Reproduction in sponges [Phylum Parazoa]
This is by both sexual and a variety of non-sexual methods. All sponges appear to bear both ova and spermatozoa, although there has recently been a suggestion that this is not invariable, and fertilization may be internal or external.
With internal fertilization the embryos develop inside the sponge, which is then said to be viviparous; with external fertilization, the sponge is regarded as oviparous or egg-laying. When fertilization is internal the spermatozoa are presumably carried into the body by the inhalant currents. In the breeding season, viviparous sponges bear large numbers of embryos scattered through the body.
Non-sexual reproduction includes purely vegetative methods, in which the body may fragment, each piece being capable of forming a new sponge, or buds may be formed, either on the surface of the body or internally; the seed-like gemmules of freshwater sponges belong to this second category. In addition, asexual larvae identical in appearance to sexually formed larvae may be produced.
In oviparous sponges, the ova can be seen lying in the choanosome, but sperms and larvae have never been identified. Presumably, the ova are fertilized in the sea after leaving through the exhalant canals and vents.
We know more about the sexual process in the viviparous species. Arthur Dendy was the first to investigate fully the origin and growth of the ovum. According to him, the first step is the migration of a collared cell from the flagellated chamber into the tissues surrounding it.
As it migrates it withdraws its collar and flagellum and becomes amoeboid. Other amoebocytes heavily laden with nutrients move towards it, and these Dendy named ‘nurse-cells’, assuming that they pass their nutrients into the developing ovum, which becomes extremely large and is filled with yolk granules. There is some doubt whether the ova may not, in some species at least, be derived from amoebocytes already within the choanosome.
The segmentation of the embryos is sometimes regular and sometimes irregular, but in all instances, a larva leaves the parent body by one of the vents. There are no special reproductive organs or special sex cells, and in the few instances investigated sperm mother cells and ova appear to be derived from modified collared cells or amoebocytes.
The larva breaks free from the parent at this time and the
Thus it is not possible to speak with precision about an ‘individual’, nor is it justifiable to speak of a colony. Some authorities have sought to get over the difficulty by referring to the ‘sponge person’ which is only another way of calling it an individual, or of the ‘sponge body’, meaning a sponge that looks like a single individual, although it may be composed of several that have coalesced, or maybe one of many that originated from a single ovum.
This plasticity is reflected in other ways, one of which can be demonstrated by the classic experiment of squeezing a piece of the living sponge through fine silk. The tissues become dissociated and pass through the meshes of the silk as a milky fluid, consisting of hundreds of cells separated from their fellows and amoeboid in shape and behavior. Usually, the collared cells withdraw their collars and flagella and cannot be recognized as such. All the cells wander at random, moving by means of pseudopodia, but after a lapse of several hours, they begin to gather together in clusters.
In some species, the individual cell walls seem to disappear as the clusters grow so that syncytia
continuous masses of cytoplasm with many nuclei are formed. These behave like giant multinucleate amoebas and in their turn wander, meet and coalesce. Eventually, rounded masses are formed in which the cell walls have reappeared, so that they do not differ from normal post-larvae. Such regenerative cell masses have been kept in aquaria for several months until they were sexually mature and producing their own larvae.
Under natural conditions multiplication by regenerative cells, and masses take place in several ways. One is the formation of internal buds or gemmules. These are most typical of freshwater sponges but have also been found in a few marine sponges. The formation of a gemmule is preceded by the streaming together of numerous small amoebocytes, heavily charged with granules, presumably nutrients.
These eventually form a spherical mass around which other amoebocytes lay down a horny capsule. The third set of cells secretes special spicules in the parent choanosome and transports these to the developing gemmule where they are laid down in the walls of the capsule and so strengthen.
In freshwater sponges the parent body dies and disintegrates in due course, leaving the gemmules to tide over the winter and provide the crop of sponges for the ensuing year. In the spring the contents of each gemmule flow out from the shelter of the capsule and proceed to behave like normal sexually produced post-larvae.
There is evidence that when spicules are extruded some of the amoebocytes transporting them are carried to the exterior of the spicules. They fall with them to the sea bed, move about, meet, and form regenerative cell masses, which it seems may develop into adult sponges.
Another variation of reproduction by means of a regenerative cell mass is seen in external budding. The Sea-orange (Tethya aurantium), which is oviparous, regularly reproduces by means of buds. The skeleton in this species is composed of bundles of long, needle-shaped spicules radiating from the center of the body to the surface. Amoebocytes migrate along with these bundles and collect beneath the ectosome, which is consequently pushed up into low warts.
Eventually, these grow out into stalked buds, the amoebocytes inside meanwhile becoming organized into tissues complete with collared cells. As the buds ripen their stalks become enormously long and attenuated, and when they break the miniature sponges float away to settle on a rock surface and grow into adults.
Perhaps the most remarkable method of reproduction involving regenerative cell masses is the formation, non-sexually, of larvae that simulate sexually produced larvae. Instead of an embryo produced by segmentation of an ovum, there is formed a precisely similar structure through an aggregation of amoebocytes, resulting in a larva that is indistinguishable from one derived from the fertilized ovum.
This ability of separate or separated sponge cells to come together to form multicellular bodies supports the views of those who regard sponges as little more than colonial protozoans. However, recent research has shown that certain
mammalian cells grown in culture media behave
in a similar way, coming together to form rounded masses surrounded by a single-layered capsule. The implications of this are not easy to see.
Non-sexual reproduction is not peculiar to sponges, but they are remarkable for the variety of ways in which they multiply without the intervention of the sexual process. Although non-sexual reproduction is more characteristic of the plant kingdom than the animal kingdom, this multiplicity of non-sexual methods in sponges was unknown in the days when they were mistakenly regarded as plants.
The remaining non-sexual methods of reproduction involve fragmentation. That seen in the sponge Halichondria quality, for instance, is a simple natural example. This sponge often forms reticulations of slender branches and is most abundant just offshore. During violent storms, it may be broken up into many pieces, and occasionally tens of thousands of these may be cast up on the beaches, their damaged surfaces already beginning to heal. Doubtless many become attached to pebbles and rocks and grow into new sponges.
Similarly, commercial sponges can be artificially propagated by cutting a mature sponge into a number of small ‘seed sponges’, fastening each to a concrete disc, and returning them to the sea.
There are several species that can be said to adopt this process naturally. The first in which this was fully demonstrated was the Purse Sponge (Garantia compressa), which grows between tide marks, and is usually one or two inches high. It has also been seen in Scypha Ciliata, another common littoral species of calcareous sponge, and the indications are that it may be more widespread.
In these two species fragmentation takes place by the formation of lines of weakness across the sponge body, associated with folding and splitting, the end result being that the sponge breaks up in a variety of stereotyped patterns, sometimes accompanied by budding, the two processes proceeding simultaneously.
It has been possible to show that such fragments reform to produce a shape like that of the parent, with the formation of a vent and a stalk for attachment. And although it has not been satisfactorily demonstrated that all such fragments become attached and thereafter grow in the normal manner, enough is known to postulate reasonably that a high percentage of such fragments may survive.