role in the etiology of ARBS remains elusive. We used a combination of histology and transmission electron microscopy to describe ultrastructural and cellular differences between healthy and diseased individuals of Aplysina cauliformis. Tissue from the red band adjacent to spongin fibers devoid of living tissue was compared to healthy‐appearing tissue on the same sponges and to healthy sponges. ARBS‐affected tissue exhibited changes in cell densities, with increases in archaeocytes, collencytes, and unidentified round eukaryotic cells; decreases in exopinacocytes, granular cells, and spherulous cells; and loss of intact choanocyte chambers. Archaeocytes associated with diseased tissue appeared to be more active, as indicated by increased numbers and lengths of pseudopodia, and by a higher density of engulfed bacteria and symbiotic cyanobacteria, some of which were in various stages of disintegration. Filaments of Leptolyngbya sp. tended to be associated only with the ectosome of the red band portion of the lesion itself, and were rarely observed infiltrating intact tissue. These descriptions of anatomical and cellular responses to ARBS in A. cauliformis add to our understanding of the pathological effects of this widespread disease.