Monterey Bay Aquarium Research Institute
Marine Botany
SEAGRASS Courtship

Papa Phyllospadix said to Mama Phyllospadix: "My, what long green hair you have! "

Phyllospadix can reproduce both asexually via monomorphic vegetative plants and sexually by flowering. The peak reproductive months are from May through August although flowering plants can be found all year round. A dioecious perennial, it is one of the seagrass genuses that can perform completely submerged pollination.

The threadlike pollen (5 microns in diameter, 1000 microns in length) is released from the male flowers during the flood after a low tide. There are no known animal vectors for cross-pollination (Williams, p.1954) and it is believed that dispersal of the pollen is due solely to water movement. The seeds themselves are negatively buoyant and contain no endosperm.

The male or female flowers grow in a zig-zag pattern on a linear, sessile spadix contained within a spathal sheath.

surf grass seeds

Upon maturity, the spadix project out of the spathe. The fruit is crescent shaped with a pericarp composed of a soft exocarp and a hard fibrous endocarp (den Hartog, p.98). The fruits mature on the female spadix after fertilization and it seems as if development (growth in size) occurs either fastest or earliest in the center of the spadix, causing it to curl backwards and reveal the fruits. Perhaps this is what causes the spadix to tear through the spathe.

When the fruit detaches from the spadix, the soft spongy exocarp decays and the endocarp is laid bare. The endocarp deposits along the insides of two proximal processes a large number of long, thin walled cells which become stiff and bristle-like when the endocarp desintegrates. The two small "arms" with bristles allow the mature seed to hook on to another alga like a coralline.

Germination occurs immediately after attachment to an alga. Upon growth of the cotyledon, leaves sprout from the ensheathed plumula and the cotyledon dies. A pair of adventitious roots develop and when they reach a few millimeters in length, they grow a dense, woolly covering of root-hairs near the tips that adhere firmly to the first solid object they encounter.

More Females than Males?

A study in 1995 (Williams) showed that there is a strongly female biased sex ratio in flowering shoots across the entire depth of a surf grass bed and proposed some ideas about this phenomenon of male reproductive rarity.
  • While males are scarce, pollen is not, because of a high pollen to ovule ratio across all depths (the highest average pollen/ovule ratio occurred at a water depth of 4.5 m and was measured to be 58,375, Williams, 1995, Table 6). The ocurrence of fertilized ovules is thus highly abundant so fertilization is not limited by pollen availability.
  • Female fitness, measured by the amount of seeds produced, is best at shallow depths where light intensity is highest (Williams 1995). Reproduction and growth are therefore limited by light and interestingly enough, also by sediment levels on the roots (Plechner 1996).

Why this reproductive bias? The current view on sex allocation is that in dioecious plants, adult sex ratios will be biased toward the sex that possesses the least biomass in reproductive and vegetative structures and therefore requires the least energy to produce and maintain(Lloyd and Webb 1977 as cited in Williams 1995). The most "expensive sex" in terms of metabolic energy will be produced only when indispensable. Williams (1995) found that the male mean biomass allocation to flowering shoots relative to the total biomass was slightly but not statistically higher than in females. Hence surf grass provides an example of how very subtle resource allocation differences can result in a dramatic population bias.

Although males are far less numerous than female plants, they are more abundant at deeper depths than in shallow waters within the seagrass beds. One hypothesis that tries to explain the spatial distribution of male Phyllospadix, is that males are more weakly attached than females and thus can survive better in less turbid waters farther from the tidal and breaking wave action. Williams showed (1995) that the mean force required to detach 30 shoots from male clones was 57.4 +-27 (n=46 clones) while the force for female clones was 78.4kg/m2s +-42.3 (n=46 clones).

There seems to be no obvious morphological explanation for the weakness of the male sex since rhizome size is the same in both sexes. The female plant has longer leaves which would hypothetically increase the drag force. Perhaps there exists a difference in the composition of the "glue" that fix the root hairs to the rocky substrata. Why the males are more weakly attached is yet a mystery. One possible explanation postulated by Williams is that the drifting of the male plants caused by uprooting might increase pollen dispersal. This, however, needs to be tested.


Phyllospadix pages copyright Erika Marin-Spiotta 1996.
Last updated: Feb. 05, 2009