Reproductive Tissues Respond to Favorable Conditions

close up photos of green plant on top of brown sand

Biological Strategy

Rockweed seaweed

Jeanette Lim

Image: Steve Sullivan / Flickr / Some rights reserved

Regulate Reproduction or Growth

Reproduction and growth are two physiological processes that occur in all living systems. There are situations when conditions are right for both, and other situations when continuing either harms the living system because both have a very high energy cost. Reproduction and growth are unique in that both can stop until conditions improve, although stopping either for an extended time can cause problems. An example of regulating reproduction is a process called delayed implantation or embryonic diapause found in some mammals, such as otters. An otter’s embryos sometimes temporarily cease developing and won’t develop further until the female senses that conditions are suitable.

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Sense Chemicals (Odor, Taste, etc.) From the Environment

Chemicals are important for signaling and communication among living systems, either intentionally (such as when two living systems try to find one another) or unintentionally (such as when a plant emits a chemical signal that an herbivore can use to find a tasty bite). They are also important for other uses, such as navigating or finding sources for minerals. But chemical signals are often relatively weak and disperse when moving through water or gases. Therefore, detecting them requires specialized abilities, including a way to determine where they are coming from. A well known example of sensing chemicals can be seen in ants following a pheromone trail laid down by others in their colony to locate a quality and abundant food source.

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Respond to Signals

To interact with its environment, a living system must not only sense a variety of signals, but also respond to them. To be energy- and material-efficient, those responses must be appropriate to the signal. This generally requires sensing thresholds to trigger an appropriate level of response (for example, hiding under a shrub versus running away to avoid a predator). Response strategies are tied to a specific signal and often have a response threshold, which determines how strong a signal must be to warrant expending energy to respond. One example is a plant that lives in arid regions in South Africa. Its seed capsules remain closed until rainfall triggers them to open to release the seeds. But the plant only responds to a second rainfall, thus protecting against releasing its seeds before there is enough sustained water for them to grow.

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Plants

Phylum Plantae (“plants”): Angiosperms, gymnosperms, green algae, and more

Plants have evolved by using special structures within their cells to harness energy directly from sunlight. There are currently over 350,000 known species of plants which include angiosperms (flowering trees and plants), gymnosperms (conifers, Gingkos, and others), ferns, hornworts, liverworts, mosses, and green algae. While most get energy through the process of photosynthesis, some are partially carnivores, feeding on the bodies of insects, and others are plant parasites, feeding entirely off of other plants. Plants reproduce through fruits, seeds, spores, and even asexually. They evolved around 500 million years ago and can now be found on every continent worldwide.

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Reproductive tissues of rockweed seaweed release eggs and sperm in favorable conditions by chemically sensing calm waters.

The rockweed, Fucus distichus, is a branching brown seaweed that lives on rocky shores in the Northern Hemisphere. It makes its home in pools of seawater formed among rocks high in the intertidal zone, which is the sloped shoreline exposed between high and low tides. When the tide is high and waves are crashing onto the rocks, water often flows in and out of these high tide pools. During low tide, however, these tide pools are often calm.

Calm waters are important for reproduction, as the rockweed releases sperm and eggs into the surrounding water where external fertilization occurs. If a tide pool is being flushed with water, this can prevent successful fertilization by making it hard for the eggs and sperm to come into contact, or by carrying eggs and sperm away. How does the rockweed sense water motion that’s ideal for reproduction?

It appears that sperm and egg release is triggered by low amounts of carbon-containing compounds (like carbon dioxide) dissolved in the water, indicating little mixing with seawater outside the pool. The rockweed takes up and uses this carbon during . When high tide pools are cut off from the ocean during low tide, this carbon uptake depletes the available carbon in the pool. Furthermore, when the pool water is calm, a largely stagnant layer of water builds up around the surface of the rockweed. Carbon dissolved in the water diffuses very slowly across this stagnant layer, reducing the rate at which it’s absorbed through the rockweed’s surface. This reduction in carbon uptake is sensed by tissues that perform photosynthesis in the seaweed. Through a mechanism that has yet to be fully understood, this signal is conveyed to the reproductive tissues at the tips of the rockweed’s branched blades. Internal compartments in these tissues contain sperm and eggs that are then released through holes in the seaweed’s surface into the calm tide pool water. In this system of relayed signals, the low level of usable carbon serves as a signal that the tide pool is calm and under favorable flow conditions for external fertilization.

Image: Steve Lonhart / NOAA MBNMS / Public Domain - No restrictions

Image: NOAA Office of Response and Restoration / CC BY - Creative Commons Attribution alone

Image: Gnexxnews / Public Domain - No restrictions

Last Updated April 20, 2018

References

“The release of gametes by Fucus distichus occurred in still water under high light and was associated with the depletion of dissolved inorganic carbon (DIC) in tide pools isolated from the ocean during daytime low tides…” (Pearson et al. 1998:1725)

“…our field and laboratory studies provide support for a model in which hydrodynamic conditions are sensed chemically via changes in the supply of inorganic carbon across the boundary layer.” (Pearson et al. 1998:1726)

Journal article

Control of gamete release in fucoid algae: Sensing hydrodynamic conditions via carbon acquisition

Ecology, 79(5): 1725-1739 | 01/01/1998 | Pearson GA; Serrão EA; Brawley SH

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Reference

“The reproductive tissue of most fucoids (sensu Fucaceae) develops as apical swellings termed receptacles (Fig. 1A), within which form many spherical or flask-shaped conceptacles containing the gametangia (both antheridia and oogonia in hermaphroditic species). Each conceptacle has an ostiole opening to the surface through which the gametangia are released during spawning (Fig. 1B). The gametangia then sink to the bottom. The gametangial membranes loosen and break down rapidly in contact with seawater to release eggs and sperm.” (Pearson and Serrão 2006:588)

Journal article

Revisiting synchronous gamete release by fucoid algae in the intertidal zone: fertilization success and beyond?

Integrative & Comparative Biology, 46(5): 587-597 | 01/01/2006 | Pearson GA; Serrão EA

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