Presentation on theme: "Phylum Porifera: Sponges Chapter 6. Gemmules Large mass of archaeocytes Formed during harsh conditions Environmentally resistant When ready, archaeocytes."— Presentation transcript:
All sponges reproduce sexually and produce motile larvae. 1) Viviparous or incubating- eggs develop in the sponge cells until the larval stage. 2) Oviparous release gametes (oocytes and spermatozoa) into the water and fertilization and larval development takes place in the open water.
The spermatozoids are derived from choanocytes. The choanocytes (C) start reabsorbing the microvilli of their collars while the cell body fills out in the choanocyte chamber (Cch).
The transforming choanocytes (C) divide many times, and detach themselves progressively from the inner wall of the chamber.
Spermatic follicles (Fo) are disseminated among the choanocyte chambers (Cch) in the mesohyl.
The origin of the oocytes in sponges remains uncertain. They may have a choanocyte origin or a archeocyte origin. At an early stage it has a central nucleus (N) and is partially surrounded by follicular cells (Cf).
Throughout its maturation the oocyte grows and becomes very large. It accumulates enormous quantities of vitelline (Vg) in the cytoplasm.
At the end of oocyte formation the cytoplasm is completely taken over by vitelline (Vg) reserves and the follicular envelop is a thin layer of flattened cells (Cf).
Fertilization The Smoking Sponges Mature sperm and oocytes are released into the environment through the aquiferous system. Fertilization takes place in the water (ovipery).
Fertilization in Viviparous Sponges After release the sperm is taken in to the aquiferous system. Indirect fertilization in viviparous sponges constitutes one of the most unusual phenomenon in the animal kingdom.
Fig 6.14 The sperm never penetrates the oocyte directly, but instead it penetrates an intermediary cell usually a choanocyte.
Fig 6.14 The choanocyte transforms itself by reabsorbing its collar and flagellum while the sperm enclosed in a vacuole is modified to become a nonflagellar sperm. This carrier cell assures the transport of the sperm to the oocyte
After fertilization the fertilized egg begins to divide, and the cells begin digesting the vitelline (Vg) reserves.
The Larva Three basic larval types have been described in sponges. 1) Coeloblastula larva 2) Parenchymula larva 3) Amphiblastula larva
The Parenchymella larva is generally ciliated and solid.
The amphiblastula larva is a hollow larva made up of two distinct areas: small ciliated cells at the anterior pole (Pa), and large cells at the posterior pole (Pp). The larva is located in one of the large tubular choanocyte chambers (Cch). It is released through the osculum, swims freely for a few days and attaches to a substrate. The anterior ciliated cells differentiate and become choanocytes.
The Big Picture Sponges are one of the most primitive metazoans with cellular organization. It is their aquiferous system and totipotent nature of cells that allows them to achieve such large sizes and diversity of body sizes.