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Ecological Implications of a Complex Life History

There are different ecologies associated with the varying morphologies of the Mastocarpus life history. In fact different stages actually prefer different growth temperatures. The optimal growth temperature for the tetrasporophyte is 12-15 degrees C, while the gametophytes experience optimal growth at 17-25 degrees C (Zupan and West 1988).

In order to look into the ecological implications of the Mastocarpus life history, I walked two mid-intertidal transects here at Hopkins Marine Station in Pacific Grove, CA. One transect was rocky and wave exposed, while the other was composed of rocks interspersed with sand. The second transect was less wave exposed due to the gradual drop in depth off the beach. I stopped at random points along the transects and recorded the first two Mastocarpus forms I saw as either tetrasporophyte (tspt) or gametophyte (gpt). Only female gpts were seen.

On the rocky transect, I counted 36 tspts and 4 gpts. On the sandy transect, I counted 6 tspts and 30 gpts. Essentially, there was a very obvious difference between tspt and gpt abundance on the two transects.

The likely explanation for this difference comes from the 'bet hedging hypothesis' presented by Paine et al in 1979, which is to say that the tspt is poor at establishing, yet rather permanent and long lived once established, while the gpts are good establishers, yet short lived. This would explain the relative abundances of tspt and gpt on the sandy plot, where sand scouring could prevent tspts from getting established. However, it does not explain the low abundance of gpts on the rocky transect.
To explain this is more complicated and less definite. I propose that there may be a difference in life history over the short distance (30 meters or so) between the two transects. Mastocarpus is known to follow both sexual and apomictic life cycles (see life history page). Suppose the two transects represent two populations.
The rocky population may be following a sexual cycle, while the sandy population follows a primarily apomictic cycle. The rocky population would then see an increase in gpts when the majority of tetraspores are released and then an increase in tspts as the carpospores are released. There were many young tspts in the rocky area along with some older ones (up to 67 cm dia.).
The sandy population could see high female gpt abundance year round. With few tspts successfully establishing, sexual reproduction would be low and only seasonal. Measurements would have to be taken for a year to validate this theory.

What this could mean is that the two life histories could have a temporal variance, operating essentially independent of each other. This could also mean a genetic disposition to one or the other life history, though entirely female populations have not been found. To Mastocarpus, this variance in life histories means it can very successfully reproduce asexually in the absence of tspts. This enhances dispersal capabilities and survival of the species.

Zupan and West made a study to investigate the variation in the relative amount of sexual development and direct development (apomictic) in Mastocarpus papillatus over the southern half of the alga's range. From the Oregon-California border south to El Rosario, Baja California, they collected mature papillae off of female gametophytes and cultured them in the lab. What they found was that relative amount of direct development rose as they moved north along the coast. They also saw very little of the tetrasporophyte (source of male gametophytes) in some of the southern field sites. The pattern of more direct development heading north coincided with an earlier study of Mastocarpus papillatus from Monterey Bay, CA up to Alaska.

There is no hard explanation for the patterns seen. Warmer waters in the southern sites could inhibit tetrasporophyte proliferation, but tetrasporophytes obviously exist because sexual development occurs. It is important to note that in Zupan and West's study, they found some southern sites with all sexual development but none with all direct development. So sexual life history is a part of all populations. Colder waters up north could have inhibitory effects on fertilization, accounting for the decrease in sexual development in the more northern sites. It would be interesting to investigate genetic differences amongst these populations.

Mastocarpus pages copyright W. Ludington 1999