Abergasilus amplexus is a species of parasitic copepod endemic to euryhaline habitats in New Zealand.

This species is unique among ergasilids in that it only has three pairs of legs: the fourth and fifth pairs are reduced to single spines or are completely absent.

[1] The male is free-living, forming part of the plankton.

[2] The main host appears to be Anguilla australis: in Lake Ellesmere / Te Waihora, infestation rates on this eel approach 100%, sometimes with over 100 parasites on the gills of a single fish.

Other recorded host species include Anguilla dieffenbachii, Arripis trutta, Carassius auratus, Galaxias maculatus, Perca fluviatilis, Pseudophycis bachus, Retropinna retropinna, Rhombosolea leporina, Rhombosolea plebeia and Rhombosolea retiaria.