Monterey Bay Aquarium Research Institute
Marine Botany
Seagrasses....

...The face and shoulders appeared of human form, and of a reddish colour; over the shoulders hung long green hair; the tail resembled that of a seal, but the extremities of the arms he could not see distinctly (Weddell, J. in Campbell, D.).

The emerald green blades of the seagrasses sweeping through the water do remind one of mermaidsí tresses flowing in the sea. Seagrasses are found in temperate and tropical seas of the world, mostly in the low zone of the intertidal but sometimes also completely submerged far from land (den Hartog, 1970, maps p.19-28). This paper will concentrate on the genus Phyllospadix which is restricted geographically to the Northern Pacific.

Although seagrasses flourish in the aquatic realm of the algae, they are true Angiosperms, and have several unique characteristics that distinguish them from the terrestrial vascular plants and allow them to thrive in a saline environment.

Ecology of Seagrass meadows

Dawes (Dawes, p.481) assigns six distinct roles to seagrass meadows as important ecosystems:

ïSeagrasses contribute to the primary productivity of the oceanís photosynthetic biomass.

ïThe marine angiosperms are highly efficient in removing nutrients from surface sediments and from water. Additionally, they replenish nutrients and oxygen in the soil via the roots.

ïAs true plants, they possess a highly specialized root system that traps sediments, improving water clarity. Their importance in bottom stabilization has also prompted recent interest in their transplantation to coastal areas prone to erosion.

Seagrasses can also function as a natural sewage filtration. A case was reported in Australia where the removal of an eelgrass meadow resulted in the poisoning of the benthic biota that had been unexposed to the raw sewage by the shelter of the dense vegetation (Phillips & McRoy, 300).

ïSeagrasses are a direct food source to many herbivores, (ie. Strongylocentrotus purpuratus ) and also indirectly at the base of many food webs. In one study, over 340 animals analyzed showed some level of seagrass consumption (Fry and Parker, 1979).

ïTheir dense vegetation offer shelter and different habitats to a myriad of organisms by creating a calm and shaded micro-environment. Seagrass beds serve as permanent and seasonal residencies for many species of fish, and often simply as forage areas. The plantsí protected bases are home for juvenile lobsters in southern California. The need for the conservation of seagrass meadows was not fully recognized until one study showed that a decline in an eelgrass bed, one of the most commonly studied genera, was followed by a sharp decline in the numbers of decapods and both juvenile and young adult stages of many commercially important fish (Dawson & Foster).

ïThe vegetative mass of the seagrass provides an abundant substrata for attachment, especially for algal epiphytes which themselves contribute to the primary productivity and are food for many small fish and invertebrates. Often seagrasses appear purplish or pink, this is due to the profuse growth of two of its most common obligate epiphytes: Smithora naiadum, a fleshy Bangiale and Melobesia, a crustous red algae (Dawson, p.178). Colonies of the bryozoan genus Membranipora are also common dwellers on the blades of seagrass (pers. observ.).

In the intertidal environment where space is a limiting factor, the importance of seagrass in providing substrata for other organisms to live on cannot be overstated. All parts of the plants are maximized by other algae and animals: the leaves, stems, rhizomes, and the leafage cover (Phillips and McRoy).

Phyllospadix: Surf-grass

Of the sea grasses, Phyllospadix, commonly known as surf-grass, is unique because it is the only flowering plant genus that grows predominantly on hard substrates in the littoral zone exposed to the most violent surf (Cooper & McRoy, 1988).

Taxonomy: Marine Angiosperm, Division Magnoliophyta, Family Potamogetonaceae, SubFamily Zosteroideae (eel-grass family), Order Helobiae

There are five described species of the genus Phyllospadix: :

scouleri, torreyi, and serrulatus commonly found on the North American Pacific coast, and japonicus and iwatensis, on the coasts of northern Japan and eastern Korea.

Distinguishing Characteristics of the 3 N. American Pacific species

ïTorreyi - (locally found in Elkhorn Slough, Asilomar state beach) Commonly grows on less exposed areas. Generative axis has 4-6 internodes, upper 2-4 nodes bearing 1-5 pedunculate spathes, leaves are 1.5-5 mm wide. The retinaculae of the female spathes are narrow at the base while that of scouleri is not.

ïSerrulatus - (Alaska)

The generative axis is reduced to single pedunculate spathe, rhizome has internodes covered with a mass of yellowish brown fibres with 2 roots, leaves have 3-7 nerves. The female spathe is obtuse, truncate or retuse. The leaftip is truncate.

ïScouleri Hook- (from Dundas Isl, in the north to Baja California, reaching tropic of Cancer, locally in Pacific Grove, Pt. Pinos and Pt. Lobos) From low intertidal to 6m deep, common in the north but in the south almost replaced by torreyi. Generative axis 0.25 -5(10)cm long, 1-2 creeping internodes with 6-10 roots, upper node bearing 1 or rarely 2 pedunculate spathes, leaves 1-4mm wide with 3 nerves : 1 in midrib to apex, two intramarginal, also cross veins. The sheath is 4-30cm long, the leaf blade 0.5-2m long. The tussock can grow up to 0.5m in diameter.

Marine Angiosperms

The seagrasses are herbaceous vascular plants that have such reduced vascular bundles it is often difficult to distinguish the xylem and phloem. The xylem is located towards the upper epidermis in the center of the vascular bundle surrounded by the phloem which is found toward the lower epidermis.

As marine angiosperms, sea grasses successfully fulfill the

requirements for life at sea outlined by den Hartog (1970, p.12):

1. They are adapted to life in a saline medium. Osmoregulation is achieved by specialized epidermal cells and the leaves have no need for stomates, as opposed to terrestrial plants, because all of the gas-exchange is accomplished through the epidermis.

2. Seagrasses must have the ability to grow when completely submerged.

3. Surf grass have a successful anchoring system to withstand tidal currents and moderate wave action.

4. Are able to reproduce in an aquatic medium. This adaptation called hydrophilly, which is unique to aquatic plants, allows the surf grass to perform both surface and completely submerged pollination.

The leaves of surf grasses are especially adapted to living

in a turbid aquatic medium. As mentioned above, they have no stomates that open to the exterior. The interior of the leaves is composed of specialized parenchymous tissue called aerenchyma that has regularly arranged air spaces or lacunae. These internal air spaces serve for flotation and gas exchange purposes (Arber, p.187). Furthermore, the fully aquatic angiosperms have chloroplasts in the leaf epidermis (not found in terrestrial plants). The leaves are flat and supple. The reduction of vascular bundles and the absence of lignification renders the leaves the ability to remain erect in strong water action. The natives of the Pacific coast used to weave baskets with seagrass because it is so flexible and and stiffens when dries. It also resists rotting and for this same reason it was used as stuffing material in the former U.S.S.R. (Phillips & McRoy, p.301). The leaves are also very strong. The stress force for a Phyllospadix leaf blade was measured to see how much force the blade could withstand before breaking. The expected tensile strength for macroalgae is between 0.7-10 MN/meter sq. (Denny et al, 1989) and that of Phyllospadix was calculated to be 10.2 which is highly beneficial in the surf-exposed zone where the plant grows (pers. observ.).

A 1988 study by McRoy & Cooper described the anatomical adaptations that allowPhyllospadix to thrive on rocky substrates in the surf exposed zone in comparison to Zostera marina , more commonly found in sheltered areas of the coast. Phyllospadix shows significantly more root hair growth than Zostera which suggests that it is able to absorb inorganic nutrients through the roots and might also provide the extra attachment force needed in its habitat. The roots and rhizome of Phyllospadix are also characterized by thicker outer epidermal walls. The lacunae are reduced because the plants live in a highly oxic, well-mixed environment. As would be expected for a plant that needs to be adapted to water motion in a turbulent surf-zone, Phyllospadix shows more extensive hypodermal nonlignified leaf fibers than Zostera.

The leaf epidermis and cuticle thicknesses of the three eastern Pacific species were also compared in two sites: Cape Arago, Oregon (CA) and Sitka, Alaska (SI). Leaf thickness was found to be negatively correlated with tidal height; the thinner leaves appearing at higher zones (McRoy & Cooper, 1988). Epidermal wall thickness also followed this pattern of variance at Cape Arago, but not at Sitka, perhaps because the of lower air temperatures in the high intertidal areas in Alaska and SI plants might need more protection against the cold air (McRoy & Cooper, 1988).

Reproduction

Papa Phyllospadix said to Mama Phyllospadix:

"My, what long green hair you have! "

(For a long-lasting perm, apply the dry treatment.)

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 performs completely submerged pollination. The threadlike pollen (5 microns in d, 1000 microns long) 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. Upon maturity, the spadices 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 (pers. observ.). 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 a red coralline alga or Gracilaria (pers. observ.). Germination occurs immediately after attachment to an alga. Upon growth of the cotyledon, leaves sprout from the ensheathed plumula and the cotyledon dies (den Hartog, p.98). 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.

Williams (1995) confirmed the fact 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, the ocurrence of fertilized ovules is highly abundant which is explained by the 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) so fertilization is not limited by pollen availability. (It would be very interesting to study the hydrodynamic processes and probabilities that govern the dispersal of pollen in a turbid aquatic medium (Denny & Shibata 1989).) 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 (Lloyd and Webb 1977 as cited in Williams 1995). The most expensive sex will then be produced scarcely and 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.

Seagrass communities are important ecosystems that need to be studied and conserved because currently they are in danger of being decimated by human activities on the coast. And when seagrasses disappear, the habitats of many kinds of fish, algae, and invertebrates also disappear and an incredibly diverse biofauna is lost.

Phyllospadix pages copyright Erika Marin-Spiotta 1996.


AuthorErika Marín-Spiotta1996. The legal stuff. You gotta love the weeds. Check out the rest of the Hopkins algae pages. Gracias to people.
Last updated: Feb. 05, 2009