Prionitis,
so laden with epiphytes that you can hardly see it!
For
all its shifty nature, other algae love to hang out with Prionitis
lanceolata anyway! The suspect has been spotted in close
association with Microcladia, Polysiphonia,
and Hymenena,
as well as a collection of diatoms. These associations have
been determined only from a preliminary survey. Its network
of accomplices may extend much wider; however, preliminary
mug shots are available below as well as additional information
on why
epiphytes are attracted to P. lanceolata, how
they attach,
and how P. lanceolata defends
against them.
Hymenena growing
over a Prionitis blade. Prionitis is
definitely getting less light now!
Why on earth would other plants want to grow on Prionitis? Algae generally like to anchor to rocks, but such substratum* can be scarce, especially in the intertidal. Thus, epiphytes have adapted to be able to anchor to other algae, such as Prionitis. While this may be all fine and dandy for the epiphyte, the indirect effects can be detrimental to the basiphyte*. The epiphyte does not derive any nutrition from the basiphyte, only support. (Otherwise, it would be a parasite!) But even so, poor Prionitis has to compete with its epiphytes for sunlight. Having epiphytes hanging off it also means there is a greater surface area for drag to act upon, thereby increasing the likelihood of Prionitis being ripped from its anchor in rough water. The one possible advantage to being covered in epiphytes is that the basiphyte is less likely to be grazed upon. However, this is probably not much of an advantage to Prionitis since it is generally not grazed upon much anyway.
Left:
A young epiphyte begins its life on P. lanceolata
Right: Hymenena grows attached to P. lanceolata
Tetraspores or gametophytes settle on the surface of Prionitis and germinate there. The young epiphyte, such as the one above left, attaches to Prionitis by way of rhizoids, root-like structures that anchor either to the surface of Prionitis or actually penetrate the basiphyte’s cortical layer, depending on the species of epiphyte.
Hymenena's rhizoids attached to Prionitis
At
this point, Prionitis’ defenses kick in. P.
lanceolata employs two known methods to control its
epiphytes: cuticle peeling and a chemical toxin. P. lanceolata sloughs
off its outermost layer of cells. When it sheds this layer
of cells, it will also shed any young epiphytes that have
attached to the surface. P. lanceolata is also thought
to secrete a chemical toxin. While there are no direct studies
of this in the literature, several observations suggest this
may be the case. Young epiphytes (e.g., M. californica)
on P.
lanceolata were observed to be dead long before cuticle
peeling occurred, with death occurring between 2-5 days after
initial settlement, leading to the supposition that a chemical
toxin was at work. Also, P. lanceolata shows no
signs of grazing, and food-choice experiments with four herbivores
and a selection of algae show Prionitis to be ranked
low by all species. These observations also suggest that Prionitis secretes
some sort of toxic, or at the very least, repellant chemical.
These defenses work
for some species, such as M. californica. However, M. coulteri has
found ways around Prionitis’ defensive efforts by sending
rhizomes into the basiphyte’s cortex. Since M. coulteri actually
penetrates the host tissue, it is unaffected by cuticle peeling. It has also
apparently evolved an ability to grow in spite of Prionitis’ chemical
toxin. This is very evident by the luxurious growth of M. coulteri on
the Prionitis specimen
at the top of the page!
Hymenena is a beautiful member of the order Ceramiales, Rhodophyta. Close-ups are available in the sections above.
Polysiphonia is another member of the Ceramiales. Be prepared to be stunned by its microscopic polysiphonous construction. For a full dossier on Polysiphonia.
Microcladia totally
overgrowing Prionitis
Yet another Ceramiales, two species of Microcladia, M. californica and M. coulteri, have played special roles in expanding our knowledge about the relationships between epiphytes and their basiphytes. See Microcladia in all its fractaled glory.
Diatoms
attached to a young epiphyte on Prionitis (left)
and on Prionitis itself (right)
Diatoms
are golden-brown algae of the division Chrysophyta. They
are unicellular and have a two-part cell wall made of silica. More
about diatoms.
Created by Alice Chiu, 2003. Images may be used with permission.