From head to rootlet: comparative transcriptomic analysis of a rhizocephalan barnacle Peltogaster reticulata (Crustacea: Rhizocephala)

Crustacean endocrinology: Sexual differentiation and potential application for aquaculture

Crustaceans, which represent a significant subset of arthropods, are classified into three major classes: Ostracoda, Malacostraca, and Branchiopoda. Among them, sex manipulation in decapod species from the Malacostraca class has been extensively researched for aquaculture purposes and to study reproductive physiology and sexual plasticity. Some decapods exhibit sexual dimorphism that influences their biological and economic value. Monosex culture, in which only one sex is cultivated, increases production yields while reducing the risk of invasiveness, as genetic leakage into natural waters is less likely to occur. Differences in yield are also observed when cultivating different sexes, with all-male cultures of Macrobrachium rosenbergii being more profitable than both mixed and all-female cultures. Research on decapod sexual differentiation has led to a better understanding of sex determination and sexual differentiation processes in arthropods. Similar to most mammals and other vertebrate classes, Malacostraca crustaceans, including decapods, exhibit a cell-non-autonomous mode of sexual development. Genetic factors (e.g., sex chromosomes) and endocrine factors (e.g., insulin-like androgenic gland factor and crustacean female sex hormone) play pivotal roles in the development of sexually dimorphic traits. This review synthesizes the existing understanding of sex determination mechanisms and the role of sex hormones in decapod species. Additionally, it provides an overview of the methyl farnesoate, which has been suggested to be involved in male sex differentiation in some crab species, as well as the phenomenon of male-to-female sex reversal in host decapods caused by parasitic crustaceans.

Tricks of the puppet masters: morphological adaptations to the interaction with nervous system underlying host manipulation by rhizocephalan barnacle Polyascus polygeneus

Background

Rhizocephalan interaction with their decapod hosts is a superb example of host manipulation. These parasites are able to alter the host's physiology and behavior. Host-parasite interaction is performed, presumably, via special modified rootlets invading the ventral ganglions.

Methods

In this study, we focus on the morphology and ultrastructure of these special rootlets in Polyascus polygeneus (Lützen & Takahashi, 1997), family Polyascidae, invading the neuropil of the host's nervous tissue. The ventral ganglionic mass of the infected crabs were fixed, and the observed sites of the host-parasite interplay were studied using transmission electron microscopy, immunolabeling and confocal microscopy.

Results

The goblet-shaped organs present in the basal families of parasitic barnacles were presumably lost in a common ancestor of Polyascidae and crown "Akentrogonida", but the observed invasive rootlets appear to perform similar functions, including the synthesis of various substances which are transferred to the host's nervous tissue. Invasive rootlets significantly differ from trophic ones in cell layer composition and cuticle thickness. Numerous multilamellar bodies are present in the rootlets indicating the intrinsic cell rearrangement. The invasive rootlets of P. polygeneus are enlaced by the thin projections of glial cells. Thus, glial cells can be both the first hosts' respondents to the nervous tissue damage and the mediator of the rhizocephalan interaction with the nervous cells. One of the potential molecules engaged in the relationships of P. polygeneus and its host is serotonin, a neurotransmitter which is found exclusively in the invasive rootlets but not in trophic ones. Serotonin participates in different biological pathways in metazoans including the regulation of aggression in crustaceans, which is reduced in infected crabs. We conclude that rootlets associated with the host's nervous tissue are crucial for the regulation of host-parasite interplay and for evolution of the Rhizocephala.

Sacculina-Induced Morphological Feminization in the Grapsid Crab Pachygrapsus crassipes

Rhizocephalan barnacles (Thecostraca: Cirripedia) are parasitic crustaceans that lack appendages, segmentation, and a digestive system in adults, while instead infiltrating their hosts with a nutrient-absorbing system of rootlets. Sacculinids, belonging to the Rhizocephala order, are known for their various parasitization-induced effects on their decapod hosts, such as parasitic castration, reduction in the growth of secondary sexual characteristics, feminization of male crabs, and alteration of host behavior. In this study, we conducted field surveys in Japan at Manazuru Town (Kanagawa) on the Pacific coast, and on Sado Island and Noto Peninsula on the Sea of Japan side, and found that sacculinid-parasite-ratios on the grapsid crab Pachygrapsus crassipes were particularly high on the Sea of Japan coast. Molecular phylogenetic analysis revealed that the Manazuru population forms a single clade with Sacculina yatsui, and both Sado and Noto populations form a single clade with S. confragosa. We further demonstrated that external morphologies of male P. crassipes parasitized by sacculinids were changed to female phenotypes. This host-parasite interaction will be a useful model for understanding molecular mechanisms underlying rhizocephalan-driven morphological and behavioral feminization and castration.

Functional role of lacunar and muscular systems in the externa of Peltogasterella gracilis (Cirripedia: Rhizocephala)

One of the most conspicuous traits of parasitic organisms is a well-developed reproductive system. In Rhizocephala ("Crustacea": Cirripedia) it is believed to be nested in the externa-a "reproductive part" located outside of the host. However, it is not clear how nutrients are transported to the externa. Several authors described a system of lacunae in the externa, and muscular contractions probably enable transport through these cavities. The aim of our study was to visualize (using microcomputed tomography and confocal laser scanning microscopy) and describe lacunar and muscular systems in the externa of Peltogasterella gracilis (fam. Peltogasterellidae). The lacunar system consists of "ventral" lacuna and several protrusions. The "ventral" lacuna is probably responsible for visceral mass nutrition, and mantle protrusions are associated with the mantle nutrition. The gross organization of the muscular system mostly corresponds to previous descriptions in other rhizocephalan species. Nonetheless, we observed several features of the externa morphology that had not been described before such as a muscular thickening in the proximal externa's part and a stalk plug disk. The muscular thickening might play a role of a propulsatory organ, helping to transport liquid through the lacunar system. The plug disk might fill the hole in the host's cuticle after the old externa drop off. The results allow us to make first assumptions on transport mechanisms in Rhizocephala.

Cancer spares no one: first record of neoplasm in parasitic barnacles (Arthropoda: Rhizocephala)

Cancer-like neoplasms are extremely rarely present in arthropods, particularly in crustaceans. Thus, it is assumed that these animals have some efficient cancer-preventing mechanisms. However, several cases of cancer-like neoplasms are described in crustaceans, though only for the Decapoda. We identified a tumor in the parasitic barnacle Peltogaster paguri (Cirripedia: Rhizocephala), and described its histological structure. A spherical cell mass, consisting mostly of roundish cells with big translucent nuclei, prominent nucleoli, and sparse chromatin, and of cells with condensed chromosomes, was found in the main trunk of the P. paguri rootlet system. Numerous mitoses were observed in this area. Such tissue organization is utterly uncharacteristic of the Rhizocephala. Based on acquired histological data, we assume that this tumor is a cancer-like neoplasm. This is the first report of a tumor identified in the rhizocephalans, as well as in non-decapod crustaceans as a whole.