Physa acuta, Draparnaud, 1805

We identified physid specimens from the Bullileo reservoir as Physa acuta View in CoL ( Fig. 2 View Figure 2 ).

They were characterized as follow: shell elongate-ovate (2.0 to 10.1 mm), light brown-translucent, thin and without ornamentations ( Fig. 2a View Figure 2 ). Mantle black with irregular large grayish-white spots ( Fig. 2b View Figure 2 ) and two digitated lobes, one on each side of the body. Foothead gray, with irregular small black spots. Penial complex ( Fig. 2c View Figure 2 ) composed by the prepuce, penis sheath, and penis. Prepuce with a conspicuous lentil shaped gland on the proximal section of the preputial wall ( Fig. 2c View Figure 2 ). Penis sheath enclosing the penis. Penis elongated, pointed, with the widest, rounded distal end ( Fig. 2d View Figure 2 ); it is white in color.

Of the 23 individuals of P. acuta examined, nine of them contained individuals of Chaetogaster limnaei ( Fig. 3a View Figure 3 ), accounting for 39.1% prevalence. The worms isolated in a petri dish supplied with fresh water were quite active, stretching and shrinking rapidly, making it difficult to estimate size. However, two specimens measured 500 and 530 µm, respectively. The worms were characterized by having a discrete prostomium and clusters of setae (bristles) on their ventral side ( Fig. 3 View Figure 3 b-d). These clusters were unevenly distributed throughout the body. They were frequent in the mid-posterior part of the body, but not in the anterior end, lacking setae, except for a couple of clusters developed at the level of the pharynx. Internally, by stretching the body, the worms also stretch the stomach and intestine, which then shrink. A pharynx was observed after the mouth. In some cases, the worms could be seen through the shell moving in the mantle cavity. The range of infestation varied from one up to four worms per snail. The smallest snail with worms measured 2.0 mm, containing three worms.

MATERIALS AND METHODS

Twenty-three specimens of Physa acuta were collected in 2017 from the Bullileo reservoir (36°17’42.10” S, 71°24’35.09” W) ( Fig. 1 View Figure 1 ). Bullileo reservoir is an artificial lake created for agricultural irrigation located about 50 km east of Parral city in the Maule Region. The reservoir is relatively isolated from human influence on the western edge of the Andes Mountain range, although in the warmer months it is used for sport fishing and recreation. The environment is made up of hills covered by exuberant native vegetation. On the shore of the lake, it is possible to find stones and abundant macrophytes that harbor snails. Physa acuta was identified using shell characters, soft body characters and anatomy of the reproductive system (see Collado 2017). The shell and soft body of the snails were photographed using a stereomicroscope (Motic). Chaetogaster specimens were isolated by microdissection of the snails and photographed with a light microscope (Motic). Worm images were also obtained using a Hitachi SU3500 scanning electron microscope (SEM). The Chaetogaster worms were identified as C. limnaei based on Collado et al. (2019), taxonomic keys and anatomical descriptions from various sources (Brinkhurst 1971, 1986, Kathman and Brinkhurst 1998, Cichy et al. 2016, Gelder 1989, Gruffydd 1965a,b, Khalil 1961). Voucher specimens are deposited at the Laboratorio de Malacología y Sistemática Molecular, Universidad del Bío-Bío, Chillán, Chile (LMSM UBBCC- Chaetogaster limnaei 1,2,8; LMSM UBBCC- Physa acuta 1,7,9) GoogleMaps .