Created by Alice Chiu, 2003. Images may be used with permission.
Prionitis lanceolata, like all good red algae, has a three-phase life history that involves a gametophyte stage, a carposporophyte stage, and a tetrasporophyte stage. Other characteristics include non-motile gametes and a carposporophyte reliant on the gametophyte.
gametophyte stage is what you would likely see of Prionitis if
you're just taking a stroll (or scramble) through the intertidal. Reproductive
structures are concentrated on the lateral pinnules* closest
to the tips of the blades. Gametophytes are haploid and produce haploid
gametes. Male gametes (spermatia) and female gametes (carpogonia) are
produced on the same plant; however, Prionitis cannot self-fertilize.
None of Prionitis' gametes have flagella. Thus, spermatia are simply released into the water and are at the mercy of the waves to carry it to a carpogonium. Carpogonia remain in the gametophyte, but they do their part by sticking out a long, skinny trichogyne to "catch" the spermatia as they float by. The spermatium then travels down the trichogyne to fuse with the egg cell (carpogonium). The fertilized carpogonium sends filaments into the medulla to find the auxiliary cell, which is a separate structure involved in reproduction. Once the auxiliary cell is found, a tube called the gonimoblast connects the auxiliary cell to the fertilized carpogonium so that the two can fuse. From here, the carposporophyte begins to develop.
diploid carposporophyte is never a free-living organism. It remains embedded
in the thallus of the gametophyte and draws some nutrition from the gametophyte.
As the carposporophyte matures, nearly all the cells will become diploid
carpospores. When the carpospores are mature, they are expelled through
the ostiole, a pore that connects the carposporophyte to the outside
world. The released carpospores will settle and grow into the tetrasporophytes.
The tetrasporophyte stage is the other apparent form in the intertidal (or subtidal). It is also diploid. Besides the different reproductive structures, tetrasporophytes and gametophytes look about the same, thus they are considered isomorphic. Tetrasporangia are concentrated on lateral pinnules toward the tips of the blades. They are located just beneath the surface of the thallus. Prionitis has cruciate tetraspores, meaning they undergo meiotic division to result in four tetraspores that form a "cross" shape. These haploid tetraspores are then expelled into the water and settle to become gametophytes, starting the cycle over again.