Prohaska, J. R.
INTRODUCTION
Tidepools are one of the more interesting habitats of the rocky
mid-intertidal zone, existing as almost isolated communities when
exposed by the tide. Most studies have focused on tidepool organisms
such as cottid fishes (Nakamura, 1971, 1976) and the active hermit crabs
(Fotheringham, 1976; Gerlach, 1976). However, there appears to have been
few studies done on tidepools as complete entities (Canning, 1971).
In my study, I did not look so much at the tidepool itself and its
inhabitants, but rather I focused on the area immediately surrounding
a tidepool and tried to determine the general effects of a tidepool on
its adjacent community.
The questions I was trying to answer were: In what way does a
tidepool affect the area around it? What species are found around
tidepools that would not or could not be there were it not for the
presence of that pool? Is the algal composition around tidepools dif¬
ferent from the mid-intertidal zone in general? Is there more or less
algae around pools? I was looking for effects both in the fauna and in
the algae. Although I anticipated that the pressures exerted by a tide-
pool might be subtle, I considered that over time an enduring tidepool
would effect an equilibrium condition where the pool's effects would
be discernible.
Prohaska, J. R.
MATERIALS AND METHODS
This study involved two sampling programs. The first was designed
to compare the algal composition of areas adjacent to a pool and areas
more removed from a tidepool's influence. The second was designed to
contrast the fauna found in the turf algae immediately surrounding a
pool with that of the non-adjacent turf.
The study area was a portion of the protected rocky intertidal
zone at Hopkins Marine Station, Pacific Grove, California. The area
ranged in tidal height from approximately 1.0-3.0 feet above mean lower
low water and was characterized by the algae Gigartina papillata,
Endocladia muricata, and Rhodoglossum affine. The tidepools found at
the study aite generally had volumes of less than five liters.
The sampling program for comparing the algal compositions in¬
volved running two 20-meter transects across the mid-intertidal zone
parallel to the shoreline. The transect line was fitted closely to the
terrain, and random points were selected along the transect. If the
point was at least 50 centimeters distant from a tidepool, it was con¬
sidered a non-pool site, and a 10 X 10 centimeter quadrat was positioned
with the transect point in the center. Algal cover was then estimated
to the nearest five percent for each species present. Tidepool sites
were selected by taking the pool nearest a given random point along the
transect, roughly determining the center of the pool, and then taking a
random direction and placing the quadrat on the face encountered in that
direction so that the bottom of the quadrat was at the water level of
the tidepool. Again the algal cover was estimated to the nearest five
percent for each species present.
Prohaska, J. R.
To examine the fauna above tidepools, samples were taken at 1, 5.
and 9 centimeter levels above the pool. Four samples were taken at each
level above pools which had a long interface with homogeneous algal
cover on the slope above the pool. A short piece of 32 millimeter
diameter brass pipe was used to core a sample down to the substrate, and
then everything was scraped off the substrate as completely as possible
and included in the sample. The sample was then covered with isopropyl
alcohol and analyzed in the lab. Snails and hermit crabs were separated
and scored without further categorization. Mesobiota were sub-sampled
in a finger bowl by taking six 20X fields of view on a dissecting scope.
The counts for each field of view were then summed and used for compari¬
son with other samples. The types of mesobiota scored were copepods,
isopods, amphipods, acarina, and clams. Only those algal fronds which
were intact from the holdfast were measured for length.
RESULTS
The results of the algal composition sampling program are pre¬
sented in Figure 1. There were a total of 20 tidepool sites and 21
non-pool sites sampled. The data demonstrates a significant difference
in algal composition between areas adjacent to a tidepool and areas at
least 50 centimeters removed. The Student's t-test for comparison of
the means of two populations was made for the seven species of algae,
and for each the difference of means for the two areas was significant
(pe.05). Gigartina papillata was much less abundant around tidepools,
as were Endocladia muricata and Rhodoglossum affine. The last two
species were found around tidepools very rarely. Gelidium coulteri
and the crust Hildenbrandia were more abundant in areas adjacent to tide
pools than in non-pool area, and Lithothamnium californicum and Corallina
were generally found only around tidepools.
Prohaska, J. R.
Separating the algae into two types, crustose forms and turf
algae, also showed interesting results. As can be seen in Figure 2.
there is strikingly less turf algae in the areas surrounding tidepools.
Again, the difference proves to be statistically significant (p..05).
The data also indicates that there is much more of the crustose algae
such as Lithothamnium californicum, Hildenbrandia, and Petrocelis
around the pools.
I also wanted to examine the effects of a tidepool on the fauna
of the adjacent community, and Figure 3 shows the results of the fauna
sampling program. It shows a substantial gradient of small hermit
crabs above the pool while no hermit crabs were found in the control
area less than one meter away sampled at the same heights. Though no
gradient appears among the snails, the data does indicate that there
were fewer snails around the pool than in the control area. There was
a sharp gradient in the mesobiota in the area adjacent to the pool, and
the mesobiota were much more abundant there than at the control site.
There was also a slight gradient in the frond lengths of the algae
(G. papillata), but this gradient was also present at the control site.
Figure 4 presents the fauna sampling data for an area with rela-
tively homogeneous algal cover adjacent to another pool. There was no con¬
trol forthis data, but it is useful because the associated tidepool was
very different from the first one. The pool itself was much larger, and
the face sampled had a different orientation, slope, and substrate.
There was less sediment and many calcareous tube worms. Still, gradients
of snails and mesobiota are clearly demonstrated, and even more inter¬
esting is the total lack of hermit crabs at the site.
Prohaska, J. R.
DISCUSSION
My first observations encouraged me that there were indeed effects
by tidepools upon the adjacent areas. Observations on pools while the
tide changed indicated that Tegula funebralis and especially hermit
crabs closely followed the rising and receding tides. Exposure to
high density feeding pressure would be expected to show effects, and
the results of the sampling programs definitely support this conjecture,
The first sampling program clearly demonstrates that the composi¬
tion of algal species around tidepools is much different from what one
would expect in that area of the mid-intertidal zone. Especially
noteworthy is the almost total absence of Rhodoglossum affine around
tidepools, for it has been reported that Tegula funebralis, one of the
most prominent inhabitants of the tidepools I looked at, has a marked
grazing preference for this alga (Best, 1964). High grazing pressure
would allow few sporelings to escape.
Generally, it is clear that there is much less turf algae around
tidepools. This is perhaps contrary to what one might expect, since
a tidepool's physical effects (increased light, greater humidity) seem
advantageous to algae. But the summed grazing effects of all the tide-
pool inhabitants, including the cottids and Pachygrapsus crassipes, may
be the overriding factor here. Of course, there are more crustose
forms around the pools, but these are perhaps more favored by the
pool's physical effects while being less subject to the effects of
grazing. The same can probably be said for Corallina.
The study on a tidepool's effect upon the adjacent faunal community
is far from exhaustive, yet some general effects have been discerned.
There is more abundant fauna around tidepools, and the fauna tend to be
organized in sharp gradients. Figure 3 shows that there was a very
Prohaska, J. R.
sharp gradient of small hermit crabs (shell length less than 6 millimeters),
largely Pagurus hirsutiusculus but also juvenile Pagurus samuelis. None
were found at the non-pool sites. Conditions above the pool in the algal
turf must be suitable for them, and predation by the cottids may legislate
against their occupancy of the tidepool. Their exploitation of the niche
may be responsible for the observed reduction in small gastropods.
Another observation on tidepools is that though there were many
barnacles all about the pools, there were none found in the pools them¬
selves or in a zone of 1-2 centimeters above the pool's water level. While
it has been reported that the gastropod Acanthina punctulata is an im-
portant predator on barnacles (Sleder, 1979), this does not adequately
account for the sharp demarcation observed, since the snails freely range
in and out of the pools. Predation by other tidepool species may account
for the absence of barnacles in pools. However, Conover and Sieburth (1966)
reported that tannins secreted by the brown alga Ralfsia verrucosa are
toxic to Balanus cyprids, and Magre (1974) indicates that a species of
Ulva has deleterious effects on barnacles in tidepools. Another possible
explanation is that the barnacles simply require the periodic exposure
which tidepool inhabitants are protected from.
There are many factors operating in the mid-intertidal zone, and
it is difficult to disentangle them all and determine what is causing
what. The effects of tidepools seem to be related mostly to its in¬
habitants. The inhabitants of tidepools vary with size, among other
things. For example, the bigger the pool, the less extreme are the physical
conditions of it, and the more abundant is its associated algae. These
pools also support larger fish and crab predators. In the second pool
sampled for fauna above it, no hermit crabs were found. Much larger
fish were seen in the pool, however, and perhaps there is a correlation.
Prohaska, J .R.
As there were no hermit crabs in the area adjacent to the pool, there
was a clear gradient of small gastropods.
Finally, from both the algae sampling program and the fauna sampling
program, it seems that the effects of tidepools do not exceed about 10
centimeters from the pools. It is in this region that further studies
should be focused.
SUMMARY
I- The composition of algal species in areas adjacent to tidepools is
significantly different from the algal composition of areas at least
50 centimeters away from a tidepool.
2- Rhodoglossum affine is particularly rare around tidepools, and this
is consistent with the reports of the grazing preference of Tegula
funebralis for this alga.
3- Mesofauna is generally more abundant around tidepools and decreases
over a 10 centimeter distance above the water's edge.
4- It appears that the effects of tidepools do not generally exceed a
range of 10 centimeters.
ACKNOWLEDGEMENTS
I wish to thank the faculty Hopkins Marine Station and all the
students of the Spring class for making the last ten weeks so enriching.
My special thanks to Bill Magruder and to Chuck Baxter, my advisor and
an ole tidepool watcher from way back.
Prohaska, J. R.
LITERATURE CITED
Best, Barbara. 1964. Feeding activities of Tegula funebralis (Mollusca:
Gastropoda). Veliger 6(supplement): 42-45.
Conover, J. T. and J. Sieburth. 1966. Effects of tannins excreted from
Thacophyta on plankton and animal survival in tidepools. In
E. G. Young and J. L. McLachian (eds.) Proceedings of the Fifth
International Seaweed Symposium, pp. 99-100.
Fotheringham, Nick. 1976. Hermit crab shells as a limiting resource
(Decapoda, Paguridea). Crustaceana (Leiden) 31(2): 193-199.
Ganning, Bjorn. 1971. Studies of baltic rockpool ecosystems.
University of Stockholm, Sweden.
Gerlach, G. A., D. K. Ekstrom, and P. B. Eckardt. 1976. Filter feeding
in the hermit crab, Pagurus bernhardus. Oecologia (Berl) 24(3):
257-264.
Magre, Eugene J. 1974. Ulva Lactuca L. negatively affects Balanus
Balanoides (L.) (Cirripedia Thoracica) Crustaceana (Leiden)
27(3): 231-34.
Nakamura, Royden. 1971. Food of two cohabiting tide-pool Cottidae.
J. Fish. Res. Board Canada 28(6): 928-932.
Nakamura, Royden. 1976. Temperature and the vertical distribution of
two tidepool fishes (O. maculosus, O. snyderi) Copeia 1976(1): 143-151.
Sleder, J. A. 1979. Acanthina punctulata (Neogastropoda, Muricacea): Its
distribution, activity, diet, and predatory behavior. Unpublished
student manuscript on file at Hopkins Marine Station library,
Pacific Grove, California.
C
Prohaska, J. R.
FIGURE LEGENDS
Figure 1. Results of algal composition sampling program for the seven
most abundant species.
Figure 2. Results of algal composition sampling program with algae
divided into two types: turf algae and crustose algae. The
turf algae include G. papillata, R. affine, Corallina,
E. muricata, and G. coulteri.
Figure 3. Data from fauna sampling program for first pool and its control.
Figure 4. Data for fauna sampling at the second pool, for which there
was no control site.
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