Heterosaccus papillosus, Boschma, 1933

Morphological characteristics of Heterosaccus papillosus

Distributed location: Nha Trang Bay, Khanh Hoa Province.

Species examinations: 10 externae ( Tab. 1 View TABLE 1 ).

Host: Charybdis anisodon ( Fig. 1A, B View FIGURE 1 ).

Rhizocephala on Charybdis anisodon displayed typical features of Heterosaccus papillosus : triangular-shaped externa, external cuticle with small, short papillae excrescences, wide mantle opening, separate receptacles, and numerous colleteric gland canals.

Re-description

The externa exhibits a triangular shape, with the widest portion located posteriorly, near the stalk ( Fig. 1C, D View FIGURE 1 ). The mantle opening is relatively large, measuring approximately one-fifth to one-fourth of the externa’s width, and is slightly elevated above the surrounding mantle ( Fig. 1C1 View FIGURE 1 ). A short stalk is present, with the opening positioned along the middle longitudinal axis of the externa ( Fig. 1C View FIGURE 1 2 View FIGURE 2 ). The external cuticle of the mantle is covered with bar or non-barbed papillae that are somewhat evenly spaced, measuring 5–8 µm in height and 3–4 µm in width ( Fig. 2A View FIGURE 2 ). The internal surface of the mantle bears numerous retinacula ( Fig. 2B View FIGURE 2 ), which may or may not be enclosed within a thin, transparent cyst ( Fig. 2C2 View FIGURE 2 ). Each retinaculum consists of a base (3–4 µm high) and 4–12 non-barbed spindles, each 10–12 µm in height ( Fig. 2C, D View FIGURE 2 ).

The visceral mass is attached to the stalk, and a mesentery is absent ( Fig. 3A View FIGURE 3 4 View FIGURE 4 ). The paired receptacles of H. papillosus reside within the visceral mass, crescent-shaped, separate, and exhibit identical morphology in terms of shape ( Fig. 3D View FIGURE 3 ). The curvature is positioned posteriorly, near the junction with the receptacle duct. Notably, the degree of curvature varies among individuals, with the angle ranging from acute ( Fig. 3B View FIGURE 3 ) to obtuse ( Fig. 3C View FIGURE 3 ). The dorsal portion of the receptacle duct, adjacent to the receptacle, may be either straight, or form a helical configuration comprising up to three left-to-right spirals. The butterfly-shaped colleteric glands are situated centrally on the left and right surfaces of the visceral mass, branching into non-linear canals, with a maximum observed number of 76 canals ( Fig. 3E View FIGURE 3 ). The inner surface of the colleteric canals is covered by a layer of villi ( Fig. 3F View FIGURE 3 7).

Remarks: The present study generally corroborates Boschma’s (1954) morphological description of H. papillosus parasitizing C. anisodon from the same geographic region. However, minor morphological variations were observed, particularly in the structure of the retinacula and receptacle ducts. The retinacula in the specimens examined herein consisted of 4–12 spindles measuring 10–12 µm in height, whereas Boschma (1954) reported 7–12 spindles approximately 7.5 µm in height.Additionally, Boschma (1954) described a twisted course exclusively in the left receptacle duct, while the present specimens exhibited similar morphology in both the left and right receptacle ducts, displaying either straight or spiral configurations. The reliance on a single specimen in Boschma’s study may have limited the observed intraspecific variation of H. papillosus . Besides, the presence of minute hairs covering the excrescences, as reported in Boschma (1938), and the broader range of spindle numbers on the retinacula (4–17) described by Shiino (1943) are subtle morphological differences from the specimens analyzed in the current study. More recently, Jung and Park (2024) documented H. papillosus on C. japonica in Korea, noting a smooth to slightly wrinkled external cuticle, representing another deviation from previous and present descriptions.

Among the 15 described Heterosaccus species, H. papillosus most closely resembles H. dollfusi (Boschma) , H. ruginosus , and H. multilacinensis (Phillips) . However, H. papillosus is distinguished from other Heterosaccus species by variations in receptacles, excrescences, and retinacula. Specifically, H. papillosus differs from H. dollfusi in having two separate receptacles, whereas the latter possesses two receptacles fused into a large sac ( Boschma 1960) or communicates by a lateral tube ( Eldeen et al. 2019). H. papillosus also differs from H. ruginosus by its covered external cuticle with excrescences and is distinguished from H. multilacinensis by the absence of numerous retinacula that share a common base.