Those of us who can't pass by some bubble-wrap without popping at least a few bubbles are on good terms with seaweed this time of year. The tips of the seaweed fronds have swollen into balloon-like structures that are inordinately satisfying to pop. These little balloons are the reproductive part of the seaweed. The receptacles are readying to broadcast sperm and eggs into the water, in hopes of generating baby seaweed.
The seaweeds with swollen tips are members of the genus Fucus (commonly called rockweeds). The different Fucus species have a variety of common names that are often used interchangeably.
Some of the Fucus species have swollen air bladders in addition to the receptacles; these are usually called bladderwrack.
One species grows in a lovely, twisting spiral — this is spiral wrack.
All grow attached to intertidal rocks and piers, and so all are often referred to as rockweed. Whatever their common name, you can recognize this group of macroalgae (seaweeds are in fact algae, members of the Protista Kingdom, not the Plant Kingdom as is commonly believed) by their brownish-tannish-green color, their habit of growing on intertidal rocks, their flattened, dichotomously branched blades, and their swollen receptacles.
In the summer, the tips of the Fucus species swell and form rounded, hollow receptacles that contain the sex organs of the seaweed. If you look carefully you can see that the swollen part is studded with pores — openings that lead into spherical cavities that contain the structures that produce sperm and eggs. If you were to squeeze the contents of a fresh receptacle onto a slide and examine it with a microscope, you might see the motile sperm swimming about.
Some species of Fucus, like Fucus spiralis (spiral wrack), are hermaphrodites, producing both sperm and eggs.
Other Fucus species, typically ones that grow lower in the intertidal zones, are either male or female, producing either sperm or eggs.
The sperm and eggs are usually released on the rising tide. During low tides the receptacles dry out, which causes them to contract and squeeze the gametes (sperm or eggs) out through the pores onto the surface of the receptacle. As the tide comes in, the gametes are washed off the receptacle into the water. The eggs release a chemical that attracts the sperm, which cluster around it, with one lucky sperm fertilizing the egg.
The newly formed embryonic seaweed settles to the bottom and starts to grow.
Many other marine organisms time reproduction with the tidal cycles: the clam worms that I have written about, grunion (small fish that come in on the high spring tides to lay eggs in the sand), and sponges, to name a few.
Reproduction in the ocean can be so creative. Contact between the genders isn't always necessary when one can just broadcast sperm and eggs to the tides.
The next time you consider popping a rockweed, think about what you are doing to all of that potential for life that you are holding in the palm of your hand.
Sue Pike of York has worked as a researcher and a teacher in biology, marine biology and environmental science for years. She teaches at York County Community College and St. Thomas Aquinas High School. She may be reached at spike3@maine.rr.com.
