Emergence, Development, and Maturity of the Gonad of Two Species of Chitons “Sea Cockroach” (Mollusca: Polyplacophora) through the Early Life Stages

Abstract

This study describes and recognises, using histological and microscopical examinations on a morphometrical basis, several gonad traits through the early life stages of Chiton articulatus and C. albolineatus. Gonadal ontogenesis, gonad development stages, sexual differentiation, onset of the first sexual maturity, and growth sequences or “early life stages” were determined. In addition, allometry between lengths and body weight pooled for both sexes per each chiton were calculated using equation Y = aXb. A total of 125 chitons (4≤TL≤40 mm, in total length “TL”) were used. All allometric relations showed a strong positive correlation (r), close to 1, with b-values above three, indicating an isometric growth. Gonadal ontogenesis and gonad development stages were categorised into three periods (“Pw” without gonad, “Pe” gonad emergence, and “Pf” gonadal sac formed) and four stages (“S0” gametocytogenesis, “S1” gametogenesis, “S2” mature, and “S3” spawning), respectively. Compound digital images were attained for each process. Periods and stages are overlapped among them and between species, with the following overall confidence intervals in TL: Pw 6.13–14.32 mm, Pe 10.32–16.93 mm, Pf 12.99–25.01 mm, S0 16.08–24.34 mm (females) and 19.51–26.60 mm (males), S1 27.15–35.63 mm (females) and 23.45–32.27 mm (males), S2 24.48–40.24 mm (females) and 25.45–32.87 mm (males). Sexual differentiation (in S0) of both chitons occurs first as a female then as a male; although, males reach the onset of the first sexual maturity earlier than females, thus for C. articulatus males at 17 mm and females at 32 mm, and for C. albolineatus males at 23.5 mm and females at 28 mm, all in TL. Four early life stages (i.e., subjuvenile, juvenile, subadult, and adult) are described and proposed to distinguish growth sequences. Our results may be useful to diverse disciplines, from developmental biology to fisheries management.

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

“…After dissection and gonad extraction, each gonad was cut into several transverse‐sections that were dehydrated in ethanol series, cleared three times in Citrisolv ® , and infiltrated three times and embedded in paraplast ® , following the procedure of Benítez‐Villalobos et al . () and Avila‐Poveda and Abadia‐Chanona (). Serial transverse‐sections were cut at 5‐mm thickness using a manual rotary microtome (LEICA ® RM2145) and mounted on glass slides.…”

“…A total of 384 adult (according to Avila‐Poveda and Abadia‐Chanona larger than 40 mm in total length) specimens of Chiton (Chiton) articulatus were collected monthly from January to December 2011 in the rocky intertidal shore of Puerto Angel (15°39′N – 96°29′O), Oaxaca, Mexico. The whole specimens were relaxed, fixed, and preserved following the protocols established by Avila‐Poveda (), and so ready for the dissection and extraction of the compact gonad.…”

“…Serial transverse‐sections were cut at 5‐mm thickness using a manual rotary microtome (LEICA ® RM2145) and mounted on glass slides. The modified Crossmon's trichrome method (i.e., Groat's hematoxylin, erythrosine B‐Orange G, and trypan blue and/or light green, derived from Crossmon ; Gray ; Gutiérrez ) was used to contrast connective tissue, as well as to reveal vitelline reserves (i.e., yolk granules) and vacuolar processes (Avila‐Poveda and Abadia‐Chanona ). Staining was performed in a targeted manner towards females (i.e., ending with trypan blue) and towards males (i.e., ending with light green), because in polyplacophoran females the early phases of the gametes (i.e., oogonia, primary and secondary oocytes) have acid cytochemical components with a strong affinity for basic dyes, i.e., intensely basophilic (Cowden , ; Aquinas‐Nimitz and Giese ; Davenport and Davenport ; Deshpande and Nagabhushanam ).…”

“…It is noteworthy that for the members of Nuttallochiton ( N. hyadesi and N. mirandus ) it has been indicated that the original paired gonads remain completely separated from each other in both sexes, although so tight together almost like one sac (Lang ). From a morphogenetic viewpoint, the gonad organogenesis in mollusks starts as two lobes during the subjuvenile‐juvenile stages (Coe ; Hyman ; Pearse ; Avila‐Poveda and Abadia‐Chanona ), as it develops its shape, the genital arteries (i.e., blood vessels) enter, hanging freely into the lumen of the gonadal lobes, but originally they were located between the two halves of the pair of gonadal lobes forming a septum, which then anastomose into a network to freely protrude into just one lumen of one single gonad in the subadult‐adult stages (Plate , ,b; Lang ; Hyman ). Therefore, it is assumed that this organogenic process does not occur in Nuttallochiton and more studies of comparative anatomy along the life stages are required in these chitons.…”

“…— F . Chiton (Chiton) albolineatus (taken and digitally redrawn from Avila‐Poveda and Abadia‐Chanona ). Drawings and pictures are not shown to scale.…”