Morphological, biochemical and histological analysis of mud crab ovary and hepatopancreas at different stages of development

Abstract

This study describes the fatty acids, total carotenoids, and cell diameter characteristics of the female ovary and hepatopancreas of the mud crab, Scylla olivacea, with comparisons at different ovarian maturation stages. Seventy-one S. olivacea individuals at all stages of ovarian maturation were sampled from the Setiu wetlands, Terengganu, Malaysia. The ovary and hepatopancreas of each crab were used for morphological studies, histological and biochemical analyses (fatty acid composition and total carotenoids). Morphological observations indicated there was an increase in ovarian gonado-somatic index (GSI), with color changes from translucent to dark red; however, a relatively consistent hepato-somatic index (HSI) in the hepatopancreas, with the color ranging from yellow to yellowish-brown. Histological analysis indicated that oocyte diameter was positively correlated with GSI. Hepatopancreatic tubules had a relatively constant diameter from Stage 2 to 4, with increased proportions of R- and B-cells. Biochemical analysis indicated there was a significant increase in total carotenoids in the ovary during maturation. The hepatopancreas, however, had relatively consistent total carotenoid concentrations that were greater than those of the ovary. Overall, the lipid analysis results indicated there were lesser concentrations of fatty acids in the hepatopancreas, while in the ovary there were increasing concentrations during maturation. The lesser concentrations of fatty acids in the hepatopancreas than ovary suggested that energy was transferred to the ovary for future embryonic and larval development. The relationship between the hepatopancreas and the ovary in nutrient content is an important finding in providing a baseline to formulate an optimal diet for improved mud crab hatchery practices.

Introduction

Fundamental research on reproduction is important to improve the understanding and prediction of the biology and manipulation of the reproductive process in broodstock, especially in captive aquatic animals. The mud crab (genus Scylla) is an economically important portunid crab that is becoming important globally because of its market value resulting from the desirable eating experience when used as a food source (i.e., sweet taste, delicacy qualities, and meat yield; Azra and Ikhwanuddin, 2016; Sreelakshmi et al., 2016; Zeng et al., 2016).

In crustaceans, the ovary and hepatopancreas are the major organs for lipid storage. The lipid concentrations in both the ovary and hepatopancreas vary as the mud crab matures (Ravichandran et al., 2009a, b). Changes in the lipid concentrations in the ovaries support the hypothesis that lipids are important for ovarian maturation in crustaceans (Ando et al., 1997). In addition to the capacity of the hepatopancreas to store energy for use during molting and starvation (Sarapio et al., 2017), this organ also produces vitellogenin (Wang et al., 2014; Liu et al., 2018). Lipids have a major effect on the metabolism and reproduction of decapod crustaceans (Das et al., 2015). Lipid sources, primarily from carotenoids and fatty acids, have been reported to be major nutritional factors regulating the success rates of reproduction and egg quality of crabs (Middleditch et al., 1980; Millamena, 1989; Alava et al., 2007). Findings from studies on fatty acids have been widely applied and necessary in the field of aquatic reproduction. These findings have focused on tracing the source of dietary intake in aquatic organisms and the understanding of the adaptive physiological reactions has been enhanced (Abdulkadir and Tsuchiya, 2008; Azra et al., 2018). There has also been an interpretation of the responses to nutrient consumption at different stages of the reproductive cycle (Galap et al., 1999) and changes in habitat (Freites et al., 2002). Carotenoids in aquatic animals are especially important for tissue pigmentation (de-Carvalho and Caramujo, 2017).

Although there is considerable information on the morphological progression of tissue development, it is difficult to obtain precise information regarding the overall processes that take place at the cellular level in the ovary and hepatopancreas through morphological observations alone. Much research is required into the function of various cells that are present within different tissues and organs. Furthermore, studies into the biochemical changes of carotenoids and fatty acids in the ovary and hepatopancreas during ovarian development of female mud crabs that are residing in their natural habitat may provide useful knowledge to improve the understanding of the reproductive process (Tantikitti et al., 2015).

In crustaceans, vitellogenesis, or yolk deposition, is a key factor for successful reproduction and embryonic development (Chen et al., 2014; Hidir et al., 2018). This process was initially thought to take place in the ovary; however, several studies have found that vitellogenesis can also occur in the extra-ovarian tissues including the hepatopancreas (Girish et al., 2014). There, however, is limited research assessing the relationship between ovarian maturation and the hepatopancreas in decapod crustaceans. No information is available concerning the relationship between ovarian maturation stages and the hepatopancreas in the mud crab, S. olivacea. The objective of this study, therefore, was to investigate the ovarian maturation stages of the female orange mud crab, S. olivacea, as well as its relationship with the hepatopancreas through morphological, histological, and biochemical analysis.