Histological Description of Oogenesis in Chiton virgulatus (Mollusca: Polyplacophora)

Abstract

SUMMARY: This paper describes the oogenesis of Chiton virgulatus, based on histological observations under transmission and scanning electron microscopy. Three oocyte types were identified: i) previtellogenic oocytes with a mean diameter of 50±20.5 µm, surrounded by elongated follicular cells of approximately 5 µm, ii) immature vitellogenic oocytes with a mean diameter of 113±15.3 µm and small cytoplasmic projections denoting the onset of the oocyte hull development; adjacent to each projection are pores approximately 0.7 µm in diameter, and iii) mature vitellogenic oocytes with a mean diameter of 146±24.8 µm; the oocyte cytoplasmic projections grow and its apical zone becomes trident-shaped; follicular cells adopt a bulbous shape due to the growth of the elongation and can reach up to 20 µm in length. The morphology and ultrastructure of the projections of the mature vitellogenic oocyte, as well as the size of pores at their base, are specific to C. virgulatus; therefore, these features could be used in taxonomic or fertilization studies.

First comparative assessment of the reproductive cycle of three species of Chiton on a temperate rocky shore of the southeastern Pacific

“…Thus, based on the range of lengths of ripe individuals sampled in northern Chile (Table 1), the onset of sexual maturity would be ~27 mm for females and 25 mm for males in C. granosus , and 25 mm for females and 32 mm for males in C. cumingsii , although this species could mature even earlier, as suggested by the size of ripe individuals recorded in previous work from central Peru (≥21 mm, although sex was not reported; Sotil, 2004). Overall, these values are rather similar to those reported for other species of the genus, such as C. albolineatus (28 mm for females, 23.5 mm for males) and C. articulatus (32 mm for females, 17 mm for males; Avila‐Poveda & Abadia‐Chanona, 2013), or C. virgulatus (31.4 and 37.5 mm at two different localities, without indicating sex; Vélez‐Arellano et al., 2014).…”

“…However, we lack data from small individuals of C. magnificus , and we cannot yet confidently assert that there is not a positive allometry between egg size and body size in this species (in fact, the scatterplot in Figure 7 suggests that further data corresponding to the left‐truncated portion of the x ‐axis might strongly affect the slope of the relationship). On the other hand, the mean size of oocytes of the three species was within the range observed for other species of Chiton (e.g., Abadia‐Chanona et al., 2016; Schwabe, 2010; Vélez‐Arellano et al., 2014), or for the same species in other coasts (e.g., C. cumingsii in central Peru; Sotil, 2004).…”

“…In general, the reproductive cycles of C. cumingsii , C. granosus, and C. magnificus showed no strict or direct relationship with seasonality at the temperate area of Coquimbo, in northern Chile, despite this zone exhibiting significant seasonal changes in key environmental variables such as sea surface temperature (Figure 2) and primary productivity (Thiel et al., 2007). Moreover, the fact that we usually observed oocytes in different developmental stages suggests that the three species may reproduce throughout the year, unlike common patterns observed in other temperate zones, where the spawning stage occurs mainly during spring (Himmelman, 1975, 1979, 1980; Sakker, 1986), or in tropical zones, where it occurs primarily during summer (Barbosa et al., 2009; Deshpande & Nagabhushanam, 1983; Pearse, 1978; Vélez‐Arellano et al., 2014). Nonetheless, some tropical species such as Onithochiton lyelli (Pearse, 1978) and C. articulatus (Abadia‐Chanona et al., 2016), and temperate species such as Mopalia muscosa (Boolootian, 1964), may also be reproductive throughout the year.…”

“…Moreover, the fact that we usually observed oocytes in different developmental stages suggests that the three species may reproduce throughout the year, unlike common patterns observed in other temperate zones, where the spawning stage occurs mainly during spring (Himmelman, 1975(Himmelman, , 1979(Himmelman, , 1980Sakker, 1986), or in tropical zones, where it occurs primarily during summer (Barbosa et al, 2009;Deshpande & Nagabhushanam, 1983;Pearse, 1978;Vélez-Arellano et al, 2014). Nonetheless, some tropical species such as Onithochiton lyelli (Pearse, 1978) and C. articulatus (Abadia-Chanona et al, 2016),…”

“…For female individuals, we counted the relative number of primary, secondary, follicular, and mature oocytes, following literature criteria (Abadia‐Chanona et al., 2016; Avila‐Poveda & Abadia‐Chanona, 2013; Gaymer et al., 2004; Ituarte & Arellano, 2015; Sotil, 2004; Vélez‐Arellano et al., 2014). We also observed the space between oocytes and epithelia in each of the oocyte developmental stages.…”