Zoothamnium duplicatum infestation of cultured horseshoe crabs (Limulus polyphemus)

[Display omitted] •Mass mortality event in a pilot, commercial scale horseshoe crab hatchery.•A protozoan infestation of juvenile horseshoe crabs.•The protozoan was identified as the sessile peritrich, Zoothamnium duplicatum.•Zoothamnium duplicatum colonised extensively the carapace and gills.•Treatments with 10ppm chlorine removed the epibiont burden. An outbreak of the sessile peritrich Zoothamnium duplicatum in a pilot, commercial-scale Limulus polyphemus hatchery resulted in the loss of ∼96% (40,000) second/third instar larvae over a 61day period. peritrich growth was heavy, leading to mechanical obstruction of the gills and physical damage. The peritrichs were controlled without resultant loss of juvenile crabs by administering 10ppm chlorine in freshwater for 1h and the addition of aquarium grade sand; a medium into which the crabs could burrow and facilitate cleaning of the carapace. Peritrich identity was confirmed from a partial SSU rDNA contiguous sequence of 1343bp (99.7% similarity to Z. duplicatum).