Stegotricha enterikos gen.nov., sp.nov. (class Phyllopharyngea, order Rhynchodida), a parasitic ciliate in the digestive gland of Pacific oysters (Crassostrea gigas), and its distribution in British Columbia

Abstract

Stegotricha enterikos gen.nov., sp.nov. is found in the lumen of the digestive gland tubules of Pacific oysters (Crassostrea gigas (Thunberg, 1793)), where it multiplies by transverse binary fission. Nondividing stages have an elongated ovoid body (49.8 ± 10.3 μm long and 18.1 ± 3.4 μm wide) covered by about 16 evenly spaced, slightly oblique kineties (ciliary rows), except for a small naked area on one side of the posterior end. One kinety, associated with the naked area, is unusual, with cilia spaced two to three times as far apart as in the other somatic kineties. A cytopharyngeal structure composed of radially arrayed overlapping sheets of microtubules occurs at the anterior end and occasionally encompasses extrusomes. The centrally located macronucleus is large (about 16 × 12 μm) and the micronucleus (about 2.5 μm in diameter) is either posterior or anterior to the macronucleus. Except for the extent of ciliation, S. enterikos is most closely affiliated with the family Ancistrocomidae. Instead of creating a new family, we suggest that the diagnosis of the Ancistrocomidae be expanded to include ciliates with complete ciliation. Stegotricha enterikos was found in 2.5–72.5% of the oysters (n = 40) sampled between November 1985 and March 1990 from 35 localities in British Columbia. Most infections (86%) were light (1–9 ciliates per wet mount of digestive gland tissue). However, there was no evidence of pathogenesis in 22 oysters with heavy infections (more than 25 ciliates per wet mount).