202
Reef Zoobenthos
The ascidians are crude filterers. They filter out from water phytoplankton, bacteria in flakes and detritus particles (Sorokin 1978b). The
stream of water driven by a ciliated epithelium enters into the pharynx. The
food particles are stuck together by mucus and then swallowed.
Besides this main source of feeding the ascidians, as many other
sedentary reef benthic animals, feed also autotrophically, having symbiotic
algae in their tissues (Lewin et al. 1980; Kott 1982). The symbiotic genera of
ascidians belong to the most common family Didemnidae (Eldredge 1966).
These are: Didemnium, Trididemnium, Diplosoma, Lissoclinum, and
Eehinoclinum. The algae are situated in them extracellularly in tissues of the
gastral, gill and cloacal cavities. The algae symbiotic to didemnid ascidians
initially were indentified as prokariotic blue-green chroococci (Newcomb
and Pugh 1975). But later it was discovered that they have another pigment
composition, different from the blue-greens. They have no phycobilin, but
chlorophyll b. So, though prokariotic, they have a light-absorbing system
similar to that in the eukariotic green algae (Alberte et al. 1986). Therefore,
the symbionts of didemnids were described as Prochloron, a group
intermediate between the green and the blue-green algae (Lewin 1977, 1984;
Lewin and Cheng 1983; Paerl et al. 1984). They have round cells of 10 to
3011 (Kott 1982; Alberte et al. 1986; Griffith and Thinh 1987). Recently it
was found that some of the dedemnids could have as symbionts also a real
coccoid cyanobacterium, Syneehoeystis trididemnii, which was found also in
sponges (Larcum and Cox 1988).
The content of chlorophyll a in symbiotic ascidians is 2-5mgg- 1 of their
dry weight. The ratio of chlorophyll a and b is close to 3-6 (Lewin and
Cheng 1983). The rate of photosynthesis in symbiotic ascidians at midday is
2- 5 mg O 2 h -I g -I of dry weight or about 9 mg O 2 mg -I h -I of chlorophyll a.
Their gross photosynthesis is around 70 mg O2 day-I per 1 mg of chlorophyll
a. At midday, photosynthesis slightly exceeds respiration. In a 24-h balance
the PIM ratio is about 0.6. Thus the photosynthesis of symbiotic ascidians
makes up for nearly half of their daily energy loss (Lewin and Pardy 1981;
Kott et al. 1984; Olson and Porter 1985). Some species of the symbiotic
didemnids may crawl with a speed of 0.5-1 cm day-I, in selecting the bestilluminated place (Birkeland et al. 1981).
The total number of species of sedentary tunicates on reefs of the IndoPacific exceeds 200 (Eldredge 1967). Their mass genera are: Diplosoma,
Tridemnium, Didemnium, Lissoclinum, Rhopalea, Eudistoma, Aplidium,
Aseidia, Perophore, Polyearpa, Pyura, and Molqula. The ascidians are
significant components of the reef zoobenthos, being a food source for
different bottom fish, predatory gastropods, crabs, polychaetes and
echinoderms.
Reef Zoobenthos
The ascidians are crude filterers. They filter out from water phytoplankton, bacteria in flakes and detritus particles (Sorokin 1978b). The
stream of water driven by a ciliated epithelium enters into the pharynx. The
food particles are stuck together by mucus and then swallowed.
Besides this main source of feeding the ascidians, as many other
sedentary reef benthic animals, feed also autotrophically, having symbiotic
algae in their tissues (Lewin et al. 1980; Kott 1982). The symbiotic genera of
ascidians belong to the most common family Didemnidae (Eldredge 1966).
These are: Didemnium, Trididemnium, Diplosoma, Lissoclinum, and
Eehinoclinum. The algae are situated in them extracellularly in tissues of the
gastral, gill and cloacal cavities. The algae symbiotic to didemnid ascidians
initially were indentified as prokariotic blue-green chroococci (Newcomb
and Pugh 1975). But later it was discovered that they have another pigment
composition, different from the blue-greens. They have no phycobilin, but
chlorophyll b. So, though prokariotic, they have a light-absorbing system
similar to that in the eukariotic green algae (Alberte et al. 1986). Therefore,
the symbionts of didemnids were described as Prochloron, a group
intermediate between the green and the blue-green algae (Lewin 1977, 1984;
Lewin and Cheng 1983; Paerl et al. 1984). They have round cells of 10 to
3011 (Kott 1982; Alberte et al. 1986; Griffith and Thinh 1987). Recently it
was found that some of the dedemnids could have as symbionts also a real
coccoid cyanobacterium, Syneehoeystis trididemnii, which was found also in
sponges (Larcum and Cox 1988).
The content of chlorophyll a in symbiotic ascidians is 2-5mgg- 1 of their
dry weight. The ratio of chlorophyll a and b is close to 3-6 (Lewin and
Cheng 1983). The rate of photosynthesis in symbiotic ascidians at midday is
2- 5 mg O 2 h -I g -I of dry weight or about 9 mg O 2 mg -I h -I of chlorophyll a.
Their gross photosynthesis is around 70 mg O2 day-I per 1 mg of chlorophyll
a. At midday, photosynthesis slightly exceeds respiration. In a 24-h balance
the PIM ratio is about 0.6. Thus the photosynthesis of symbiotic ascidians
makes up for nearly half of their daily energy loss (Lewin and Pardy 1981;
Kott et al. 1984; Olson and Porter 1985). Some species of the symbiotic
didemnids may crawl with a speed of 0.5-1 cm day-I, in selecting the bestilluminated place (Birkeland et al. 1981).
The total number of species of sedentary tunicates on reefs of the IndoPacific exceeds 200 (Eldredge 1967). Their mass genera are: Diplosoma,
Tridemnium, Didemnium, Lissoclinum, Rhopalea, Eudistoma, Aplidium,
Aseidia, Perophore, Polyearpa, Pyura, and Molqula. The ascidians are
significant components of the reef zoobenthos, being a food source for
different bottom fish, predatory gastropods, crabs, polychaetes and
echinoderms.