Dispersal & reproduction
Surprisingly, many species of sponges are actually capable of some locomotion, just very slowly. And of course sponge larvae are highly mobile, capable of crawling across the sea bed, but generally adult sponges are firmly attached to the seabed and the phylum is considered to be ‘sessile’ or ‘sedentary’.
Oceanapia sagittaria, upper portion of otherwise burrowing tubular body with soft capitum, thought to be a asexual reproductive mechanism for dispersal.
Cinachyrella sp., showing asexual budding at the ends of long filamentous processes, where each bud will drop and settle elsewhere as a new individual.
Hard bodied ‘sclerosponge’, Astrosclera willeyana, a prominent species in the coral reef cave fauna of the Indo-Pacific.
Haliclona (Reniera) chrysa, showing small exhalant breathing tubes (fistules) growing from the sponge, which can develop very rapidly in an aquarium.
Many or most sponges reproduce sexually at some stage of their life cycle, producing larvae that can disperse and recolonise new habitats. These larvae are either incubated within the parent and born live (vivipary), or the eggs and sperm (gametes) are broadcast directly into the seawater where fertilisation takes place (ovipary). Oviparous sponges may have separate sexes (gonochoric), or may be capable of changing sex, from male to female and back again, in sequence (sequencially hermaphroditic). The ability of oviparous sponges to broadcast gametes directly into the seawater theoretically gives them greater potential for dispersal, although these gametes do not remain viable in the water column for very long, and where known, eggs are apparently ‘nested’ close to parent.
Sponges that reproduce through vivipary have gametes fertilised inside their body, and larvae are incubated by the parent. Larvae swim or crawl away from their parent. They are apparently very short-lived, and there is no known larval stage that feeds in the plankton (unlike corals for example). Therefore viviparous reproduction is likely to have limited impact on sponge dispersal.
In contrast, asexual reproduction is common in sponges (clonality). This includes budding and fragmentation, with fragments reattaching to the seabed somewhere else. Thanks to the possession of totipotent cells it theoretically takes only a single cell (archaeocyte) to start a new sponge, and consequently asexual reproduction may be the predominant method for dispersal and colonisation amongst sponges. Catastrophic storm events might have a major role in sponge dispersal at large spatial scales through fragmentation.
Sponge growth rates and their life spans can vary enormously, but generally this is still poorly known. Some soft-bodied species are very fast growing in an aquarium, capable of producing tendrils at rates of centimeters per week. Other species, perhaps only the hard-bodied sponges, have been measured to have lived for a very long time, some dated as over 1000 years old.
The limited capabilities for larvae to disperse, and the predominance of asexual reproduction in sponge recruitment generally contribute to their inbreeding at small spatial scales (local or regional faunas). The lack of gene flow between even small-scale or geographically close populations (as shown by increasing genetic evidence of ‘heterozygote deficits’ in populations), leads to new species evolving by being genetically isolated from other populations (allopatric speciation). However, these new species are not necessarily morphologically very different from the other related species (sibling species). Consequently, many sponge species that appear to be widely-distributed (so-called ‘cosmopolitan’ species), scattered across the vast Indian and Pacific Oceans, probably consist of related but genetically distinct species that show no or little difference in their morphology across their vast range. These are called cryptic species, and there is increasing genetic evidence that shows cryptic species are high amongst the sponges.
Therefore, our present estimates of numbers of species of sponges, currently based mostly on their morphology (‘morphospecies’), are probably very conservative. Their true genetic diversity is likely to be much higher.
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