Hydractinia is a genus of clonal/colonial marine cnidarians, members of the class Hydrozoa. Other cnidarians include sea anemones, corals, and jellies, the latter being Hydractinia’s closest relatives. Clonal animals are defined by being able to reproduce clonally, i.e. asexually, thereby generating genetically identical copies (clones) of themselves. Hydractinia clonemates remain attached to each other, forming a colony of genetically identical members who share a common digestive space and a nervous system. Hence, in a certain way, a Hydractinia colony acts as a meta-individual because its members function in a coordinated way and are physiologically integrated. As a result, feeding one polyp feeds the entire colony; gently pinching one polyp would result in other polyps contracting simultaneously – many polyps, one functional unit. A colony can grow indefinitely if well fed; cutting a small piece of it and letting it attach to a new glass slide or Petri dish would result in a new, genetically identical colony: probably the easiest way to clone an animal!
Among the many Hydractinia species known worldwide, two North Atlantic ones have been best studied: The European Hydractinia echinata and its American congener H. symbiolongicarpus. In the wild, Hydractinia forms sessile colonies on the surface of gastropod shells that are inhabited by hermit crabs. This mobile substratum provides excellent protection against exposure to low tides and prevents the colony from being covered by sediment. In the laboratory, however, Hydractinia happily grows on any glass or plastic surface; most commonly used are microscope glass slides.
The basic unit of Hydractinia is the polyp that is structured as a tube with a head on one end (the oral end) and is connected to the colony at the other end (the aboral end). The head consists of a mouth that is surrounded by a whorl of tentacles, used to capture prey (mostly small crustaceans) and inserting it into the mouth. The polyp body wall is simple, consisting of an epithelial bilayer that sandwiches a thin basement membrane (extracellular matrix), called mesoglea. The outer epithelial layer, the epidermis, is ciliated; the inner layer, the gastrodermis, consists of phagocytic cells that can secrete digestive enzymes into and engulf food particles from the gastric cavity. A specialized tissue, called stolon, interconnects polyps and attaches to the substratum. The stolonal tissue consists of a network of gastrovascular tubes whose wall is constructed of an epithelial bilayer, similar to polyps’ body walls. Other than polyp tissue, the stolonal epidermis secretes a chitinous layer, the periderm, that provides structural rigidity. The stolon’s epidermis tends to fuse with neighboring stolons’ epidermis, resulting in a so called stolonal mat, consisting of an upper and lower epidermal layers with a network of gastrodermal tubes running between them. Polyps are connected to the stolons at their aboral end.
Most polyps can feed and are hence called gastrozooids. As the colony matures, other polyp types emerge. The most prominent of which is the sexual polyp or gonozooid. Gonozooids do not feed. Their oral end does not contain a mouth and only rudimentary tentacles. Along their body wall, special structures develop called gonophores. A gonophore is a reduced medusa (a medusoid) that never fully develops and remains attached to the polyp. This is different than other hydrozoans, where the medusa detaches and forms the pelagic, sexually reproducing life stage. In Hydractinia, instead, the rudimentary jellyfish (gonophore) functions as a gamete container. Sexes are separate and genetically determined by an XY sex determination system. There are at least two other polyp types that occur more rarely and probably function in defence and food gathering, but do not catch prey or reproduce.
Hydractinia has attracted the attention of scientists since the late 1800s for several reasons. First, it is easy to work with - culturing the animal in the lab is relatively easy; second, it readily provides access to all life stages on a daily basis; finally, its biology is exciting: a non-aging, highly regenerative animal with unmatching growth plasticity. Hydractinia’s adult stem cells, known as i-cells, have been the first animal stem cells to be studied and represent a model for animal stem- and germ cell research. Hydractinia possesses a genetically determined allorecognition system that can discriminate self from nonself cells with high precision. Hydractinia can be genetically modified through random integration transgenesis or using CRISPR-Cas9. Protocols for gene knockdown and knockout are well established. Current studies involving Hydractinia include neurogenesis, embryonic development, metamorphosis, regeneration, and ecology.
With the development of further experimental tools, Hydractinia research enters a renaissance more than 130 years after being used for the first time in scientific research. We, the Hydractinia research community, are happy to support newcomers with material and advice. Contact us!