Date Published: April 19, 2019
Publisher: Public Library of Science
Author(s): Daming Chen, Fang Liu, Zhihuang Zhu, Qi Lin, Chaoshu Zeng, Haihui Ye, Gao-Feng Qiu.
The peppermint shrimp Lysmata vittata (Caridea: Hippolytidae) is a marine caridean shrimp popular in marine aquarium trade. The species is known to display the sexual system of protandric simultaneous hermaphrodite. In this study, based on captive bred specimens, the complete ontogenetic gonad development of L. vittata was studied both morphologically and histologically, from newly settled juveniles until they reached euhermaphrodite phase. It was found that in all specimens examined (carapace length: 1.8–8.5 mm), including the newly settled juveniles, possessed ovotestes, which comprised of an anterior ovarian and a posterior testicular part. Based on both morphological (e.g., size, color and shape) and histological features (e.g., oogenesis and spermatogenesis), four gonadal development stages were defined and described for L. vittata. From Stage I to III, the testicular part of the gonad became gradually mature but the ovarian part was still immature, which is defined as the male phase. At the male phase, cincinulli (5–8 hooks) presented at the tips of the appendix interna on the first pair of pleopods while appendices masculinae (AM), in a form of a stick structure with spines, presented at the inner edge of the appendix interna (AI) on the second pair of pleopods. At Stage IV, both the testicular part and the ovarian part were mature and hence is defined as euhermaphrodite phase. At the euhermaphrodite phase, most individuals lacked cincinulli and appendices masculinae on the first and second pair of pleopods respectively. This is the first time that complete ontogenetic gonadal and external sexual character development have been described and staged for a species from the genus Lysmata from newly settled juveniles to euhermaphrodite phase.
The vast majority of crustacean species are gonochoristic, however, the caridean shrimp species belonging to the family Hippolytide display more diverse sexual systems of both gonochorism and hermaphroditism. For instance, in the genus Thor, T. manningi is hermaphroditic while T. dobkini and T. floridanus are gonochoristic [1, 2]. In the genus Hippolyte, both H. obliquimanus and H. williamsi are reportedly gonochoristic [3, 4]. Moreover, all known species from the genus Lysmata have so far been confirmed as protandric simultaneous hermaphrodite (PSH) , i.e., the shrimp first mature as male (male phase), and then gain female function at a later stage as the shrimp grow, when it remains functional male with gonad produces both eggs and sperms simultaneously (euhermaphrodite phase).
L. vittata used for this study were captive bred at Fisheries Research Institute of Fujian Province, China located in Xiamen city. The shrimps were transported to the laboratory of Xiamen University, Fujian, China and acclimated in salt water aquaria (salinity: 34±1 ppt; water temperature: 28±1°C) when they were fed a commercial formulated diet daily. The shrimps were sampled continuously from early juveniles to the sexual maturity as euhermaphrodite individuals (carapace length ranged from 1.8 to 8.5 mm) for the study of ontogenetic gonadal and external sexual characters development.
Despite the unique sexual system of PSH is widespread in species from the genus Lysmata , full ontogenetic development of the gonad from newly settled juveniles to euhermaphrodite individuals has not be described previously. Such knowledge is important as baseline information for further study of this unique sexual system in crustaceans and its underlying molecular regulating mechanisms. To fill this knowledge gap, in the present study, based on captive bred specimens, ontogenetic gonadal development of L. vittata were studied both morphologically and histologically from newly settled juveniles.
Based on both morphological (e.g., size, color and shape) and histological features (e.g., oogenesis and spermatogenesis), four gonadal development stages were defined and described for L. vittata. From Stage I to III, the testicular part of the gonad became gradually mature but the ovarian part was still immature, which is defined as the male phase. At Stage IV, both the testicular part and the ovarian part were mature and hence is defined as euhermaphrodite phase.