Briarosaccus tenellus, Boschma, 1970

Briarosaccus tenellus from Sitka (Alaska)

The carapace width of the infested Hapalogaster mertensii ranges from 11.1 to 17.2 mm. All crabs are singly infested. The externa of Briarosaccus tenellus varies from 8.5 to 15.5 mm in length and from 2.5 to 4.5 mm in height. It is attached to the basal part of the soft ventral side of the host abdomen via a short stalk ( Fig. 10A View Fig ). Its long axis is at right angles to that of the host. The mature externa is elongated, cylindrical and slightly curved with the ventral outline being convex, and the dorsal outline concave. Anterior part of the externa is slightly bilobed and somewhat thicker than posterior one ( Fig. 10B View Fig ). It is oriented to the left side of the host. The opposite, posterior end of the externa is rounded. The stalk occurs near the central part of the dorsal side. The chitinous shield around the stalk is fusiform, has growth rings, and covers from 1/4 to 1/3 of the externa. The mantle opening is placed in the anterior part on the right side of the externa facing the host. It is not elevated, slit-like and surrounded by lips ( Fig. 10B, G View Fig ).

The external cuticle is smooth, without papillae or excrescences ( Fig. 10E View Fig ). It is covered with longitudinal grooves. A dorsal strip of transversal grooves from the mantle opening to the stalk is visible ( Fig. 10D View Fig ). In SEM, this strip and area around the mantle opening is cellular ( Fig. 10F View Fig ). The mantle opening is densely covered with numerous spines of 8–10 μm in length ( Fig. 10H View Fig ).

The internal cuticle is wrinkled and covered with sparse hairs of 4–8 μm in length which may be united in groups with a common base ( Fig. 10I View Fig ). Retinacula of 10–20 μm in length, densely barbed, single or united into groups of 2–4 are rarely found ( Fig. 10J View Fig ).