Abstract
For the first time, this study has systemically investigated the male reproductive system in a sessile broadcast-spawning marine invertebrate, Galeolaria caespitosa (Polychaeta: Serpulidae), which has significant potential as a bio-indicator species of coastal marine pollution. The abdomen of G. caespitosa was divided by intersegmental septa into over 80 trunk segments. Each segment served as a germinal chamber with a C-shaped gonadal arrangement consisting of several distinct compartments: a seminiferous epithelium (SE) compartment located in the centre of the chamber, with each of its two ends connecting to a nurse cell (NC) compartment and then an efferent duct (ED) compartment. The SE compartment contained a multilayered seminiferous epithelium where spermatogenesis was initiated. Spermatids were released in pairs into the lumen of the SE compartment and then transported to the NC compartment where they underwent spermiogenesis with the support of secretory vesicles released by the nurse cells. Spermatozoa were stored in the ED compartment and subsequently released into the seawater through the vas deferens. Unlike vertebrates where germ cells differentiated in close proximity to the nurse cell population (i.e. Sertoli cells), the spermatogenic cells of G. caespitosa exhibited no direct contact with supporting cells at any spermatogenic stage. This finding suggested that the spermatogenesis in G. caespitosa was more dependent on intrinsic developmental programming than most species. Notwithstanding such differences, there were clear parallels between the male reproductive system of G. caespitosa and mammals, in terms of the structure and function. The independence of spermatogenic cells from supporting cells in G. caespitosa raised the possibility of inducing spermiogenesis in vitro, which would provide a useful tool to dissect the mechanisms underlying this complex cell differentiation process in invertebrates and other higher order animals.
Highlights
Galeolaria caespitosa is an Australian native marine polychaete that ubiquitously inhabits along the south-eastern coast, with a continuous geographical distribution over 4,500 km [1]
This study revealed that motile spermatozoa of G. caespitosa could actively accumulate at the lower surface of tissue culture plates, while the immotile cells were unable to adhere to this structure
Except for the first five to six segments next to the thorax, all other abdominal segments served as germinal chambers (GC) where spermatogenesis occurred
Summary
Galeolaria caespitosa is an Australian native marine polychaete that ubiquitously inhabits along the south-eastern coast, with a continuous geographical distribution over 4,500 km [1]. Numerous characteristics that G. caespitosa possesses endow this species with the potential to serve as an excellent experimental model for toxicity testing and a bio-indicator species of coastal marine pollution To be specific, this species can be collected from the intertidal region of the seashore, where marine organisms are subject to a comparatively intensive pressure derived from human activity. This invertebrate is still much smaller than most marine vertebrates and, as a consequence, only low amounts of laboratory space and toxicants are required to implement toxicity testing Such a small body size is associated with an increase in the vulnerability of the host species to pollutants [6]. G. caespitosa is a filter-feeding species, which is able to accumulate hazardous substances through the uptake of suspended particulate matter during the water exchange involved in foraging activities [3, 7]
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