Agarum is often called colander weed because the blades are
perforated by numerous holes. No one really knows why.
A brown seaweed with a broad leafy blade, perforated by numerous holes, should be familiar to most divers on both the east and west coasts of Canada. In Newfoundland (Canada), I have heard it called "holey kellup," but there seems to be a tendency among divers to use the name "colander weed." Biologists, however, recognize it as a member of the genus Agarum. On the Pacific coast of Canada there are two similar species, Agarum fimbriatum and Agarum cribrosum, while the Atlantic has only the latter species.
Agarum belongs to the brown seaweed order Laminariales, or kelps. Like all kelps, Agarum shows an alternation of two separate generations - that its, two completely different types of free- living plants. The large blades which we see are the diploid (they have two complete sets of chromosomes, so all chromosomes occur in pairs - we humans are diploid) generation called the sporophyte. You may have noticed dark patches on Agarum blades. Within these dark patches numerous single-celled spores are produced by meiosis (type of nuclear division, in which the chromosome number is halved in the same way is is when we humans produce gametes). These spores are able to swim about by means of tiny hair-like structures called flagella. The spores settle to the bottom, where they germinate into haploid (have half the diploid chromosome number) microscopic plants called gametophytes. When they have reached a size of only few cells, the gametophytes produce motile (free swimming by means of flagella) sperms and non-motile eggs. Sperms must first swim to the egg which, after fertilization, develops back into the next sporophyte generation, thus completing a cycle of events known as a "life cycle."
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Agarum is often called colander weed because the blades are
perforated by numerous holes. No one really knows why.
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