Development of Raphidascaris acus (Nematoda, Anisakidae) in paratenic, intermediate, and definitive hosts

Abstract

Larval development and morphology of Raphidascaris acus were studied in experimentally infected invertebrates and fish. Previous studies disagreed on the roles of these hosts. Raphidascaris acus develops to the second stage in the egg. Eggs are then infective to both invertebrates and fish. Larvae did not develop in experimentally infected amphipods or chironomid larvae. In experimentally infected darters (Etheostoma spp.) and yellow perch (Perca flavescens), larvae migrated to the liver and developed to the fourth stage, not the third as reported by others. The larvae moulted at 1.3 and 2.2–2.5 mm in these fish and grew to 5–6 mm. Therefore, invertebrates act as paratenic hosts and make the larvae available to fish which act as intermediate hosts. Definitive hosts (northern pike, Esox lucius; rainbow trout, Salmo gairdneri; brook trout, Salvelinus fontinalis) were infected experimentally using fourth-stage larvae. The parasite attained the adult stage in the gut of these fish, moulting at about 6 mm and maturing in about 1 month at 12–14 °C. The stages of R. acus are readily distinguished by reproductive organs and cephalic structures. A boring tooth is present in the second and third stages, nonpedunculate lips are present in the fourth stage, and pedunculate lips are present in the adult. Fourth-stage larvae have a ventral interlabium.