Reproduction And Life History

Sponges utilise a number of reproductive strategies based around their characteristic cellular totipotency. Asexual reproduction involves the production of prop-agules such as buds and fragments containing a sufficient number of cells from which complete sponges can develop. A few euryhaline species of the genus Mycale produce gemmule-like bodies, but true gemmules are restricted to freshwater sponges of the Order Haplo-sclerida. Most groups have considerable means of asexual propagation, such as fragmentation from storm events, which is thought to be an important mechanism for sponge recruitment, and all have extensive regenerative powers that appear to be vital for sustaining local populations. Sponges have sexes that are separate, or sequencially hermaphroditic, producing eggs and sperm at different times. Although there are no gonads or reproductive ducts, sexual reproduction involves the production of gametes by the choanocytes and totipotent archaeocytes, with fertilisation often (but not always) internal. Individuals release sperm externally via the exhalant current, whereas their oocytes reside in the incurrent aquiferous system to minimise self fertilisation. Sperm are engulfed by choanocytes, which become amoeboid, travelling to and transferring them to the oocytes. Cleavage leads to a solid steroblastula or hollow coeloblastula, with internally brooded, viviparous embryonic development in many cases, and larvae leaving the parent for dispersal. Other sponges are oviparous, with females shedding their eggs externally as zygotes or early embryo stages, rarely as unfertilised oocytes, although the details of embryology still remain unknown for most species. Other forms of development have also been recorded, such as elimination of the free-swimming larval stage and embryos brooded in the maternal sponge before being expelled as young adults. Eight different larval types are known but few of these have been adequately investigated. Most embryos develop into free-swimming (lecithotrophic) or demersal crawling larvae, ciliated to a greater or lesser extent, 0.05-5.00 mm long, with a brief planktonic phase, short longevity (maximum of 72 hrs recorded), and, unlike most marine invertebrates, have no plank-totrophic stage.

Release of propagules (gametes, zygotes or embryos) is asynchronous in viviparous species but highly synchronous in oviparous sponges, triggered by factors such as temperature and lunar cycle. A prominent member of the GBR sponge community, Xestospongia testudinaria, is oviparous and broadcasts eggs in spawning events that were synchronised among populations of the same species, with timing found to be correlated with the lunar cycle. Molecular studies of individuals in local sponge populations show that most have high levels of genetic variability, not high genetic relatedness as would be expected if asexual recruitment was predominant, with some evidence that both asexual and sexual propagules are important for population structure, whereby sponge fragments that disperse and reattach may contain incubated sexual propagules.

Larval settlement and metamorphosis is thought to be influenced by a variety of environmental stimuli (such as light, gravity, physical and chemical features of the substrate), with the former best studied to date. There are examples of both photonegative and photopositive responses among the phylum. Larval competence (the threshold and duration of larval maturity required for settlement) is not thought to be as important for sponges as for many invertebrates since the high cellular totipotency allows fragmented larvae to attach unselectively. Growth rates, regenerative abilities after damage, and longevity is still poorly understood, but what little is known to date demonstrates that these vary considerably across groups of sponges and the habitats they occupy. Using various direct (C14) and indirect measurements (e.g. growth rate extrapolation indices), some species are known to reproduce and die in less than one year (such as some soft-bodied Haliclona spp.), or are highly seasonal in their growth, biomass and ultimately survival (Chon-drilla australiensis); some species live for many decades (Aplysilla sp.), to over 400-500 years (Astrosclera wil-leyana) and it is claimed that sponges belonging to the hexactinellid family Rosellidae living in Antarctica are among the oldest living animals on the planet, with individuals estimated at 1515 years old.

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