Stylactis inermis is a species of the Hydractiniidae family, belonging to the Anthoatheca order within the Hydrozoa class. Until the middle of the second decade of the XXI century this species was known under the name of Hydractinia inermis, however, several studies later placed it within the genus Stylactis. It is a species endemic to the Mediterranean Sea, and that we will find up to 40 meters deep, always associated with various species of algae and to a lesser extent to bivalves and other hyzdozoans. Very difficult to distinguish morphologically from Stylactis fuciola and Clava nana, as they are evolutionarily very close species that we can only distinguish with genetic studies.
During the polyp or hydroid phase of Stylactis inermis, the polyps form colonies of tens to hundreds of specimens. Each of the hydroids is connected to the rest by a network of stolons. There are several specialized hydroids (gastrozooids and gonozooids), although with little or no morphological differences between them. Both have a basal cup from which arises an elongated column ending in a crown of approximately 20 fine tentacles forming two or more concentric crowns. The color of the polyps is translucent brown or pink, becoming whitish at the apical end, although the tentacles maintain the same translucent brown / pink color.
The diet of Stylactis inermis is mainly composed of small microorganisms that are captured by the tentacles of its polyps, thanks to the action of paralyzing substances.
Stylactis inermis is a dioecious species, that is to say that they are male and female specimens. During the hydroid phase, the colony is completely of the same sex, and resorts to asexual reproduction to make the colony grow by gemmation of new individuals. During the months of February to May it reproduces sexually, by releasing gametes into the water column, which are captured by female colonies to fertilize their eggs (visible inside the polyps due to their transparency).
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