Halofolliculina corallasia
First protozoan and eukaryote known to cause a coral disease.
Halofolliculina corallasia is a single-celled organism from the heterotrich ciliate group that causes skeletal eroding band (SEB), a disease affecting corals. It stands out because it is the first protozoan—and the first eukaryote—known to be a coral disease pathogen; all previously identified coral pathogens are bacteria.
Like other members of the folliculinid family, this species is sessile and lives inside a protective structure called a lorica, which is like a tiny house. When disturbed, the cell can retreat into this lorica. The organism’s mouth is flanked by two wing-like projections, each fringed with polykinetids—clusters of cilia that work together to create a water current that pulls food toward the mouth.
So far, *H. corallasia* is the only known cause of skeletal eroding band, which is the most common coral disease in the Indian and Pacific Oceans and also occurs in the Red Sea. A very similar disease was later found in the Caribbean Sea, but it is caused by a different, unnamed species of the same genus and appears in a different type of environment.
The species was first spotted in 1988 on reefs near Motupore Island in Bootless Inlet, Papua New Guinea, and was formally named in 2001. Its name, *corallasia*, combines “coral” with the Greek word *lasios*, meaning “densely overgrown.” As a ciliate, it shares three key traits with other members of that group. First, it reproduces both by cell division (splitting into two) and by conjugation, where two organisms temporarily join to exchange DNA. Second, it has two cell nuclei: a larger macronucleus that handles everyday cell functions by transcribing DNA into RNA, and a smaller micronucleus used only for reproduction. Third, it has cilia at some point in its life cycle.
Heterotrichs are among the largest protozoans and have a spiral of polykinetids around their mouths. In the family Folliculinidae, members are sessile and anchor the rear of their bodies inside a lorica—the word comes from Latin for “cuirass,” a type of flexible armor. The lorica is attached to a surface, and the cell can retract into it. *H. corallasia* is smaller than other species in its genus. Its lorica is flask-shaped, typically 220 micrometers long and 95 micrometers wide, and its wing-like projections extend 175 to 200 micrometers when fully out.
Quick Facts
- Genus
- Halofolliculina
- Species
- corallasia
Facts from the source article.
Lore & Background
Halofolliculina corallasia was first observed in 1988 on reefs near Motupore Island in Bootless Inlet, Papua New Guinea, and was formally named in 2001. The species name corallasia combines 'coral' with the Greek word lasios, meaning 'densely overgrown'. It is a member of the ciliate group of protozoans, which are among the most complex single-celled eukaryotes. Ciliates are distinguished by three characteristics: they reproduce both by cell division and conjugation; they have two cell nuclei (a macronucleus for normal cell function and a micronucleus for reproduction); and they have cilia at some stage in their life cycle.
As a heterotrich, H. corallasia is among the largest protozoans and has a spiral of polykinetids around its mouth. It belongs to the family Folliculinidae, whose members are sessile and live in a lorica—a flask-shaped, dark gray or black house made of a chitin-like substance, typically 220 μm long and 95 μm wide. The cell can retreat into the lorica when disturbed, and flaps close the opening. The mouth is flanked by a pair of wing-like projections fringed with polykinetids, which produce a current to draw in food. H. corallasia is smaller than other members of the same genus, with wings 175 to 200 μm long when fully extended.
Reader's Guide
Halofolliculina corallasia is significant as the first protozoan and the first eukaryote identified as a coral disease pathogen, breaking the pattern of bacterial causes. It is the only known agent of skeletal eroding band (SEB), the most common coral disease in the Indian and Pacific Oceans, also found in the Red Sea. SEB appears as a black band that advances over corals, leaving dead coral behind. The disease spreads through cell division, producing worm-like larvae that settle on undamaged coral ahead of the band, secrete a lorica, and spin to shape it. This spinning, combined with hardening chemicals, crumbles the coral skeleton and kills polyps. Discarded loricae of parent cells leave a spotted region in the band's wake, distinguishing SEB from black band disease, which leaves a completely white dead area.
A 2004 survey in the Caribbean Sea, published in 2006, reported a disease with very similar symptoms affecting 25 coral species across 6 families. Although initially suspected to be H. corallasia, further examination revealed a different, unnamed species of the same genus, Halofolliculina. The Caribbean infestations were most common in oceanic waters, while Indian and Pacific Ocean occurrences were more prevalent in coastal waters, leading to the name 'Caribbean ciliate infection'. This highlights the genus's broader ecological impact and the need for continued study.
Did You Know?
- Halofolliculina corallasia is the first protozoan and first eukaryote known to cause a coral disease; all other identified coral disease pathogens are bacteria.
- Its lorica is dark gray or black, made of a chitin-like substance, and has flaps that close when the cell retracts.
- Skeletal eroding band is distinguished from black band disease by the spotted region left behind by discarded loricae.
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