A light and electron microscopic study of tissue interactions between a parasitic copepod, Scolecodes huntsmani (Henderson), and its host ascidian, Styela gibbsii (Stimpson)
Authors: Dudley Patricia L. 
Published: February 2005

The structure of the cellular cyst which encapsulates the parasitic copepod, Scolecodes huntsmani, in the subendostylar blood vessel of the ascidian, Styela gibbsii, is described from light and electron microscopic studies.

The cells comprising the cyst are contributed by the ascidian. The cells are columnar, contain large central reservoirs of glycogen and lipid, and have a conspicuous Golgi apparatus, many small cisternae of smooth endoplasmic reticulum and peripheral mitochondria. The cells are held together by complex basal interdigitations and a short apical zonula occludens. Long cilia emerge in circular clusters from the cell apices and beat in the lumen of the cyst. As atypical of a columnar epithelial layer, the nuclei are staggered in position in the cells and there is no basal lamina. One end of the cyst is blind, but the other end, which may be either anterior or posterior with respect to the longitudinal axis of the host, narrows to a profusely ciliated duct which opens through the wall of the blood vessel to the atrium of the ascidian by a ciliated funnel. The effective beat of the cilia of the duct and the funnel is outward toward the atrium.

The first nauplii of the copepod emerge from the incubatory pouch of the adult and pass to the exterior sea water through the cyst funnel and the atrium and atrial siphon of the ascidian. As in other notodelphyid copepods, the life cycle of this incarcerated form also involves free‐living naupliar stages followed by two free‐living copepodid stages. The provision of an egress for the first nauplii is, therefore, important to the survival of the species.

The adult females of Scolecodes, which range in length from 2 to 14.6 mm, are sluggish when removed from the cyst and fail to survive in sea water for more than 24 hours. The males, which have only been obtained when parasitic fifth copepodids molt in culture, are much smaller, averaging 0.8 mm, and are very active. Since one dead male has been found inside the cyst of an adult female and females are often found with attached spermatophores, it is suggested that the funnel of the cyst may also serve as an entrance for the males.

Evidence is presented for the formation of the cyst as an accumulation of totipotent lymphocytes around the copepod. Cysts of parasitic developmental stages (third through fifth copepodids) are also described. All of these cysts and those of immature adult females lack funnels to the atrium. The funnel of the cyst of mature females is formed, in part, by modified cells of the wall of the blood vessel, but is induced after the major portion of the cellular cyst has been formed.

Cells in the general circulation of the ascidian and those inside the lumen of the cyst are compared. The cells in the lumen of the mature cyst do not arise by diapedesis of blood cells from the subendostylar blood vessel, but by conversion and migration of cells composing the cyst proper. These cells have been found in the guts of the copepods and they may serve as a nutritive source.

The ascidian appears not to be harmed by the association, but the copepod gains in many ways.