200
Reef Zoobenthos
biofilter, they channel the organic matter input from passing oceanic waters
to its bottom communities. The lime skeletons of crinoids correspond to
its biogenic carbonate sediments. Many crinoids are hosts to various
commensals: polychaetes, ophiuroids, small gastropods and crustaceans
(Finshelson 1974).
5.5.5 Holothurians
The holothurians are the most important element of vagile macrobenthos in
lagoons and in reef-flat shallows covered with sand and rubble. Some of
them live under stones and boulders. They are the second object (after fish)
of reef fishery. The weight of large holothurian specimens might be 1-2 kg.
On average, their biomass in these biotopes varies within 50 to 500 g m -2,
and their numbers within 0.2-40 sp. m- 2 (Bakus 1968; Webb et al. 1977;
Levin 1979; Shelley 1985). Their maximum number has been recorded at the
shallow sandy flat of the Enivetok atoll at night: 200 sp. m- 2 of Holothuria
difficilis (Bakus 1973). Most holothurians are sediment eaters, able
somewhat to select the food particles. With the aid of their mouth tentacles
they select from sand its more nutritious components: forams, detritus,
microalgae and microbenthos. Therefore, the food material consumed by
holothurians is enriched with carbon and nitrogen and also with the
microbial biomass, as compared with surrounding bottom sediment. The
main source for their feeding is the microbial biomass of the sediments. Its
assimilability by holothurians was found to be rather high: 30-40% (Sorokin
1978b; Moriarty 1982). The holothurians are active bioturbators of soft reef
sediments. In areas of their high density they pass the whole upper layer of
sediments through their guts during a month. For example, 5 million of the
holothurians Holothuria atra and H. leucospilota pass 250000 tons of sand
per year through their guts (Bonham and Held 1963). The acid excreted
within their guts dissolved about 1 % of sand consumed. Thus, by their
average biomass of 200 g m -2 they will dissolve per day 2-4 g of carbonate
sediment, which is close to 10-20% of the average rates of production of
carbonaceous material by corals, forams and coralline algae (Webb et al.
1977). When passing the sediment consumed through the guts, holothurians
stimulate in it the development of the benthic microalgae and control the
abundance of meyobenthos (Renaud-Momant 1977). They grow very fast.
An average sized holothurian grows 0.5-1 cm month -1, increasing its weight
by 15-25 g. Thus because of their high density they are often responsible for
a major portion of the total benthic production in the biotopes of soft
sediments, which usually dominate on reefs of their area.
The total number of species in the Indo-Pacific reefs is about 140, while
within a particular reef system it varies from 10 to 60. For example, in the
GBR area it is 21, and near Madagascar 48 (Hammond et al. 1985; Conand
1985). Their fauna is composed of three main families: Holothuriidae,
Reef Zoobenthos
biofilter, they channel the organic matter input from passing oceanic waters
to its bottom communities. The lime skeletons of crinoids correspond to
its biogenic carbonate sediments. Many crinoids are hosts to various
commensals: polychaetes, ophiuroids, small gastropods and crustaceans
(Finshelson 1974).
5.5.5 Holothurians
The holothurians are the most important element of vagile macrobenthos in
lagoons and in reef-flat shallows covered with sand and rubble. Some of
them live under stones and boulders. They are the second object (after fish)
of reef fishery. The weight of large holothurian specimens might be 1-2 kg.
On average, their biomass in these biotopes varies within 50 to 500 g m -2,
and their numbers within 0.2-40 sp. m- 2 (Bakus 1968; Webb et al. 1977;
Levin 1979; Shelley 1985). Their maximum number has been recorded at the
shallow sandy flat of the Enivetok atoll at night: 200 sp. m- 2 of Holothuria
difficilis (Bakus 1973). Most holothurians are sediment eaters, able
somewhat to select the food particles. With the aid of their mouth tentacles
they select from sand its more nutritious components: forams, detritus,
microalgae and microbenthos. Therefore, the food material consumed by
holothurians is enriched with carbon and nitrogen and also with the
microbial biomass, as compared with surrounding bottom sediment. The
main source for their feeding is the microbial biomass of the sediments. Its
assimilability by holothurians was found to be rather high: 30-40% (Sorokin
1978b; Moriarty 1982). The holothurians are active bioturbators of soft reef
sediments. In areas of their high density they pass the whole upper layer of
sediments through their guts during a month. For example, 5 million of the
holothurians Holothuria atra and H. leucospilota pass 250000 tons of sand
per year through their guts (Bonham and Held 1963). The acid excreted
within their guts dissolved about 1 % of sand consumed. Thus, by their
average biomass of 200 g m -2 they will dissolve per day 2-4 g of carbonate
sediment, which is close to 10-20% of the average rates of production of
carbonaceous material by corals, forams and coralline algae (Webb et al.
1977). When passing the sediment consumed through the guts, holothurians
stimulate in it the development of the benthic microalgae and control the
abundance of meyobenthos (Renaud-Momant 1977). They grow very fast.
An average sized holothurian grows 0.5-1 cm month -1, increasing its weight
by 15-25 g. Thus because of their high density they are often responsible for
a major portion of the total benthic production in the biotopes of soft
sediments, which usually dominate on reefs of their area.
The total number of species in the Indo-Pacific reefs is about 140, while
within a particular reef system it varies from 10 to 60. For example, in the
GBR area it is 21, and near Madagascar 48 (Hammond et al. 1985; Conand
1985). Their fauna is composed of three main families: Holothuriidae,