Spongillidae are a family of freshwater sponges from the Animalia kingdom. In contrast to their flashy seawater relatives, our native freshwater species are inconspicuous. Yet their way of life offers fascinating perspectives on an ancient life form.
Description
Both the Spongilla lacustris and the Ephydatia fluviatilis freshwater sponge species are relatively common in Central European waters. The shrub-like branched fingers of the Spongilla lacustris can grow up to one meter long, but only if ideal conditions prevail in the summer.
The freshwater sponge has no pre-defined shape. It is often characterised by forms like small ribs or humps which are covered by pores. Sponge colour varies from whitish to stark green, the latter being the case when algae has been absorbed. The body of a sponge typically consists of a very tough and resistant collagen protein called spongin which is reinforced by tiny silicic acid spicules permeating the fibrous spongin. (In contrast, a sponge which can be purchased for household use lacks these spicules, or small needle-like pieces, which is why it is soft and flexible.)
The unspecialised cells of this organism have varying functions; unlike higher organisms, they are not collected in organs. Furthermore, a sponge has no nerve or sensory cells.
Other types of sponges belonging to the Spongilla species may only be determined under the microscope in an examination of the spicules. The second species of sponge, Ephydatia, is able to probe the deepest into the water of all native sponges, namely up to 20 metres below the water's surface.
Should one encounter a living sponge, it is the intensive iodine-like smell it exudes which is most noticeable.
Distribution
Freshwater sponges can be found in many large lakes and rivers, including in the Donau-Auen National Park. Yet they avoid ponds. Sponges may spread by way of bottom-feeding ducks, which carry the Spongillidae into other bodies of water.
Endangerment and Conservation Status
No endangerment exists as long as a good water quality is maintained. Freshwater sponges are excellent natural water filters and thus play a significant role in keeping waters clean.
Behaviour
Spongillidae require firm substrates such as stones or wood in order to settle in currents, but once they have settled, they no longer change their location. Thanks to the constant water flow from the current, they are continually provided with nutrients and oxygen to breathe. Water is absorbed through the fine pores on their outer surfaces and oxygen is absorbed in their labyrinthine inner parts. Food particles such as dissolved organic substances, bacteria or protozoa are also filtered out. The water is then diffused through thicker collection channels.
The filtered nutrients are absorbed by free-moving scavenger cells and distributed among the other cells.
Although there are male and female sponges, sexual reproduction appears to be of minor interest to the freshwater sponge. Higher success rates are achieved through vegetative reproduction by way of so-called gemmules. Produced in autumn, these round reddish-yellow gemmules contain stem cells which are dormant over the winter. In the spring the cells emerge from the capsule, which is around 2mm large, and form a new sponge.
Special Characteristics
Spongilla lacustris is collected for medicinal purposes in the autumn and is used in dried form for treatments. It is commonly used to treat thyroid ailments. It is said to alleviate heart palpitations, insomnia and hyperthyroidism. Other forms of the sponge are available as drops, for example to treat eye infections.