Wave Exposure and Fucus

Introduction:  Brown algae of the genus Fucus are quite
common in the intertidal zone of rocky coastlines. It can be identified by
its brownish to olive-green coloration and fronds that branch
dichotomously with a clearly defined midrib and reproductive receptacles
at the tips (Amos et al. 1985). It has alternation of generations,
characterized by a diploid sporophyte generation and a haploid gametophyte
generation. The spores are produced in structures called sporangia.
(Keats, 1997) Gametes are produced within the receptacles in structures
called conceptacles. (Note that these are very similar terms and are
easily confused.) The conceptacles are lined with the male reproductive
organs, the antheridiophores, which are branched and produce biflagellate
antherozoids. The female reproductive organs are called oogonia; each
produces 8 eggs. When the specimen is exposed to air at low tide, it
desiccates and the gametes are released onto the surface of the
receptacle. There the gametes unite and form a zygotic cell, which will
eventually settle onto the substrate and form a new thallus. (Guiry, 1996)
Hydrodynamic forces have a significant effect on the relative sizes of
Fucus growing in wave-exposed and wave-sheltered areas of the intertidal
and subtidal zone. Specimens growing in sheltered areas are capable of
attaining a larger size than those in exposed areas attain because in the
latter locale wave forces tatter the thallus of the kelp. This pruning
prevents or reduces detachment from the substrate (Blanchette, 1997).
There is a possibility that this pruning also serves a reproductive
purpose; the pieces that break off might float away and establish new
populations in another location. (Norton et al. 1982) If this tattering is
a reproductive benefit, then the size of the Fucus would be
optimized to provide the most tattering, and thus the greatest
reproductive potential, while still maintaining a grip on the substrate
and avoiding lethal tissue loss. In Blanchette's experiment, she found
that exposed Fucus specimens had fewer receptacles, while sheltered
Fucus had a greater number of receptacles. If my theory were
correct, then a sheltered Fucus would have more receptacles in
order to maximize the potential for them to float away. This receptacle
raft could then transport spores or gametes and zygotes to a substrate
site farther away from the parent site than would be reached without
breakage of the Fucus thallus. If breakage is an advantage, then
one might expect to find that the structure of the plant provides for
breakage in an optimal location on the thallus, probably directly below
the receptacles. This would minimize general tissue loss while maximizing
receptacle release potential.

Purpose/Significance: If one of the goals in Fucus growth is
attaining "pruning size", then Blanchette's optimal size argument is
complicated. The "pruning size", the size that the plant must grow to in
order for pieces to start to break off, would be slightly larger than the
"optimal size", since part of the process the Fucus undergoes to
reach this optimal size involves pruning. The Fucus is growing not
only to a size that will prevent detachment from the substrate but will
also enable the Fucus to release rafts of receptacles. It is a
prime example of the tradeoff between survival and reproductive potential.
If the plant remains small, it is more likely to survive but will have
lower reproductive potential.
Conversely, if the plant grows large, its reproductive potential will
increase, but its chances for survival will decrease. (Blanchette, 1997)
The Blanchette study did not consider the possibility of a receptacle raft
as a vector for reproduction and only used the number of receptacles
remaining on the plant to calculate reproductive output. If the receptacle
raft theory is feasible, then the reproductive output of the plant may
actually be higher than first calculated, because there would be viable
zygotes produced on both the surface of the sessile plant as well as the
receptacles that were floating loose. This might even provide an advantage
to fertilization in close proximity to the parent plant because it would
allow a rapid long distance spread of the species and therefore would
benefit the overall population.
If evidence that Fucus has structural weaknesses that would
encourage breakage is found, then this supports the theory that the
detachment of a raft of receptacles is a viable method of reproduction
along with in situ zygote formation. If such a weakness is not found it
does not completely disprove the receptacle raft theory, because it may
still be feasible for long distance transport of reproductive material to
be successful, even if breakage occurs randomly along the thallus.

Question/Hypothesis: I hypothesize that pruning offers reproductive
benefits for Fucus by enabling zygotes to settle farther way from the
parent plant. I expect the wave-exposed plants to be more likely to break
and release receptacles. I also hypothesize that opportunities for tissue
loss by tattering are optimized by development of a breakage point.

Methods: I will first test to see ifFucus receptacles that
have been separated from a sessile specimen and left to float freely will
propagate and grow into mature Fucus plants. As a control I will
have ten tanks, each with six mature Fucus plants attached to rock
substrate. Five of the tanks will have wave-sheltered Fucus, and
the other five will have wave-exposed Fucus.  I will then have
twenty more tanks that will serve as the experimental tanks. Each tank
will contain a rocky substrate so that the zygotes will have a place to
settle. In ten of these I will place one kilogram of tissue lost from
Fucus that have been pruned using a high-powered water jet. Again,
five of the tanks will contain tissue from wave-sheltered Fucus,
and five will contain tissue from wave-exposed Fucus. The last ten
tanks will also be divided into five wave-sheltered and five 
wave-protected, and be filled with one kilogram of pruned Fucus tissue.
The difference will be that theFucus in these last ten
tanks will have been tattered by hand (and therefore not at the natural
breaking point). Because desiccation is a crucial stimulus for the release
of gametes, tides will be simulated by a periodic change in water level,
with the plants completely submerges and exposed at the extremes. The
average natural breaking point of both wave-sheltered and wave-protected 
Fucus will be calculated by analyzing percentage loss of length and
mass, and the breakage location of the water jet pruned Fucus. 

Possible problems: The most obvious problem that would occur with
this experiment would be that none of the plants produce another
generation. This could be due to a variety of problems that could occur at
any step in the reproductive process. The gametes may not have been
produced properly, the zygote may not have formed, and if the zygote did
form it may not have settled safely and begun growth. The experiment would
have to be performed again, perhaps with one large tank for each category,
as opposed to five separate tanks. Another problem that might be
encountered would be that the water jets would not be powerful enough to
break apart the Fucus, in which case perhaps naturally tattered
Fucus might have to be collected. 

Interpretation of Results: Growth of second generation plants from
the
pieces broken off by either method of tattering will prove that tattering
does not injure the reproductive status of the specimen and therefore this
is a viable way for Fucus to reproduce.  The release of a greater
percentage of receptacles by the wave-exposed plants would prove that
Fucus uses hydrodynamic forces to optimize its reproductive potential.
Conversely, if the wave-sheltered plants lose a greater percentage of
receptacles, this suggests that the development of more receptacles by
this group of plants might be directly related to optimizing the amount
detached to gain a reproductive advantage. If only the controls produce
second generations even after the tattered specimens are tested again to
reduce the possibility of experimental error then the receptacle raft
theory cannot be proven. It is always possible that the receptacle raft
method of transporting zygotes, gametes, and spores works only in a
natural setting and fails in a laboratory. If the Fucus does
experience
tattering or breakage, but the breakage does not follow any particular
pattern, then the second part of my hypothesis is left unsupported, and
nothing can be said about the evolution of receptacle loss as a
reproductive method. 

 
Bibliography:

Amos, W.H., and S.H. Amos. 1985. Atlantic and Gulf Coasts. Alfred A.
Knopf, Inc. 

Blanchette, C.A. 1997. Size and survival of intertidal plants in response
to wave action:
 A case study withFucus gardneri. Ecology, 78 (5) 1563-1578.

Guiry, M.D. 1996. Fucus. Department of Phycology, University
College,
Galway, 
Ireland.

Keats, D. 1997. Algal life cycles. Department of Botany, University of the
Western Cape, 
Cape Town, South Africa.

Norton, T.A., A.C. Mathieson, and M. Neushul. 1982. A review of some
aspects of form 
and function in seaweeds. Botanica Marina 25: 501-510.