Porifera, commonly named sponges, are devoid of head, digestive tract, nervous system, muscles or any other organs. Despite this anatomical and morphological simplicity, they are true animals or metazoans. Sponges are a successful group of mostly marine filter feeders that represent a major life form of several aquatic ecosystems. Efficient water filtration is performed by a network of pores (at the origin of their name), chambers and canals. Sponges are currently considered as one of the best candidates as sister group to all other animals; these organisms have therefore a key phylogenetic position. Recent studies have shown that they possess an unexpected molecular complexity raising exciting questions about early animal evolution. These results yielded an increased interest in the biology of these strange animals, and some sponge species can be considered as biological experimental models. Key Concepts As adults, sponges are sedentary filter feeder animals. The water flow is canalised in an aquiferous system composed of inhalant and exhalant pores (ostia and oscula), canals and choanocyte chambers. At present, the number of formally described sponge species is more than 9000. Sponges are divided into four clades (rank Class): Hexactinellida, Demospongiae, Calcarea and Homoscleromorpha. As sponges are devoid of organs, their individual cells or cell layers ensure vital functions. One cell type may have several functions. Sponges have high regeneration capabilities as well as a high capacity of cell transdifferentiation. Sponges often use both asexual and sexual reproduction. Most of the key transcription families and main signalling pathways required for other metazoans development and body patterning are present in sponges. The increasing interest for sponge biology improves our understanding of animal evolution and marine ecosystems.

Vanadium (V) is the second most abundant transition metal in the oceans. Nevertheless, its concentration in organisms is generally low despite its involvement in various physiological or enzymatic functions. Because exposure to high concentrations was shown to be harmful for various organisms including animals, the unexpected very high concentrations found in urochordates and annelids yielded additional studies to decipher their biological meaning. Here we report cases of V hyperaccumulators in a distinct animal phylum (Porifera): the two homoscleromorph sponge species Oscarella lobularis and Oscarella tuberculata (up to 30 mg/g dw). These high concentrations overpass those reported previously in urochordates and annelids and are not found in the 5 other sponge species studied here. In both Oscarella species, vanadium is mainly accumulated in the 100 micrometer surface tissues, and in particular in mesohylar cells, as vanadyl (+4) before being partly reduced in V (+3) in the deeper tissues. Genomic surveys failed to find any previously described gene implicated in vanadium metabolism, suggesting that V hyperaccumulation emerged convergently in this lineage. This feature may be of interest for developing bioremediation strategies in marine ecosystems or bioinspired processes to recycle this critical metal.

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