page_title: Amyloodinium ocellatum
section_title: Life cycle
hierarchical_section_title: Amyloodinium ocellatum / Life cycle
caption_attribution_description: English: Simplified scheme of the life cycle of Amyloodinium ocellatum. (Dr P. Beraldo, University of Udine)
context_page_description: Amyloodinium ocellatum is a cosmopolitan ectoparasite dinoflagellate of numerous aquatic organisms living in brackish and seawater environments. The dinoflagellate is endemic in temperate and tropical areas, and is capable of successfully adapting to a variety of different environments and to a great number of hosts, having been identified in four phyla of aquatic organisms: Chordata, Arthropoda, Mollusca and Platyhelminthes. Moreover, it is the only dinoflagellate capable of infecting teleosts and elasmobranchs.
  The parasite represents a serious problem for both reared and aquarium fish, since amyloodiniosis, the infection caused by this protozoan, can lead the host to death in less than 12 hours, with acute morbidity and mortality around 100%. However, these two parameters vary considerably on the basis of farming typology, parasite burden, fish species and season considered. In general, amyloodiniosis is typically present in land- or lagoon-based rearing sites, where shallow seabeds and poor water exchange/recirculation allow the parasite to reach its optimal proliferation values. Especially in the warmest months, A. ocellatum causes high mortality rates and economic damages.
context_section_description: The lifecycle of A. ocellatum is direct and divided in three phases. In general, it can be completed in five to seven days when temperature and salinity are between 23-27°C and 30-35 ppt respectively. However, the parasite can also express its virulence in extreme conditions. In particular, lethal outbreaks were documented at high temperatures (more than 35°C) in both salt water (46 ppt) and brackish (7 ppt) environments .
  The parasitic stage is represented by the sessile trophont. In this phase, the protist is pear-shaped, enclosed in a cellulose wall and exhibits specific structures, rhizoids (tentacle like processes) that enable it to strictly anchor to host epithelia (gill or skin predominantly). If the infection is severe, trophonts can also be found on eyes, fins and in all oropharigeal cavity, the latter is a typical infection site in the European seabass (Dicentrarchus labrax). Constantly moving while anchored and therefore causing physical injuries to cells, trophonts inflict serious damages to the host, potentially inducing its death in 12-48 hours as a function of the parasite burden. Based on data in the literature, the trophont feeds directly from the host cells, probably using the stomopode by releasing digestive enzymes, which exacerbate the rhizoid lesions. Trophont size can vary considerably, with early trophonts measuring 27×23 µm while mature ones can reach up to 130×60 µm and more.
  Two to six days after feeding, the trophont detaches from the host and encysts on inert substrates (pond/tank bottom or seabed) transforming into the tomont: the reproductive stage. In this phase, the protozoan is round and encapsulated in a cellulose wall, which becomes thicker and confers upon it an exceptional resistance to unfavourable conditions and to several therapeutic treatments. The protozoan reproduces asexually, the first division is longitudinal while the succeeding ones are approximately regular and at right angles to each other. Potentially, in two to four days, from a single tomont, 256 new dinospores can be generated. The number of newly formed dinospores is directly correlated to the nutritive state of the trophont.
  The dinospore (8–13.5 × 10–12.5 μm), whose antero-posteriorly compressed shape resembles a hamburger, is the infective stage. In this phase, the armoured (cellulose wall) protist is capable of active swimming thanks to two flagella: one longitudinal, the other transverse. After adhesion to a new host, the dinospore transforms into a trophont within 5 to 20 minutes .
caption: Simplified scheme of the life cycle of Amyloodinium ocellatum. (Dr P. Beraldo, University of Udine)
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