First Report of Halomonhystera sp. cf. disjuncta Associated With Branchial and Cutaneous Lesions During a Mortality Event in Laboratory‐Acclimated Olive Flounder ( Paralichthys olivaceus )

This clinical report describes a mortality event associated with a free-living marine nematode provisionally assigned to the Halomonhystera disjuncta species complex (Halomonhystera sp. cf. disjuncta) in laboratory-acclimated olive flounder (Paralichthys olivaceus). Fish were acclimated at 13°C in filtered seawater with 50% daily water exchange. Affected fish developed caudal fin erosion, gill pallor, excessive branchial mucus, abnormal posture and, in larger fish, cutaneous ulcers. Cumulative losses within 1 month reached approximately 50% in the 10 g groups and 30% in the 100 g group. Microscopy revealed numerous nematodes in branchial mucus, adjacent to gill lamellae and in fin erosions and skin ulcers. Morphologically similar nematodes were recovered from the seawater filtration line but were not molecularly confirmed. VHSV was not detected by RT-qPCR at stocking or during the mortality event, and outbreak-associated PCR assays yielded no target-specific amplification for Tenacibaculum maritimum, Cryptocaryon irritans, or Ichthyobodo spp. Partial 18S rRNA and ITS sequences showed 100% identity to GenBank sequences assigned to H. disjuncta. Because this complex contains cryptic lineages, the nematode was conservatively designated Halomonhystera sp. cf. disjuncta. Although causality was not established, the findings support a clinicopathological association with branchial and cutaneous lesions.