Osmoregulation in the fresh water sponge, Spongilla lacustris

Abstract

External medium passed through the canal system of small Spongilla lacustris at the rate of 70 body volumes per hour, allowing for little or no modification of its composition by the animal. Consequently any osmotic gradient between medium and animal was experienced at the cell level. Forty or more contractile vacuoles arranged around the nucleus in each basal pinacocyte removed fluid at the rate of 24.7 k 1.0 ym3lcelUrnin in 1 . 3 milliosmolal (mosm) medium, Output of the contractile vacuoles depended in a nearly linear fashion on the osmotic concentration of the medium, decreasing to zero at 12-16 mosm. The contractile vacuoles responded to the osmotic concentration of the medium, not to ionic strength or the concentration ratios of particular ions. Osmotic concentration of the vacuolar fluid can be estimated as 10-40 mosm, leading to the hypothesis that contractile vacuoles of sponges function by isotonic water separation.

The Cell Biology of Sponges

“…Contractile vacuoles have now been clearly demonstrated in the pinacocytes of freshwater sponges (Feige, 1969;Brauer, 1975;Weissenfels, 1974); both exo-and endopinacocytes possess them (Weissenfels, 1975(Weissenfels, , 1980. Similar contractile vacuoles have also been described in freshwater sponge choanocytes (see Fig.…”

“…The output response has been shown to be related to osmolarity not ionic strength and the vacuole contents appear isotonic with the medium. While these data indicate that intracellular K+ is probably regulated independently of the contractile vacuoles, it seems probable that much of the intracellular Na+ is contained within them and they thus may act as a reservoir for this ion (Brauer, 1975). …”

“…In freshwater sponges, both the rate and the volume of output of the vacuoles can be directly affected by the osmolarity of the medium (Brauer, 1975). At 12-15 mosmolar no output occurs and at decreasing solute concentration there is a strictly linear relationship between output and osmolarity of the medium: the lower the osmolarity of the medium the higher the output.…”

Contractile Vacuoles and Associated Structures: Their Organization and Function

“…Evidence that contractile vacuole complexes eliminate water from the cell to offset the osmotic influx of water comes from several areas: ( I ) contractile vacuoles are widespread in fresh-water organisms in which the osmotic gradient is expected to be most severe, but are frequently absent from marine and parasitic forms (Finley, 1930; Kitching, 1938b; Bovee, 1953; Page, 1970; inter alia); (2) contractile vacuoles occur mainly in protists lacking rigid and entire external cell walls; and in the case of algae and fungi with walls, contractile vacuoles only occur in the naked stages of the life-cycle (Section 11. 3); (3) vacuolar output is depressed by an increase in the osmotic pressure of the medium (in effect a decrease in the osmotic gradient across the cell membrane) (Section 111); (4) the arrest of contractile vacuole activity, for example by heavy metal ions or inhibitors of metabolic activity (Section 111), frequently results in swelling of the cell; (5) mutants lacking contractile vacuoles may be restricted to media with high osmotic pressures (Guillard, 1960); (6) the rate of vacuolar output correlates well with values of the rate of water flux through cells measured by other means (Belda, 1942; Lplvtrup & Pigon, 1951; Brauer, 1973). It is thought that the osmoregulatory function can be achieved by a regulated production of fluid by the contractile vacuole and the surrounding spongiome.…”

“…The contractile vacuole fills from a radiating system of irregular lacunae. This type of organization has only been seen in some fresh-water sponges (Brauer, 1973 ;Weissenfels, 1974Weissenfels, , 1975 (Eyden & Vickerman, 1975) and possibly by the flagellate Vacuolaria virescem (Heywood, 1978).…”