We use the sea anemone Exaiptasia pallida, commonly known as Aiptasia, as a model system. The sea anemones are closely related to the stony corals and we could show that Aiptasia forms symbioses with the same algal species as corals. Aiptasia has many practical advantages over corals as a model organism in the lab, allowing deeper mechanistic insights into the functioning of the symbiosis. At the same time, many findings are transferable to its relatives, the reef-building corals, and also shed light on the evolution of host-symbiont interactions.
Reef-building corals are completely sessile in the adult phase of their lifecycle, prevented from moving by their stony skeletons. Long-range dispersal occurs following sexual
Unlike the colonial lifestyle of many stony corals, which form a colony of many polyps, the adult Aiptasia is a single, independently-living polyp. The polyp usually hosts photosynthetic symbionts in its endodermal cells, and can also use its tentacles to capture small animals as a source of food. The polyp is mostly sessile, attaching to hard substrates using its muscular foot, but can detach and move over short distances to find a new place to live.
Additionally, Aiptasia can survive without symbionts if fed regularly. ‘Bleaching’ (loss of symbionts) can be induced experimentally making Aiptasia an ideal model to study the mechanisms of coral bleaching in response to environmental change, and recovery under laboratory conditions.
As is the case with their coral relatives, Aiptasia produces swimming planula larvae through sexual reproduction that can disperse far from the parents. Also similar to corals, the larvae do not initially contain any symbionts, instead establishing symbiosis with partners that they encounter and take up from the environment.
In addition to sexual reproduction, Aiptasia is also able to reproduce asexually through the process of pedal laceration. Small buds of tissue (pedal lacerates) are produced from the foot of an adult polyp, which develop into small polyps genetically identical to the parent, thus producing clonal lines.
Coral mass spawnings are spectacular events, when colonies across the reef release their gametes at the same time to allow fertilization to happen. The moon plays a key role in this synchronization, as its phases provide a common cue that different colonies can use to coordinate spawning.
Similarly, the phase of the moon can trigger spawning in Aiptasia. Aiptasia is dioecious, that is, polyps are either females, which produce eggs, or males, which produce sperm. We feed adult polyps very well, giving them enough nutrition to produce gametes. We use an “artificial full moon” – blue LEDs shining during the night for 5 consecutive nights – to stimulate these polyps into releasing their gametes. Approximately 2 weeks after the stimulus the polyps release their gametes into the water, which can be used for experiments or fertilized directly to produce planula larvae.
Aiptasia larvae are an excellent system for studying the cell biology of symbiosis. In comparison to coral larvae, which are relatively big and, thanks to the large amounts of yolk they contain, opaque, the larvae of Aiptasia are small and very transparent. These attributes make them very well suited for light microscopy, meaning that subcellular molecules and processes can be imaged in detail.
By two days after fertilization, the larvae are competent to phagocytose symbionts and establish symbiosis. Since they are initially symbiont free, and infection rates can be easily assessed by counting the algae taken up by individual larvae, a very simple assay can be used to measure the effects of various factors on the establishment of symbiosis. Hundreds of larvae can be maintained per milliliter of sea water; therefore large numbers of larvae can be treated and exposed to a standard density of algae for a period of time. The larvae can then be fixed and the infection success measured across the population.