Modeeria rotunda is a species of hydrozoan belonging to the family Haleciidae, within the order Leptothecata. The term ‘rotunda‘ is Latin for round, referring to the distinctive shape of its bell. This species of hydromedusa is known for its delicate appearance and bioluminescence. Modeeria rotunda is found mainly in the cold and temperate waters of the North Atlantic, including the North Sea and the Atlantic coast of Europe, as well as in some areas of the North Pacific. It is a pelagic species that lives in the water column from the surface to depths of 680 meters, usually in areas with abundant plankton.
Modeeria rotunda is characterized by its round, flattened bell shape, which can reach a diameter of up to 2-3 centimeters, although it is usually smaller in many individuals. The bell is almost completely transparent and has a delicately scalloped edge. Inside, there is a central structure called the manubrium, which contains the mouth, and is bright orange in color. Four radial canals extend from the manubrium towards the edge of the bell. Modeeria rotunda has between 16 and 30 marginal tentacles arranged in a single ring around the edge of the bell, each of which is equipped with cnidocytes, stinging cells used for prey capture and defense. Unlike other hydromedusae species, there are no sensory organs (ropalia) on the margin of the bell. Bioluminescence is a distinctive feature of this species, visible as a faint blue-green glow in the dark, which is thought to be used as a defense mechanism or to attract prey.
In terms of feeding, Modeeria rotunda is carnivorous and feeds on zooplankton, including small crustaceans, mollusc larvae, and other marine invertebrates found in the water column. It uses its tentacles, loaded with cnidocytes, to capture and paralyze its prey on contact. Once the prey is captured, the tentacles transport it to the mouth located in the central manubrium, where it is ingested and subsequently digested. Like many other hydromedusae, Modeeria rotunda relies on passive movement and ocean currents to obtain its food, floating in the water with its tentacles extended to maximize prey capture.
Modeeria rotunda reproduction includes both sexual and asexual phases. Sexual reproduction occurs when individuals release gametes into the water column, where fertilization is external. The resulting larvae, known as planulae, swim freely before settling on a suitable substrate to develop into benthic polyps. These polyps can then reproduce asexually by budding, releasing new juvenile jellyfish (ephyra) that develop into adults. This alternation of phases between benthic polyps and pelagic jellyfish allows Modeeria rotunda to disperse and colonize new habitats in the marine environment.
An interesting fact about Modeeria rotunda is that, despite its small size and apparent fragility, it plays an important role in marine food networks. It acts both as a predator of zooplankton and as prey for a variety of other organisms, including small fish and other jellyfish. In addition, its bioluminescence may contribute to the fascinating light displays that occur in the oceans, especially in cold and temperate waters where this species is found. Although less well known than other bioluminescent jellyfish, such as those in the genera Aequorea or Pelagia, Modeeria rotunda remains an interesting species for scientists studying pelagic ecosystems and bioluminescence mechanisms in the ocean.
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