Observation and establishment of gonad development stages in polyplacophorans (Mollusca): Chiton (Chiton) articulatus a case study

Abstract

Gonad development stages (GDS) and, subsequently, the reproductive cycle are described by performing histology of some gonad portions. In polyplacophorans, gametogenesis is not enough to define GDS; further anatomical gonad features are relevant. In most adult polyplacophorans, the gonad is a simple anatomical structure that resembles and operates as one single gonadal acinus without glandular structure. These features have gone unnoticed causing inaccurate GDS assignment and, consequently, imprecise reproductive season in polyplacophorans. Here, dissection protocols that allow extracting a compact gonad are established. Emphasizing the anatomical structure of the whole gonad and the displacement of gametes, five GDS were assigned to both sexes of Chiton (Chiton) articulatus: I‐goniogenesis, II‐development, III‐ripe, IV‐spawning, V‐rest. Tissue platelets contribute importantly to GDS assignment and even help distinguishing between males and females. Neither a randomly selected portion of gonad nor a longitudinal section are recommended because it leads to misinterpretation higher than 50% in determining GDS and besides ignores displacement of gametes. A panoramic sweep across a complete transverse‐section of each gonad was validated as the best option for establishing GDS. This new methodology was tested on several polyplacophorans species, and seems generally applicable for histological assessment of reproductive cycle and reproductive season in polyplacophorans.

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

“…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 the species we studied, gonadal stages are apparently asynchronous, as suggested by the presence of different maturation stages and the high frequency of ripe individuals of both sexes throughout the year in each of the three species. A fifth development stage (resting) was described for other species of Chiton (e.g., C. articulatus ; Abadia‐Chanona et al., 2016), although it was not evident in any of the species examined in our study.…”

“…The fact that virtually all sampled individuals of C. magnificus were either ripe or spawning prevented us from observing the maturation process of this species, but we were able to observe the sequence of gonad maturation in C. cumingsii and C. granosus . Although some statistical procedures can be used to estimate the size at maturity of chitons for fishery purposes (e.g., Abadia‐Chanona et al., 2016), another appropriate descriptor is the onset of the first sexual maturation, which in this case is the size at which each sex first shows mature gametes (e.g., Avila‐Poveda & Abadia‐Chanona, 2013). 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).…”

“…The identification of stages of gonad development was performed following Abadia‐Chanona et al. (2016), in which transverse sections were examined and characterized as one of four sequential gonad development stages: goniogenesis, development, ripe, or spawning. Histological preparations were observed under microscope (Leica DM750), and five optical fields (10× magnification) were examined for each individual and photographed with a digital camera (Leica ICC50).…”

“…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.…”