Sponges can reproduce both asexually (e.g., by budding off parts of the sponge) and sexually (e.g., through the production and exchange of gametes).
Sponges have several means of asexual reproduction:
- regeneration from fragments – universal
- production of gemmules – freshwater species
- production of surface buds – marine species
Asexual reproduction provides mechanisms for dispersal, maintaining attachment space and extreme environment survival as well as reproduction.
Gemmules are armored small spherical structures capable of withstanding freezing and desiccation. Gemmule formation allows freshwater sponges to overwinter in waters that freeze. Under favourable environmental conditions gemmules hatch and regenerate the sponge.
Three types of buds are produced in marine species:
- stalked external buds – common in glass sponges and sponges such as some Tethya and Mycale species
- large buds from basally attached stolons
- armored gemmules – found in some boring Cliona species
Asexual reproduction of larvae is found in some intertidal demosponges (sponges whose skeletons are composed of siliceous spicules and spongin—a collagen). Larvae reproduced sexually can replicate asexually in some sponges.
This description assumes the reader is familiar with the mechanisms of sexual production—meiosis and the generation and recombination of haploid gametes.
In general sponges produce both male and female gametes, but a different times. As well gametes are not localized to specific areas of the sponge but are distributed throughout the body. In viviparous sponges, oocytes are fertilized within the sponge body (by intake of spermatozoa from the water column; often reported by divers as “smoking” sponges). Once cell division and limited specialization takes place, larvae are released. Larvae swim freely for a period and then settle. If settlement occurs on a suitable substrate, complete metamorphosis into the adult sponge form takes place. Of course, the vast majority of free-swimming larvae do not settle successfully and when larvae are released en masse they may form an important food source for small fish or larger planktonic larvae.
Most demosponges are viviparous, i.e., incubate fertilized oocytes up to larval development, but some are oviparous. Oviparous sponges release oocytes into the water column where they are fertilized by spermatozoa. Development of larvae appears to occur in the water column.
Henry Reiswig, a glass sponge specialist has been looking for signs of reproductive activity in Cloud Sponges with little success to date.
At some localities such as Senanus Reef in Saanich Inlet we have found few individuals smaller than 30 cm in size suggesting the recruitment rate is very low. For example, two divers looked for juveniles (<10cm) for fifteen minutes.
However, at Christmas Pt. (11 km to the south) 35 juveniles were found in 17 minutes. The low recruitment at Senanus Reef may be reflected in the finding of embryos only once despite diligent searches since the early 1980s (Leys et al 2007)
In the early spring of 2003 divers found what appeared to be a long “drip” of soft tissue from the edge of the osculum or exhaust opening in two sponges. A diver returned to one of the drips two weeks later and noted that it had an additional bulge midway along the “drip”. Could this be an example of asexual reproduction by budding ? It is reminiscent of buds found in some distantly related sponges (Tethya sp. and Polymastia sp. ) Divers will continue to monitor the “drips” and we will update our website as we receive new information.
Some drips may develop into fingers or mittens. Marco Stadelmaier, a graduate student at the University of Stuttgart, suggests the drips may become the root like extensions seen, at least, in another related species.