The integumental pigments of some isopod crustacea

Building on 150 Years of Knowledge: The Freshwater Isopod Asellus aquaticus as an Integrative Eco-Evolutionary Model System

Interactions between organisms and their environments are central to how biological diversity arises and how natural populations and ecosystems respond to environmental change. These interactions involve processes by which phenotypes are affected by or respond to external conditions (e.g., via phenotypic plasticity or natural selection) as well as processes by which organisms reciprocally interact with the environment (e.g., via eco-evolutionary feedbacks). Organism-environment interactions can be highly dynamic and operate on different hierarchical levels, from genes and phenotypes to populations, communities, and ecosystems. Therefore, the study of organism-environment interactions requires integrative approaches and model systems that are suitable for studies across different hierarchical levels. Here, we introduce the freshwater isopod Asellus aquaticus, a keystone species and an emerging invertebrate model system, as a prime candidate to address fundamental questions in ecology and evolution, and the interfaces therein. We review relevant fields of research that have used A. aquaticus and draft a set of specific scientific questions that can be answered using this species. Specifically, we propose that studies on A. aquaticus can help understanding (i) the influence of host-microbiome interactions on organismal and ecosystem function, (ii) the relevance of biotic interactions in ecosystem processes, and (iii) how ecological conditions and evolutionary forces facilitate phenotypic diversification.

Exoskeletal cuticle of cavernicolous and epigean terrestrial isopods: A review and perspectives

Comparative ultrastructural studies of the integument in terrestrial isopod crustaceans show that specific environmental adaptations of different eco-morphotypes are reflected in cuticle structure. The biphasic molting in isopods is a valuable experimental model for studies of cuticular matrix secretion and degradation in the same animal. The aim of this review is to show structural and functional adaptations of the tergal cuticle in terrestrial isopods inhabiting cave habitats. Exoskeletal cuticle thickness, the number of cuticular layers, epicuticle structure, mineralization, pigmentation and complexity of sensory structures are compared, with greater focus on the well-studied cave trichoniscid Titanethes albus. A large number of thinner cuticular layers in cave isopods compared to fewer thicker cuticular layers in related epigean species of similar body-sizes is explained as a specific adaptation to the cavernicolous life style. The epicuticle structure and composition are compared in relation to their potential waterproofing capacity in different environments. Cuticle mineralization is described from the functional point of view as well as from the aspect of different calcium storage sites and calcium dynamics during the molt cycle. We also discuss the nature and reduction of pigmentation in the cave environment and outline perspectives for future research.

Ommochrome deficiency in an albino strain of a terrestrial isopod, Armadillidium vulgare

In order to clarify the cause of ommochrome deficiency in an albino strain of the terrestrial isopod, Armadillidium vulgare, levels of xanthommatin, 3-hydroxykynurenine, 3-hydroxyanthranilic acid and tryptophan in whole body extracts of the albino and the wild type individuals were determined together with enzyme activities of kynurenine-3-hydroxylase, kynureninase and tryptophan-2,3-dioxygenase. Xanthommatin could not be detected in the albinos. The levels of 3-hydroxykynurenine and 3-hydroxyanthranilic acid were determined by high-performance liquid chromatography (HPLC) with electrochemical detection and were markedly low in the albinos compared with the wild type individuals. In contrast to those, the tryptophan levels determined by HPLC with fluorescence detection did not differ significantly between the two phenotypes. In the albino A. vulgare, kynurenine-3-hydroxylase activity was lower and kynureninase activity was higher than in the wild type, although the differences were not statistically significant. Tryptophan-2,3-dioxygenase activity in the albinos was less than 10% that in the wild type. Thus, ommochrome deficiency in the albino A. vulgare is considered to be caused by the extremely low activity of tryptophan-2,3-dioxygenase.

Genetic and biochemical studies on ommochrome genesis in an albino strain of a terrestrial isopod, Armadillidium vulgare

Genetic studies and quantitative determination of levels of 3-hydroxykynurenine and kynurenine were performed in an albino strain of a terrestrial isopod Armadillidium vulgare. From the results of matings between the albino and the albino, the red, the dark red, or the wild type individuals, the albino A. vulgare seems to be regulated by an autosomal gene(s) recessive to its wild allele. Litter mating of F1 progenies obtained by crossing the albino and the red mutant or the albino and the dark red mutant yielded progenies at a ratio of 3:6:3:4 for the red, the dark red, the wild, and the albino phenotypes, respectively. The albino gene(s) seems not to be allelic but to be epistatic to the red gene(s) with respect to ommochrome biosynthesis. Quantitative determination of 3-hydroxykynurenine carried out by high-performance liquid chromatography with electrochemical detection revealed that the 3-hydroxykynurenine content in the albino was significantly lower than that in the wild or the red type. The whole content of 3-hydroxykynurenine after enzymatic conversion of kynurenine to 3-hydroxykynurenine was still considerably lower than that found in the wild type, even though it increased after the conversion. The albino gene(s) seems to be associated with a blockage at distinct level(s) of ommochrome biosynthesis.

Morphological characterization of three phenotypes of the isopod Armadillidium vulgare

The morphological characteristics and ommochrome quantity in the integument of red, white, and wild type (black-grey) Armadillidium vulgare were studied. The red phenotype was found to possess two kinds of immature ommochrome pigment granules within its pigment cells, in addition to mature pigment granules. The immature granules seemed to contain uniformly distributed fibrilles, or to have an electron-dense central region surrounded by an electron-lucent outer edge. Since these immature pigment granules were typically observed to be distributed along with the mature ones, and were also more easily extractable than the wild type's, it is hypothesized that ommochrome granule maturation in the red phenotype may occur slowly due to a defect in the pigment granule internal process which combines pigments with matrix proteins. Regarding the white phenotype, although its pigment cells were undeveloped, several large-sized vesicles containing a small amount of electron-dense material appeared in the pigment cell cytoplasm. The wild and red type males of A. vulgare were found to have an ommochrome content twice as large as that of the corresponding females, with no ommochrome pigment being detected in the white phenotype. The genetic relationship between the white and red phenotypes was discussed using as a basis the observed pigment granule structure.

Pteridines in the yellow-colored chromatophores of the isopod, Armadillidium vulgare

Biochemical analyses of the dorsal integument of the isopod, Armadillidium vulgare, revealed that sepiapterin, biopterin, pterin, isoxanthopterin and uric acid accumulated in the yellow-colored chromatophores which are distinguishable from ommochrome chromatophores. The pattern of the yellow-colored chromatophores in the female is externally observable at the dorsal surface of the integument as yellow markings. In contrast, the yellow-colored chromatophores are not externally observable in the male, since they are covered by an ommochrome chromatophore layer. The content of both sepiapterin and biopterin in the male chromatophores was about two times greater than that in the female. The yellow-colored chromatophores were observable by light microscopy as pigmented granules. Electron microscopy showed that morphological properties of the granules were similar to those of pteridine granules which contain uric acid occurring in the silkworm integument. These facts indicate that both pteridines and uric acid in the integument of A. vulgare are localized in the pigmented granules of the yellow-colored chromatophores.

Morphological and biochemical characterization of the cream markings in the integument of the female isopod, Armadillidium vulgare

Cream markings aligned along the dorsal region of the female isopod, A. vulgare, were investigated with light and a fluorescence microscope and an electron microscope. Biochemical studies were also carried out. The cream markings were observed in the dorsal integument as a group of cream-colored chromatophores that emit a yellow fluorescence. These chromatophores, which are distinguishable from ommochrome chromatophores, contained numerous granules in the cytoplasm, and these granules (0.6-3.0 micron in length by 0.4-1.5 microns in width) were electron-lucent and spheroidal in shape with a concentric arrangement of membranes. Based on various biochemical analyses, the principal component of the yellow pigment isolated from the cream markings was identified as sepiapterin. These facts revealed that the cream markings are the chromatophores that contain pteridine granules. The males have no cream markings like those of the females, since the cream-colored chromatophores are externally hidden by the ommochrome chromatophore layer. The content of sepiapterin in the males was about two times greater than that in the females. This quantitative difference in sepiapterin content between males and females suggests that the pteridine formation in this pigment cell may be regulated by hormones associated with sex determination.

The dark brown integumentary pigment of a barnacle (Balanus eburneus). A comparative study

A histochemical analysis involving tinctorial and solubility tests was pursued in conjunction with electron microscopy for the purpose of identifying the dark brown epidermal pigment of a barnacle (Balanus eburneus) as melanin and/or ommochrome. Histochemically, comparisons were made with other brown pigments located in the subcarapal epidermis of another crustacean, the fiddler crab (Uca pugilator), the dorsal skin of the red-backed salamander (Plethodon cinereus), the liver and testis of a slider turtle (Chrysemys sp.) and the substantia nigra of human brain. The solubility properties of the pigment of the two crustacean invertebrates were in general similar to one another and markedly different from the pigment of the three vertebrates. Insolubility in appropriate solvents classified the vertebrate pigment as melanin. The invertebrate pigment, however, which was soluble in the ommochrome solvents, concentrated sulfuric and formic acids and 2-chloroethanol, remained insoluble in the ommochrome solvents, dilute aqueous and methanolic hydrochloric acid and dilute sulfuric acid. On the basis of these solubilities, an unequivocal classification of the invertebrate pigment as either melanin or ommochrome did not appear possible. The tinctorial and electron microscopic properties of the barnacle pigment were also equally ambiguous in regard to its specific classification.