180
Reef Zoobenthos
substances fixed in the membranes of their cells, which prevent their fouling
with bacteria (Burkholder 1973; Amede and Pesando 1985). The sponges
usually are poorly grazed by reef fauna. They have low nutritional value, are
armed with sharp needles, and some of them are toxic. Therefore, they are
eaten only by some specialized tetradontid fishes, some species of urchins,
and by nudibranchs.
5.2.3 Boring Sponges
Boring sponges are among the ecologically most important components of
bottom fauna. They complete up to a quarter of the total sponge
communities in the upper- and middle-depth reef zones. Their activity
reduces the stability of coral communities to wave-stress. The majority of
the species of boring sponges belongs to the families Clionidae, and some to
the family Adocidae (Siphonodyction) and Spirastrellidae (Speciospongia,
Anthosigmella). The most common species of this group belong to the genus
Cliona (c. lampa, C. copiosa, C. viridis, C. aprica, C. langae, C.
laticlaviola) , Mycale (M. laevis) , Anthosigmella (A. varians). The boring
sponges bore holes and small caves in living and dead coral colonies, in
molluscan shells, pieces of rubble, and rocks of reef-flats. Among the living
corals these sponges prefer colonies with dead basal parts. The larvae settle
there, grow and bore, thus weakening them and finally causing their collapse
under wave or current action (Thomas 1972; Pang 1973). Their destructive
activity is especially evident in coral communities on steep slopes, where
whole massives of colonies of foliose corals, like Agaricia or Turbinaria,
could be seen collapsing on being undermined by boring sponges (Goreau
and Hartman 1966). It was also discovered that the infestation of corals by
boring sponges increases on reefs influenced by pollution (Rose and Risk
1985). An affinity of some species of the clionid sponges to definite species
of corals was also recorded (Goreau and Hartmann 1963).
The sponges bore the hard substrate with the aid of special "scraper"
cells, they extrude the pseudopodia, which excrete the acid and thus dissolve
the calcium carbonate. The pseudopodia penetrate into the lime rock like
the scoop of an excavator and extract chips from it, which have a specific
semispherical form, easily identifiable in the bottom sediments. The chips of
boring sponges are one of the important sources of the formation of coral
sand providing 20 to 30% of material composing it (Hartmi\n 1977). The
rates of boring of hard lime materials by boring sponge communities in situ
varies within 0.3 to 3 kg m -2 year-I. The linear rate of boring by a single
sponge might be 2 to 5 mm per year (Neumann 1966; Moore and Shedd
1977). The sponges play an important role in the processes of reef genesis,
and also of bioerosion (Marshall 1983). They influence even the shape of the
growing reef and control the age of hard corals, not only of the ramose but
of the massive kind too (Goreau and Hartman 1963). Besides their function
Reef Zoobenthos
substances fixed in the membranes of their cells, which prevent their fouling
with bacteria (Burkholder 1973; Amede and Pesando 1985). The sponges
usually are poorly grazed by reef fauna. They have low nutritional value, are
armed with sharp needles, and some of them are toxic. Therefore, they are
eaten only by some specialized tetradontid fishes, some species of urchins,
and by nudibranchs.
5.2.3 Boring Sponges
Boring sponges are among the ecologically most important components of
bottom fauna. They complete up to a quarter of the total sponge
communities in the upper- and middle-depth reef zones. Their activity
reduces the stability of coral communities to wave-stress. The majority of
the species of boring sponges belongs to the families Clionidae, and some to
the family Adocidae (Siphonodyction) and Spirastrellidae (Speciospongia,
Anthosigmella). The most common species of this group belong to the genus
Cliona (c. lampa, C. copiosa, C. viridis, C. aprica, C. langae, C.
laticlaviola) , Mycale (M. laevis) , Anthosigmella (A. varians). The boring
sponges bore holes and small caves in living and dead coral colonies, in
molluscan shells, pieces of rubble, and rocks of reef-flats. Among the living
corals these sponges prefer colonies with dead basal parts. The larvae settle
there, grow and bore, thus weakening them and finally causing their collapse
under wave or current action (Thomas 1972; Pang 1973). Their destructive
activity is especially evident in coral communities on steep slopes, where
whole massives of colonies of foliose corals, like Agaricia or Turbinaria,
could be seen collapsing on being undermined by boring sponges (Goreau
and Hartman 1966). It was also discovered that the infestation of corals by
boring sponges increases on reefs influenced by pollution (Rose and Risk
1985). An affinity of some species of the clionid sponges to definite species
of corals was also recorded (Goreau and Hartmann 1963).
The sponges bore the hard substrate with the aid of special "scraper"
cells, they extrude the pseudopodia, which excrete the acid and thus dissolve
the calcium carbonate. The pseudopodia penetrate into the lime rock like
the scoop of an excavator and extract chips from it, which have a specific
semispherical form, easily identifiable in the bottom sediments. The chips of
boring sponges are one of the important sources of the formation of coral
sand providing 20 to 30% of material composing it (Hartmi\n 1977). The
rates of boring of hard lime materials by boring sponge communities in situ
varies within 0.3 to 3 kg m -2 year-I. The linear rate of boring by a single
sponge might be 2 to 5 mm per year (Neumann 1966; Moore and Shedd
1977). The sponges play an important role in the processes of reef genesis,
and also of bioerosion (Marshall 1983). They influence even the shape of the
growing reef and control the age of hard corals, not only of the ramose but
of the massive kind too (Goreau and Hartman 1963). Besides their function