First report of Thraustochytrium caudivorum isolated from the undulated surf clam (Paratapes undulatus): Morphological, molecular, and ecological insights under climate change

Abstract

Thraustochytrium caudivorum is a marine protist of the class Labyrinthulea with limited information on its ecology and host associations. Here, we report its first isolation from the gill tissue of the undulated surf clam (Paratapes undulatus) collected from subtropical coastal waters in the western Pacific. Sequencing of an 18S rDNA fragment (1225 bp) showed 99.8 % identity to known T. caudivorum strains, with three nucleotide differences suggesting intraspecific variation. Phylogenetic analysis confirmed its placement within the T. caudivorum clade, contributing to Labyrinthulea taxonomy. Morphological and ultrastructural observations revealed typical thraustochytrid traits, including lipid-rich spherical cells, multilayered cell walls, globose sporangia, and filamentous ectoplasmic network structures. Growth assays indicated optimal in vitro proliferation at 35 °C and 40 psu, suggesting strong tolerance to elevated temperature and salinity. To evaluate potential ecological interactions, Ruditapes philippinarum (Manila clam) was experimentally exposed as a surrogate host. Mortality was observed under higher temperatures, indicating possible opportunistic behavior, although no lesions or mortality were detected in the original P. undulatus specimens. These findings expand current knowledge of T. caudivorum biology and suggest its potential relevance in aquaculture and marine ecosystems, particularly under warming and salinity fluctuations associated with climate change.