DISTRIBUTION AND MICROHABITAT SELECTION OF HEMIGRAPSUS OREGONENSIS (DANA) AND PACHYGRAPSUS CRASSIPES RANDALL IN ELKHORN SLOUGH, MONTEREY COUNTY, CALIFORNIA
A Thesis Presented to the Graduate Faculty of California State University, Hayward
In Partial Fulfillment of the Requirements for the Degree Master of Science in Biology
By Mark C. Sliger March 1982 Copyright © 1982 by Mark C. Sliger
ii ABSTRACT
The vertical distribution and habitat selection of
two species of grapsid crabs Hemigrapsus oregonensis (Dana)
and Pachygrapsus crassipes Randall along the main channel
bank of Elkhorn Slough, Monterey County, California was
investigated. While the vertical distribution of the two
crab species was found to overlap, H. oregonensis typically
occupied burrows in the lower region of the bank and P.
crassipes was usually found in burrows located in the upper bank or in bank slumps located on the lower mudflat.
Substratum and tidal elevation were found to be the most important factors influencing crab distribution along the banks of Elkhorn Slough. Both H. oregonensis and P. crassipes had similar resistance to desiccation abilities, however smaller members of each crab species were more susceptible to desiccation. · Hernigrapsus oregonensis was found to be able to tolerate silty-clay water while
P. crassipes was highly susceptible to small, unconsoli dated mud particles.
iii DISTRIBUTION AND MICROHABITAT SELECTION OF
HEMIGRAPSUS OREGONENSIS (DANA) AND
PACHYGRAPSUS SIPES RANDALL
IN ELKHORN SLOUGH, MONTEREY COUNTY, CALIFORNIA
By
Nark C. Sliger
Date:
' .,.( 7. -i"-- . ...: ~-- 17 [
iv ACKNOWLEDGMENTS
There are a great number of people without whose
help this work would not have been completed. Financial
support was provided by Sea Grant #R/CZ-45. I wish to
thank the members of my committee, Drs. James Nybakken,
Pamela Roe and Gregor Caillier for their continued support
through the duration of this research. I am especially
grateful to Dr. Pamela Roe for her contagious vitality,
inspiration and encouragement during times of crisis.
Thanks to Dr. Ann Hurley for providing assistance with
statistical analysis and experimental design during the
critical preliminary phase of the study. I am grateful to
Dr. John Oliver for his stimulating conversations and
cr ical review of the initial draft.
Chris Jong deserves special thanks for providing
friendship, thought-provoking ideas and valuable assistance
in collecting data. Her undaunted spir contributed greatly to the successful comp tion of this research.
I wish to express my gratitude to numerous iends at the Moss Landing Marine Laboratories who supported and encouraged me throughout this study. In particular, I am indebted to Signe Johnsen for her help with computer analysis; Sheila Baldridge, the librarian, for locating needed references; Rosie Stelow for editing the manuscript;
v . vi
and Lynn McMasters for her exceptional illustrations.
I also greatly appreciate the assistance of Joy Milhaven,
Neal Scanlon and Fred Lauber with the field work. Special thanks to the "Benthic Bubs" for the camaraderie and help in unraveling some of the mysteries of Elkhorn Slough.
My sincere appreciation to Valerie Breda, Debbie Fellows and Melanie Mayer for their advice and emotional support.
Finally, I extend the deepest appreciation to my family for their understanding, love, and support. TABLE OF CONTENTS
ABSTRACT o o • • iii ACKNOWLEDGMENTS v LIST OF TABLES . . ix LIST OF FIGURES x· INTRODUCTION . . 1 METHODS AND MATERIALS 4 Study Area 4 Crab Distribution . 5 Vertical Bank 5 Bank Slumps 6 Physical Properties of the Channel Bank 7 Erosion 7 Exposure 10 Field Experiments 11 Tidal Height Preference 11 Bank Region Preference 13 Substratum Transferal 13 Laboratory Experiments 14 Desiccation 14 Tolerance to Silty-Clay Water 15 RESULTS 16
vii viii
Page
Crab Distribution . 16
Vertical Bank 16 Bank Slumps 17 Physical Properties of the Channel Bank 17 Field Experiments 19 Tidal Height Preference 19
Bank Region Preference . 20
Substratum Transferal 20
Laboratory Experiments 21
Desiccation 21
Tolerance to Silty-Clay Water 21
DISCUSSION . . . . 22
SUMMARY 30 LITERATURE CITED 31
TABLES . 35
FIGURES 39 -
LIST OF TABLES
Table Page
1. Mean densities and mean differences in densities of Pachygrapsus crassipes and Hemigrapsus oregonensis collected in 0.25 m2 quadrats in Elkhorn Slough from December 1979 to May 1980 ...... 35
2. Summary of stat tical analysis for the physical charac tics of the upper and lower bank regions of the main channel bank of Elkhorn Slough ...... 36
3. Summary of chi-square tests for tidal height preferences of Pachygrapsus crassipes and Hemigrapsus oregonensis . . . . . 37 ! 4. Summary of chi-square tests for the bank region preferences of Pachygrapsus crassipes and Hemigrapsus oregonensis . . . 38
'i
ix LIST OF FIGURES
Figure Page 1. Map of Elkhorn Slough showing position of study site ...... 39 2. Photograph bank slumps at the study site in Elkhorn Slough during low tide 41 3. Diagram of the device used in the substratum erodibility experiment ...... 43
4. Artificial substratum cage against the main channel bank of the study site during low tide ...... 45
5. Size frequency diagram of all Hemigrapsus oregonensis collected from mud burrows of the upper bank region ...... 47
6. Size frequency diagram of all Hemigrapsus oregonensis collected from mud burrows of the lower bank region ...... 49
7. Size frequency diagram of all Pachygrapsus crassipes collected from mud burrows of the upper bank region ...... 51
8. Size frequency diagram of all Pachygrapsus crassipes collected from mud burrows of the lower bank region ...... 53
9. Results of five Vertical Height Preference/ Species Interaction experiments . . . 55
10. Tolerance to desiccation of Hemigrapsus oregonensis and Pachygrapsus crassipes 57
11. Regression of desiccation survival time of Hemigrapsus oregonensis against crab size as determined by carapace width ...... 59
12. Regression of desiccation survival time of Pachygrapsus crassipes against crab s e as determined by carapace width 61
13. Tolerance to silty-clay water 63
X INTRODUCTION
While the study of the distribution and habitat preference of brachyuran crabs has been generally limited
i . to works which concentrated on the burrowing, depos feed- i: .r' I , I , ing ocypoid crabs of the genus Uca (for a review, see i ~ Crane, 1975), a number of investigators have focused on the crabs of the family Grapsidae. For example, Bacon
(1971) studied the distribution of Cyclograpsus insularum and Cyclograpsus lavauxi and found substratum and tidal elevation important factors for habitat selection. Sub- stratum and behavior were suggested by Abele (1973) as important factors in limiting the distribution in Florida of six species of closely associated grapsid crabs of the genus Sesarma. Salinity has been found to affect the distribution of several species of grapsid crabs
(Snelling, 1959; Jones, 1976; Seiple, 1979) and Kikuchi et al. (1981) found that the distribution of the pebble crab (Gaetice depressus was correlat with beach elevation.
Although MacGinitie (1935) f st noted the occurrence of the grapsid crabs Hemigrapsus oregonensis, Hemigrapsus nudus and Pachygrapsus crassipes among rocks or in inter- tidal mud burrows in Elkhorn Slough, California and additional studies (Knudsen, 1964; Ricketts and Calvin,
1968; Batie, 1974) have shown that Pacific coast populations of
1 2
the three grapsid crabs generally form distinct distribu-
tional patterns, few authors have explored experimentally
the biological and physical parameters influencing the
distribution and habitat selection of these crabs. Hiatt
(1948) suggested that the observed habitats of H. oregonensis,
H. nudus and P. crassipes along the coast of California
were influenced by substratum and desiccation. He in-
vestigated the relative abilities of the three grapsid
crabs to withstand siccation and found that ~· crassipes
and H. nudus apparently had greater tolerances to desiccation
than H. oregonensis. Low (1970) found that the divergent
habitat preferences of H. oregonensis and H. nudus ~ I were influenced by different physiological tolerances I~ to muddy water and low oxygen concentrations. Hemigrapsus
oregonensis outlived H. nudus when both spec s of crabs I. were placed in flasks filled with low oxygenated, muddy \ water. His results were consistent with Hiatt's (1948)
suggestion that morphological differences in the respira-
tory systems of H. oregonensis, H. nudus and P. affected the ability of the di erent cies to survive on fine particulate substratum. Willason (1981) found
that the distribution and coexistence of P. crassipes and
H. oregonensis a southern Californian saltrnar were primarily the result of predation. By preying on H.
P. crassipes restrict that crab spec s to 3
lower intertidal areas. He suggested that a possible
release mechanism for H. oregonensis was the inability
of small P. crassipes to cope with the stressful estuarine
environment.
Hemigrapsus oregonensis (Dana) and Pachygrapsus
crassipes Randall form an important part of the invertebrate macrofauna of Elkhorn Slough, Monterey County, California.
Both species of crabs occupy similar habitats, generally found in burrows located in the intertidal mudflat or pickleweed Salicornia virginica) marsh. The present study examined the abundance and distribution of H. oregonensis and P. crassipes along the intertidal mud banks of this embayment and sought to answer questions concerning the limits of their distribution and habitat selection and how these m~y be influenced by tolerances to environmental variables and crab agonistic behavior. METHODS AND MATERIALS
STUDY AREA
Elkhorn Slough is a tidally influenced coastal embay
ment and seasonal estuary located in Monterey Bay, California.
The main channel, which averages 100 meters in width, is
bordered by extensive tidal mudflats and pickleweed
(Salicornia virginica) marshland. The axial length of the
slough is approximately 10 km and the main channel is well
mixed vertically due to tidal currents (Smith, 1974).
The study area was located along an intertidal bank
bordering the main channel approximately 5 km from the harbor entrance (Figure 1). In addition to its accessibility,
this particular bank was chosen because of the uniform vertical height of 1 m. The upper edge of the bank was
located approximately at the +1.4 m tidal level. Two
25 m transects were located along the west bank about 25 meters apart.
The tidal level of the bank was determined by ob
serving on two occasions wooden stakes placed along the upper edge of the channel bank during high tide. Stakes were equipped with stripes of phenaphthazine paper placed along their lengths and changes in color of the pH paper indicated the maximum water height attained during that tidal cycle. The tidal elevation of the
4 5
bank was then estimated from the predicted high tide based
on National Ocean Survey Tables obtained from the U.S.
Department of Commerce. Results are comparable to
Smith's (1974) estimated tidal elevation of +1.45 m for
the upper edge of the Salicornia dominated channel bank.
CRAB DISTRIBUTION
Vertical Bank
Obvious differences in bank morphology and some preliminary observations on crab distribution in Elkhorn
Slough suggested that systematic sampling of the upper and lower halves of the main channel bank would yield the most useful information on the vertical distribution of Hemigrapsus oregonensis and Pachygraspsus crassipes.
Both H. oregonensis and P. crassipes are nocturnal
(MacGinitie,l935; Hiatt, 1948; pers. obs.) and generally remain in mud burrows during daylight hours at low tide.
In an earlier study at Goleta Slough, Willason (1981) found that removing crabs by burrow excavation during daylight low tides was the most suitable method of sampling these grapsid crabs.
Individual crabs were collected by placing the upper 2 edge of a 0.25 m quadrat frame along the top of the main channel bank at a random point along the transects.
All crabs occupying burrow ace within the quadrat were 6
removed by complete excavation of each burrow. Crabs were removed from the substratum in a similar manner in the lower bank after lowering the quadrat SO em.
This method collected 98 percent of the crabs s~mpled, as determined by comparing results with those obtained by screening 1 the sediment through a 0.5 mm screen in six samples.
Sampling was conducted over a six-month period, from December 1979 to May 1980. Five upper and five lower bank quadrats were taken monthly. All crabs collected were identified to species, sexed and measured to the nearest 0.1 mm. Both Hemigrapsus oregonensis and
Pachygrapsus crass es were measured across the carapace from the tips of the second anteriolateral spines. Due to handling difficulties, very small crabs ( < 10 mm) were measured using a dissecting scope fitted with a disc micrometer. All other crabs were measured using vernier calipers.
Bank Slumps
Due to tidal scouring and d ferential erosion susceptibility of the soil, the main channel bank is undercut and develops large overhangs. With time, the overhangs with their resident crab populations break away from the bank and collapse onto the mudflat and 7
form , temporarily stable islands dense root
mat surrounded by unconsolidated mudflat sediment
(Figure 2). In order to determine which crabs occupy bank
slump lands, six bank slumps were randomly selected 2 and haphazardly sampled with a 0.2S m quadrat by burrow
excavation. The six bank slumps were located in the
transect area and situated below the +0.9 m elevation
of the tide.
PHYSICAL PROPERTIES OF THE CHANNEL BANK
Erosion
A consistent pattern of soil erosion occurs along the banks of the main channel and major tidal creeks of Elkhorn Slough. As indicated earlier, the banks are generally undercut, forming overhangs that eventually collapse onto the tidal mudflat. To tigate the relative susceptibility of the channel bank of Elkhorn
Slough to erosion by water movement, the bank was divided into two vertical levels. The upper bank was that region of the channel bank extending from the uppermost edge to a dep of SO ern. The bank region from this midway point to the mudflat below, a distance of SO ern, was called the lower bank.
The susceptibility of the substratum of the bank to scouring by tidal currents was explor using a 8
simple laboratory apparatus (Figure 3). The device
allowed an equal and constant flow of water to percolate
upwards and past sediment cores collected from the
upper and lower bank levels. After wet weighing the
cores, pairs of cores were randomly placed the
cylinders of the apparatus and subjected to water move ment for 120 minutes. At the completion of the experiment, cores were removed, drained for three minutes and weighed. Since the cores varied in size, standardization was determined by calculating the percentage of weight lost by the equation:
weight of core (before) - weight of core (after) % loss = X 100 weight of core (before)
In his study of the erosional patterns of San
Francisco Bay marshlands, Pestrong (1965) suggested that the soils underneath alicornia showed increased resistance to erosion because their extensive root system and dense organic material bound soil together and their high elevation made them drier.
Root mat material, composed of Salicornia roots and detritus, was observed underneath the Salicornia in Elkhorn
Slough. In order to determine quantitative differences root mat density between the upper and lower halves of 9
the channel bank, cores (7.5 em X 7.5 em X 15 em) were collected randomly from each bank level along the two transect sites. All samples were taken to the laboratory, immediately wet weighed and wet sieved on a 0.5 mm screen. The material retained on the sieve was oven dried at 70°C and weighed. Since each sample varied in size, the ratio of dry root material to wet weight of the core from which it had been extracted was determined, utilizing the following equations:
weight dry root material root mat material weight wet core . I Moisture conten~ of the sediment may also affect the i erosion susceptibility or the soils of the channel bank. Therefore, the amount of water contained sediment cores obtained from each bank level was determined. Sediment cores (3.5 em X 15 em) were collected from the channel bank at low tide on two occasions. Cores were returned to the laboratory, immediatelywetweighed then oven dried at 70°C. Moisture content was determined by the following relationships:
wet wt. of soil - dry wt. of soil % moisture ------X 100 wet wt. of so 10
The grain composition of marsh sediment helps
determine the water retention characteristics of the
substratum and -affects the substratum's erosional behavior (Pestrong, 1965). Therefore, standard pipette analysis (for technique, see Krumbein and Pettijohn, 1938) was utilized in sediment s determination of the two bank levels. Sediment cores (3.5 em X 15 em) were randomly collected from the channel bank, returned to the laboratory and immediately frozen. After six days, the samples were thawed, processed and analyzed (for methods on particle s e determination, see Folk and
Ward, 1957).
Exposure
Due to the mixed, semi-diurnal tides of Monterey
Bay, crab burrows in the main channel bank of Elkhorn
Slough which differ in vertical elevation of only a few centimeters may have significantly different periods of exposure during the year. To determine differences in inundation rate for the upper and lower bank levels, a time series analysis of the total number of hours during the year each bank region is fully exposed was performed.
The percentage time exposed was determined from the predicted tidal curve for the year 1980 using a computer program that involved the ten most significant tidal constituents (Broenkow, pers. comm.). 11
FIELD EXPERIMENTS
Tidal Height Preference
In a previous study, Willason (1981) utilized an
artificial burrow experiment to determine the preference
of Pachygrapsus crassipes for different bank levels in an
estuarine tidal creek. Pachygrapsus crassipes consistently
preferred those burrows found in the higher tidal level
of the enclosure cage. A series of similar experiments
utilizing artificial substrate cages was performed to
investigate the tidal height preference of . crassipes
and Hemigrapsus oregonensis along a section of the main
channel bank of Elkhorn Slough.
Two styrofoam-backed cages (1 m in he and 54 em
in width) enclos with 2 mm wire mesh were placed vertically against the bank at the study area with the upper edge of the cage flush with the upper edge of the bank and the lower edge resting on the mudflat (Figure 4).
Cages were oval shaped in order to reduce the attraction of crabs to sharp corners (pers. obs.). One artificial substrate cage was designed for large Pachygrapsus crassipes (carapace width range 17-36 mm) and had 40 cylindrical holes with diameters of 54 mm and drilled
15 em o the styrofoam. Experiments with smaller P.
(carapace width range 7-15 mm) and all
Hemigrapsus oregonensis (c ace width range 13-23 mm) ......
12
utilized a second cage with 40 cylindrical holes drilled
15 em into the styrofoam but with diameters of 32 mm.
For both cages, 20 holes were randomly drilled in the
upper half and 20 in the lower half of the cage. For
each replicate experiment, ten marked crabs of one
species (for marking technique, see Kuris, 1971) were
introduced into the cage at low tide during daylight
hours, five placed randomly into holes of the upper
region and five placed into holes in the lower area.
The position of each crab was recorded at the conclusion
of the experiment, approximately 24 hours later during
the next daylight low tide. For all experiments, cages
had been completely inundated by a night high tide.
Crabs for each experiment were collected from Elkhorn
Slough prior to the initiation of the experiment and
were used only once.
To test the af t of species interaction on tidal
height preference, six Pachygrapsus and six
Hernigrapsus oregonensis were simultaneously placed into
large-holed artific 1 substrate cage. Half of the
individuals of each species were placed randomly in the
holes of the upper cage and half in the lower region.
The position of each crab was recorded 24 hours later at
the next low tide. 13
Bank Region Preference
An additional cage experiment was used to investigate which bank region Hemigrapsus oregonensis (carapace width range 12-22 mm) and Pachygrapsus (carapace width range 19-38 mm) selects without interference from the crab species. A similar enclosure cage (1 m X
0.5 m), oval shaped but without styrofoam backing, was placed directly against a cleared section of the channel bank at the study area. The upper edge of the cage was flush with the upper of the bank and the lower edge flush with the mudflat. The bank within the enclosure was c ed by excavation of all burrows and removal of all crabs. Twenty holes (3.5 em in diameter) were bored into the substratum to a depth of 15 em. Ten holes were randomly bored in the upper bank region and ten in the lower. S marked crabs were placed the holes, three in the upper bank and three in the lower. The position of each crab was recorded 24 hours later, at the next daylight low tide, after a night high t had completely inundated the cage. All crabs were col ted from
Elkhorn Slough before start of the experiment and used only once.
Substratum Transferal Experiment
A simple observational experiment was used to determine which crab ecies would occupy a newly cle 14
section of upper bank substratum transferred to the
lower bank elevation. A swath of channel bank composed
completely of upper bank substratum was provided by
clearing by excavation a 1 m wide section of the
channel bank of all crabs, digging away the lower bank
until a large upper bank section could be cut and trans
ferred downward into the space provided. Twenty holes
were haphazardly bored into the bank, ten the upper
bank elevation and ten in the lower. Daily observations
were made for six success daylight low t s .
LABORATORY EXPERIMENTS
Desiccation
abilit of Pachygrapsus crassipes, Hemigrapsus
oregonensis and H. nudus to withstand desiccation, the
relationship between desiccation resistance and crab
size was not noted. Therefore, an investigation the
effects crab size on the tolerances of P.
and H. oregonensis to desiccation was performed.
Pairs of similarly sized individuals of Pachygrapsus
crassipes (carapace width range 13-35 mm) and oregonensis (carapace width e 9-27 mm) were collected
from Elkhorn Slough and placed 500 ml polythylene
jars kept at a room temperature of 22 0 C. Crabs were 15
checked every 30 minutes and the length of time from the
initiation of the experiment to the death of each crab
was noted. Death was signified by the complete lack
of appendage movement when touched.
Tolerance To Silty-Clay Water
In a previous study, Low (1970) found that
Hemigrapsus oregonensis outlived H. nudus in flasks
filled with muddy, poorly oxygenated water. A variation
of this crab tolerance experiment was used to
investigate the effects of small mud particles on H.
oregonensis (carapace width range 13-24 mm) and
Pachygrapsus crassipes (carapace width range 11-24 mm).
Pairs of crabs of each species, collected from
Elkhorn Slough and matched for size, were placed in
500 ml polyethylene jars filled with aerated seawater.
Fifty milliliters of unconsolidated lower bank mud was
added to half of the jars. All jars were kept at a room
temperature of 22°C and the condition of each crab was
checked hourly. The 1 th of time from the initiation
of the experiment to the death of each crab was recorded.
Lack of appendage movement signified the death of the crab. RESULTS
CRAB DISTRIBUTION
Vertical Bank
Different distributional patterns were found for
Pachygrapsus crassipes and Hemigrapsus oregonensis along the main channel bank of Elkhorn Slough (Table 1).
Pachygrapsus crassipes numerically dominated the mud burrows of the upper bank region and were found in significantly fewer numbers in the lower bank (P < 0.001,
Paired-sample t-test). In contrast, H. oregonensis was found in higher numbers in the lower bank region and seldom occupied burrows of the upper bank (P <0.001,
Paired-sample t-test). An average of 24.6 P. crassipes and 1.8 H. oregonensis was found in the 0.25 m2 upper bank quadrats. A mean of 3.1 P. crassipes was found in the 0.25 m2 lower bank quadrats, while an average of 12.2
H. oregonensis occupied similar burrow space in that bank region. Very rarely were P. crassipes and H. oregonensis found in the same burrow.
Size frequency diagrams of all Hemigrapsus oregonensis sampled during the six-month period showed that relatively small, similarly sized individuals were found in both the upper (Figure 5) and lower (Figure 6) bank regions. Average carapace widths of 7.3 mrn (SD = 2.8, n =53) and 7.0 mm
16 17
(SD = 3.4, n = 365) were found for H. oregonensis in the upper and lower bank regions, respectively. Average sizes of Pachygrapsus crassipes for the upper and lower bank regions were 13.5 mm (SD = 9.9, n = 737) and 7.9 mm
(SD = 5.8, n = 92), respectively. Size frequency diagrams of all P. crassipes collected during the sampling period showed that many large individuals were found in the upper bank (Figure 7) while very few individuals larger than
16 mrn occupied burrows in the lower bank region (Figure
8) .
Bank Slumps
Pachygrapsus crassipes numerically dominated all bank slumps which had fallen onto the main channel mudflat 2 with an average of 15.0 individuals/0.25 m (SD = 6.0, n = 6). Hemigrapsus oregonensis, with a density of 2 0.7 individuals/0.25 m (SD = 1.2, n = 6), occurred in significantly lower numbers on the bank slumps
(P <0.01, Mann-Whitney U-test).
Physical Properties of the Main Channel Bank
Many of the physical characteristics of the main channel bank were found to be different for the upper and lower bank regions (Table 2). While in the erosion apparatus, the lower bank cores lost a significantly 18
greater amount of sediment than the upper bank cores
(P <0.001, Mann-Whitney U-test). In all cases, the
lower bank cores deteriorated more rapidly than the
upper bank cores and were reduced to small clumps of
mud while retained in the erosion apparatus. The upper
bank cores remained tually unaffected by the flow of
water and maintained the original shape and consistency.
The density of root mat in the upper channel bank
was significantly greater than the lower bank (P <0.01,
Mann-Whitney U-test) with the lower bank almost devoid
of organic material. Moisture content analysis of the
bank cores revealed that there was litt difference
in soil moisture between the upper and lower bank regions
(P >0.05, Mann-Whitney U-test). Soil analysis, which
·ignored weight contributed by root mat material, indicated
that the soils of both ions of the bank were composed
exclus ly of fine silty and clay particles. However,
there were some differences in the gra composition
between the upper and lower bank substrata. The lower
bank substratum had a smaller mean part le size (P <0.05,
Mann-Whitney U-test) and a greater percentage of clay
partie s (P < 0.05, Mann-Whitney U-test) than the sub
stratum found in the higher bank region.
De the slight difference in vertical tidal
height between the upper and lower bank ions, time 19
series analysis indicated that the upper half of the
bank is fully exposed 34 percent more during the year
than the lower half. While the upper bank is higher and
exposed more often, high water content is apparently
maintained by the retention of water in a dense root mat
filled with small sediment particles.
FIELD EXPERIMENTS
Tidal Height Preference
Pachygrapsus crassipes and Hemigrapsus oregonensis
utilized the art ial substratum cages placed along the main channel bank of Elkhorn Slough differently (Table 3).
Both small and large P. crassipes showed a preference for
the upper intertidal region of the channel bank (P <0.001,
chi-square test). In contrast, H. oregonensis did not
show a preference for any tidal elevation within the vertical range of the channel bank (P >0.10, chi-square
test). When both H. oregonensis and P. crass s were placed into the same cage for 24 hours, P. crassipes maintained its preference for the higher tidal elevation while H. oregonens , although sustaining high mortalities,
again showed no tidal height preference (Figure 9).
Mortalit s were assumed to result from predation by
P. crassipes because on several occasions P. crass es were observed eating H. oregonensis along the banks of 20
Elkhorn Slough. In addition, when both species of crabs were placed in aquaria, H. oregonensis consistently underwent heavy mortalities. Willason (1981) also observed this aggressive interaction between H. oregonensis and
P. crassipes.
Bank Region Preference
Pachygrapsus crassipes and Hemigrapsus oregonensis had divergent bank region preferences when placed in the enclosure cage along the main channel bank of Elkhorn
Slough (Table 4) . Pachygrapsus crassipes preferred the upper bank region (P <0.001, chi-square test), while
H. oregonensis had a distinct preference for the lower bank region (P <0.001, chi-square test).
Substratum Transferal Experiment
Daily observations showed that only Pachygrapsus crassipes utilized the burrows in the cleared experimental bank area.
Eight crab individuals were observed at low tide during the six-day period, with three large P. crassipes (20-30 mm) found in upper bank substratum placed in the lower tidal level. 21
LABORATORY EXPERIMENTS
Desiccation
Hemigrapsus oregonensis and Pachygrapsus crassipes
had approximately the same ability to survive desiccation
(Figure 10). Mean survival time for P. crassipes was
25.7 hours (SD = 12.6, n = 16) with all individuals dead
within 47 hours. Individuals of H. oregonensis survived
on the average of 24.6 hours (SD = 11.1, n = 15) with all individuals dead within 57 hours. The difference in
survival time between crab species was not significant
(P > 0.50, Student's t-test). Larger individuals of both
H. oregonensis (Figure 11) and P. crassipes (Figure 12)
were more tolerant to desiccation than smaller individuals.
Tolerance to Silty-Clay Water
Hemigrapsus oregonensis had a significantly greater
survival time than Pachygrapsus crassipes in experimental
jars containing silty-clay mud (P <0.001, Student's
t-test). Hemigrapsus oregonensis had a mean survival time of 40.6 hours (SD 16.6, n = 10), while . crassipes lived on the average of 7.2 hours (SD = 5.1, n 10)
in the muddy water. All P. crassipes were dead within
19 hours, while one H. oregonensis individual survived more than six days in the muddy water (Figure 13). Size
did not influence survival ability of either crab species. DISCUSSION
While some overlap does occur along the main channel bank of Elkhorn Slough, Hemigrapsus oregonensis is concentrated in burrows located in the lower region of the bank and Pachygrapsus crassipes is generally found in burrows located in the upper bank or bank slumps. Field and laboratory experiments have demonstrated that the observed distributional pattern can be best explained by differences in microhabitat preference. Although the relative importance of various habitat characteristics is difficult to determine, substratum and tidal elevation are major factors in the determination of suitable bank habitats for H. oregonensis and P. crassipes in Elkhorn
Slough.
Degree of tidal exposure appeared to be a major factor in microhabitat selection of Pachygrapsus crassipes.
Previous work by Hiatt (1948) and Gross (1957), respectively, demonstrated that P. crassipes was negatively hydrotactic and preferred to be out of water 50 percent of the time.
The calculated yearly exposure period of 54 percent for the upper bank zone of Elkhorn Slough would therefore fit their preference. Results of the tidal height preference experiment confirmed Willason's (1981) findings that
P. crassipes preferred the more exposed intertial elevations.
22 23
Although exposure time may be important in the
selection of a suitable burrow habitat for Pachygrapsus
crassipes, substratum characteristics also play an
important role. In Elkhorn Slough, P. crass s was
found in high densities in the lower intertidal on bank slumps and in man-made burrows bored into upper bank
substrate previously transferred to the lower bank region.
Therefore, the tidal height preference of P. crass s can be modified by the existence of stable, erosion-resistant upper bank substratum found in less exposed elevations.
The observed vertical distribution of Hemigrapsus oregonensis along the main channel bank of Elkhorn Slough appears to be the result of substratum preference. Willason
(1981) has suggested that H. oregonensis is inhibited from occupying mud burrows in the upper regions of tidal creeks due to predation pressures from Pachygrapsus crass s.
While antagonistic behavior by P. crass es may be a factor in limiting the distribution of H. oregonensis, results of the bank region preference experiment showed that H. oregonensis preferred the lower bank substratum, selecting this bank area within the enclosure cage without the presence of P. crassipes. In an earlier study, Low (1970) found that H. oregonensis preferred silty mud substratum to substratum composed of larger particles. Thus, 24
characteristics of the substratum are important parameters
for H. oregonensis in its selection of a burrow habitat
along the banks of Elkhorn Slough.
The importance of substratum composition as a factor
affecting the pattern of distribution of faunal in
vertebrates has been investigated by a number of authors
(fora review, see Grey, 1974). Earlier studies on the
distribution and ecological requirements of certain
brachyuran crabs, especially of the family Ocypodidae,
found that the distribution of many deposit feeding fiddler
crabs was correlated with substratum characteristics
(Teal, 1958; Ono, 1962, 1965; Whiting and Moshiri, 1974;
Barnes, 1974; Icely and Jones, 1978; Frith and Brunen
meister, 1980). One substratum property, density of root
mat material, was shown to have a major affect on the
distribution of several species of fiddler crabs (Genus
Uca) in a North Carolina estuary (Ringold, 1979).
The relative susceptibility of the main channel bank
of Elkhorn Slough to erosion by water movement may determine
the stability and longevity of crab burrows found in this particular habitat. Observations of medium-sized holes
(35 mm in diameter) bored into the lower bank indicated
that they deteriorated rapidly, and even if they occurred 2 in low densities (ten holes/0.25 m ), caused the collapse 25
of the lower bank surface within one or two days. In
contrast, equal-sized holes in equal densities bored into
the upper bank substratum maintained their integrity from
several weeks to several months. A major factor in the
reduced susceptibility of the upper bank substratum to
erosion is its compaction by dense root mat which binds
the soil particles together. The lower bank, devoid of
such material, is highly susceptible to erosional processes.
The difference in the erodibil and longevity of burrows of the upper and lower bank substrata may be an important factor in the habitat choice of Pachygrapsus crassipes. Substratum instability has been found to be an important influence of the distribution of many marine organisms (Nickols, 1970; Stephenson et al., 1970;
Biernbaum, 1979). In a recent study, McKillup and Butler
(1979) found that substratum stability directly affected the burrowing behavior of the grapsid crab, Helo sus haswellianus, by causing the crab population to limit the number of burrows dug in a mud bank. Burrows serve both
P. crass es and Hemigrapsus oregonensis as places of refuge from potential bird (Stenzel et al., 1976), fish
(Low, 1970; Talent, 1973; Nybakken et al., 1977) and terrestrial (Lindberg, 1980) predators while also providing insulation from various physical factors. Willason (1981) f 26 I
found that H. oregonensis readily burrows into mud banks, but P. crassipes does not. He suggested that individuals of P. crassipes utilize burrows initially constructed by H. oregonensis and enlarge them by body movements as needed. The unstable, rapidly deteriorating substratum of the lower bank may limit the life expectancy and size of burrows that can be constructed in that habitat. Large burrows were seldom observed in the lower bank and only small individuals of P. crassipes were found there. Therefore, most adult P. crassipes appear to be restricted to stable, long lasting burrows previously constructed by H. oregonensis in the organically denser and erosion resistant upper bank substratum. In contrast, Hemigrapsus oregonensis may be less affected by the burrow instability of the lower bank substratum. Hemigrapsus oregonensis is an efficient burrower that can rapidly burrow into the mud banks of Elkhorn Slough (pers. obs.). In addition, H. oregonensis found along the main channel bank of Elkhorn Slough were relatively smaller than Pachygrapsus crass es and subsequently occupied smaller burrows which the lower bank substratum can support. The degree of soil unconsolidation and the level of suspended sediment are additional substratum characteristics 27
which may affect the distribution and habitat selection of Pachygrapsus crass es and Hemigrapsus oregonensis.
The upper bank region and bank slumps, which originate from the upper bank, are composed of dense root mat material which binds the sediment particles together and reduces the amount of free silt in the burrows or near the substratum-water/or air interface. The lower bank substratum, devoid of root mat material, is less resistant to erosion and may have increased levels of suspended or loose silt and clay soil particles in and surrounding crab burrows. Results of the tolerance to silty water experiment indicated that P. crass es was less tolerant of muddy water than H. oregonensis and suggested that P. crassipes lacks some mechanism which prevents gill clogging.
Surrounding the incurrent channels of the gill chambers of H. oregonensis and P. crassipes are small setae on the branchiostegites. Generally, setae located on these structures are thought to function in attracting water to the gill chamber by capillary action (Warner, 1977).
Hiatt (1948) suggested that these setae also function in filtering fine particles from the water before it enters the gill chambers of P. crass s and H. oregonensis. Pachy-
sus crassipes has noticeably fewer and coarser setae ~~---- than H. oregonensis (Hiatt, 1948; pers. obs.), which may be the morphological basis of its reduced particle f tering 28
efficiency. In waters containing fine suspended particles,
Hiatt (1948) suggested that the gills of P. crassipes would tend to become clogged much more rapidly than those of H. oregonensis.
Upon dissection of crabs exposed to fine sediments during the tolerance to silty water test, Hemigrapsus oregonensis individuals had heavy deposits of mud on the setae of their branchiostegites, indicating they efficiently filtered out sediment particles from the water. The setae of Pachygrapsus crassipes were clean, suggesting they did not effectively filter out the silt particles. The gills and branchial chambers of P. crass s were also observed to contain a heavier concentration of mud particles than those of H. oregonensis. Therefore, while the highly erodible substratum of the lower bank is actively selected by the silty mud tolerant H. oregonensis, it may be avoided by the more sensitive P. crass es.
The selection of the lower bank microhabitat by
Hemigrapsus oregonensis may not only be determined by substratum preference but also by desiccation tolerance.
Small intertidal crabs, because of their high surface to volume ratio, lose water by evaporation at a higher rate and are less tolerant than larger crabs to desiccation
(Herreid, 1969; Grant and McDonald, 1979). Studies 29
have shown that smaller crabs are restricted to lower tidal areas or to substratum with higher moisture contents (Warner, 1969; Frith and Brunemeister, 1980; Kikuchi et al., 1981). Although H. oregonensis possesses the same ability to tolerate desiccation as Pachygrapsus crassipes, its relatively smaller size may make it less likely to choose the more exposed tidal elevation of the channel bank. Although many small P. crassipes were found in the upper bank, their occurrence here may provide a clue to estimating the relative importance of desiccation and substratum to the crab species. The silty, unstable nature of the lower bank may impose a greater physiological burden on small individuals of P. crassipes than the increased exposure to air of burrows found in the upper bank. SUMMARY
The distribution of Hemigrapsus oregonensis and
Pachygrapsus crassipes along the tidally influenced
vertical banks of Elkhorn Slough is the result of the
interaction of a variety of physical and biological
factors. While the relative importance of the factors
would be difficult to determine, substratum and tidal
elevation seem to be the most important factors in
microhabitat choice for both species of crabs. Although
P. crassipes generally prefers the more exposed levels of
the channel bank, this tidal height preference was
modified by the existence of stable substratum in the
lower intertidal. Therefore, while tidal height does play
a role in microhabitat selection, burrow stability and
level of unconsolidated silty-clay sediment are also very
important in limiting the distribution of P. crassipes.
In contrast, by selecting the less exposed, unstable lower
bank substratum, H. oregonensis avoids desiccation stress
and possibly predation by larger P. crassipes. Efficient
burrowing ability and tolerance to fine particulate mud may be adaptations related to the substratum preference
by~- oregonensis.
30 LITERATURE CITED
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Barnes, R.S.K. 1974. Estuarine biology. Edward Arnold, London. 76 pp.
Batie, R.E. 1974. Population structure of the intertidal shore crab Hemigrapsus oregonensis (Brachyura, Grapsidae) in Yaquina Bay, a Central Oregon coast estuary. Ph.D. dissertation, Oregon State University. 128 pp.
Biernbaum, C.K. 1979. Influence of sedimentary factors on the distribution of benthic amphipods of Fishers Island Sound, Connecticut. J. Exp. Mar. Biol. Ecol. 38: 201-223.
Crane, J. 1975. Fiddler crabs of the world (Ocypodidae: genus Uca). Princeton University Press, Princeton, N.J. 736 pp.
Folk, P.W. and W.C. Ward. 1957. Brazos River Bar: a study in the significance of grain-size parameters. J. Sed. Pet. 27: 3-26.
Frith, D.W. and S. Brunenmeister. 1980. Ecological and population studies of fiddler crabs (Ocypodidae, genus Uca) on a mangrove shore at Phuket Island, western peninsular Thailand. Crustaceana 39 (2): 157-183.
Grant, J. and J. HcDonald. 1979. Desiccation tolerance of Eurypanopeus depressus (Smith) (Decapoda: Xanthidae) and the exploitation of microhabitat. Estuaries 2 (3): 172-177.
Grey, J.S. 1974. Animal sediment relationships. Oceanogr. Mar. Biol. Ann. Rev. 12: 223-261.
31 32
Gross, W.J. 1957. A behavioral mechanism for osmotic regulation in a semi-terrestrial crab. Biol. Bull. 113: 268-274.
Herreid, C.F. 1969. Water loss of crabs from different habitats. Comp. Biochem. Physiol. 28: 829-839.
Hiatt, R.W. 1948. Biology of the lined shored crab, Pachygrapsus crassipes (Randall). Pacif. Sci. 2: 134-213. Icely, J.D. and D.A. Jones. 1978. Factors affecting the distribution of the genus Uca (Crustacean: Ocypodidae) on the East African shore.~stuarine and Coastal Mar. Sci. 6: 315-325.
Jones, M.E. 1976. Limiting factors in the distribution of intertidal crabs (Crustacea: Decapoda) in the Avon Heathcote Estuary Christchurch. N.Z.J. of Marine and Fresh. Res. 10 (4): 577-587.
Kikuchi, T., Tanaka, M., Nojima, S., and T. Takahashi. 1981. Ecological studies on the pebble crab, Gaetice depressus (de Haan). I. Ecological distribution of the crab and environmental conditions. Publ. Amakusa Mar. Biol. Lab. 6 (1): 23-34.
Knudsen, J.W. 1964. Observations of the reproductive cycles and ecology of the common Brachyura and crablike Anomura of Puget Sound, Washington. Pacif. Sci. 18: 3-33.
Krumbein, W.C. and F.J. Pettijohn. 1938. Manual of sedimentary petrology. Appleton-Century-Crofts, New York. 549 pp.
Kuris, A.M. 1971. Population interactions between a shore crab and two symbionts. Ph.D. dissertation, University of California, Berkeley. 477 pp.
Lindberg, W.J. 1980. Behavior of the Oregon mud crab, Hemigrapsus oregonensis (Dana) (Brachyura, Grapsidae). Crustaceana 39 (3): 263-281.
Low, C.J. 1970. Factors affecting the distribution and abundance of two species of beach crabs, Hemigrapsus nudus and H. oregonensis. M.S. thesis, University o tish-Columbia. 70 pp. r 33
MacGinitie, G.E. 1935. Ecological aspects of a California marine estuary. Am. Midl. Nat. 35: 629-765.
McKillup, S.C. and A.J. Butler. 1979. Cessation of hole digging by the crab Helograpsus haswellianus: a resource-conserving adaptation. Mar. Biol. 50: 157-161. Nickols, F.H. 1970. Benthic polychaete assemblages and their relationship to the sediment in Port Madison, Washington. Mar. Biol. 6: 48-57.
Nybakken, J. Cailliet, G. and W. Broenkow. 1977. Ecologic and hydrographic studies of Elkhorn Slough, Moss Landing harbor nearshore coastal waters. July 1974 to June 1976. Moss Landing: Moss Landing Marine Laboratories. 465 pp.
Ono, Y. 1962. On the habitat preference of ocypoid crabs I. Mem. Fac. Sci. Kyushu Univ. Series E. (Biol.) 3 (2) 143-163. Ono, Y. 1965. On the ecological distribution of ocypoid crabs in the estuary. Mem. Rae. Sci. Kyushu Univ. Series E. (Biol.) 4 (1): 1-60.
Pestrong, R. 1965. The development of drainage patterns on tidal marshes. Stanford University publications. Geological Sciences. 10 (2): 87 pp.
Ricketts, E.F. and J. Calvin. 1968. Between Pacific Tides. 4th ed. Stanford Univ. Press, Stanford, Calif. 614 pp.
Ringold, P. 1979. Burrowing, root mat density and the distribution of fiddler crabs in the eastern United States. J. Exp. Mar. Biol. Ecol. 36: ll-21.
Seiple, W. 1979. Distribution, habitat preferences and breeding periods in the crustaceans Sesarma cinereum and S. reticulatum (Brachyura: Decapoda: Grapsidae) Mar. Biol. 52: 77-86.
Siegel, S. 1957. Nonparametric statistics for the behavioral sciences. McGraw-Hill, New York. 312 pp.
Smith, R.E. 1974. The hydrology of Elkhorn Slough, a shallow California coastal embayment. M.S. thesis, San Jose State University. 88 pp. 34
Snelling, B. 1959. The distribution of intertidal crabs in the Brisbane River. Aust. J. Mar. Fresh. Res. 10: 67-83. Stenzel, L.E., Huber, H.R. and G.W. Page. 1976. Feeding behavior and diet of the long-billed curlew and willet. The Wilson Bulletin 88 (2): 314-332.
Stephenson, W., Williams, W.T. and G.N. Lance. 1970. The macrobenthos of Moreton Bay. Ecol. Monogr. 40: 459-494.
Talent, L. 1973. The seasonal abundance and food of elasmobranchs occurring in Elkhorn Slough, Monterey Bay, California. M.A. thesis, California State University, Fresno. 58 pp.
Teal, J.M. 1958. Distribution of fiddler crabs in Georgia salt marshes. Ecology 39: 185-193.
Warner, G.F. 1969. The occurrence and distribution of crabs in a Jamaican mangrove swamp. J. Anim. Ecol. 38: 379-389.
Warner, G.F. 1977. The biology of crabs. Paul Elek, Ltd., London. 202 pp.
Whiting, N.H. and G.A. Moshiri. 1974. Certain organism substrate relationships affecting the distribution of Uca minax (Crustacea: Decapoda). Hydrobiologia (4): 481-493.
Willason, S.W. 1981. Factors influencing the distribution and coexistence of Pachygrapsus crassipes and · Hemigrapsus oregonensis (Decapoda: Grapsidae) in a California salt marsh. Mar. Biol. 64: 125-133.
Zar, J.H. 1974. Biostatistical analysis. Prentice-Hall, Inc., New Jersey. 620 pp. 35
Table 1. Mean densities (+ 1 SD) and mean differences in densities (+ 1 SD) of Pachygrapsus crassipes and Hemigrapsus oregonensis collected in 0.25 m2 quadrats in Elkhorn Slough from December 1979 to May 1980. N = 30.
Density Density Mean Upper Bank Lower Bank Difference
P. crassipes 24.6 (+11.0) 3.1 (+3.7) 21.6 ( +9 . 6) *~~·k
H. oregonensis 1.8 (+2.9) 12.2 +9.1) 10.1 +9 3) 'bb~
* 1'"*Significantly different, P < 0. 001, Paired-sample t-test. 36
Table 2. Summary of statistical analysis for the physical characteristics of the upper and lower bank regions of the main channel of Elkhorn Slough. Values are mean + 1 SD with numbers in parentheses indicating the sample size. See text for methods.
Probability Upper Bank Lower Bank level a
Erodibility 3.1 + 3.2 53.2 + 35.7 <0.001 (% wt. loss) (7) (7)
Root Mat Density 9.3 + 0.38 0.75 + 0.06 < 0. 01 (Dry root wt./ (5) (5) wet wt.)
Moisture Content 51.8 + 5.4 48.2 + 2.5 >0.05 (%water wt.) (5) (5)
Sediment Analysis
Diameter 3.06 + 0.74 1.38 + 0.88 <0. 05 (microns) (4) (4) % clay 58.4 + 6.3 76.6 + 10.7 < 0. 05 (4) (4)
Exposure 54 20 (% time exposed/ yr)
a - Probability that observed differences between mean values are due to random effects as determined by Mann-Whitney U-test (Siegel, 1957). 37
Table 3. Summary of chi-square tests for the tidal height preferences of Pachygrapsus crassipes and Hemigrapsus oregonensis. Ten crabs were introduced into the cage for each experiment.a
b No. of crabs No. of crabs No. of E. upper cage lower cage experiments
P. crassipes 76 14 9 < 0. 001
c H. oregonensis 43 35 8 > 0. 05
a - Nonsignificant, P >0.05, heterogeneity chi-square test indicates pooled data justified (see Zar, 1974). b - Probability that observed tidal height preference is due to random effects as determined by chi-square with homogeneous sets of data.
c - Two crabs died during the experiment.
Table 4. Summary of chi-square tests for the bank region preferences of Pachygrapsus crassipes and Hemigrapsus oregonensis.a Six crabs were initially introauced into the cage in each experiment. Number of crabs found on cage matgrial and not actual bank substratum are in parentheses.
No. of crabs No. of crabs No. of upper bank region lower bank region experiments p_c
P. crassipes 30 (10) 5 (3) 6 <0.001
H. oregonensis d 4 (0) 39 (3) 9 <0.001
a -Nonsignificant, P > 0.05, heterogeneity chi-square test indicates pooled data justified (see Zar, 1974). b - Only crabs found on the bank substratum tested. c - Probability that observed bank region preference is due to random effects as determined by chi-square procedure with homogeneous sets of data. d - Nine crabs escaped from the cage during the series of preference experiments. 39
Figure 1. Map of Elkhorn Slough showing position of study site. 40
. . ~ .. " ~ .. ·...... ~ . . . . . "' ·1 CALIFORNIA .... I i I "·'-....._ "·"· "· ~. ··.~·.::. KIRBY . ·>:·:·:.> .. PARK .
Kilometers 1 2 % Miles
. BENNETT . ·:·. ... · SLOUGH
...... OYSTER DOCK.·<· .. 41
Figure 2. Photograph of bank slumps at the study site in Elkhorn Slough during low tide. 42 43
Figure 3. Diagram of the device used in the substratum erodibility experiment. Incoming water pressure was approximately 65 lbs/in2. Arrows indicate direction of water flow. Cylinders (10 em X 24 em) were constructed of plexiglass (3 mm in thickness).. A wire cage (6 em X 15 em) with an enclosed substratum core (3.5 em X 15 em) was centered within each cylinder during the experiment. 44
I It I It tl I I SUBSTRATE 1 I I t CORE I I 1t I t :ttj. }:
~ -= -==:: -::::::: j f11 :t/_.... -- .:::::-- 1• / ~ 1' .,. .\1' \.\ t . ~J' '\ ?
RUBBER TUBING diameter= 5mm
ttt
65psi 45
Figure 4. Artificial substratum cage against the main channel bank of the study site during low tide. 46 47
Figure 5. Size frequency diagram of all Hemigrapsus oregonensis collected from burrows of the upper bank region. Five quadrats (0.25 m2) were sampled monthly (December 1979 to May 1980) in the study area, n = 53 48
co I I I I N
-
~ - C\..1 ..., E E 1..-J
(0 - ...... r:. -o~ - •r-1 ~
(J) - 0 ...-.N o· a_ 0 L 0 0
00
1-
I I ! I
Aouenbe,..~,:t 49 '
Figure 6. Size frequency diagram of all Hemigrapsus oregonensis collected from mud burrows of the lower bank region. Five quadrats (0.25 m2) were sampled monthly (December 1979 to May 1980) in the study area, n = 365. 50
co • • • ' N
es:a !- N
- r-"1 E E .... L.....l U':) ..!::...... -+> -o..... == Q) 0 "' . N,...... a...0 0 t. 0 0
. CD
. -
I I I I
Aouenbe...Jj 51
Figure 7. Size frequency diagram of all Pachygrapsus crassipes collected from mud burrows of the upper bank region. Five quadrats (0.25 m2) were sampled monthly (December 1979 to May 1980) in the study area, n = 737. 52
~ • • • I • • ..q-
~..q- [ co ('0 I ... N - ('0 I ...... , co E - N E 1...-J I ...c. . -..:t' -+> N -o.,.., I ill'.: .(1) ~ - N 0 0 a_ I 0 co !... - ...... 0 I 0 N ...... I - . l 1- - I I I I I • I
Aouenbe..J;} 53
Figure 8. Size frequency diagram of all Pachygrapsus crassipes collected from mud burrows of the lower bank region. Five quadrats were sampled monthly (December 1979 to May 1980) in the study area, n = 92. 54
""':t ~
s ...:t'
enUJ
Nen
f""""'' m E N E I...... ! ...c ~ +> N -o..... 3: s G) N 0 0 0... 0 UJ l...... 0 0
Aouenbe-..tj 55
Figure 9. Results of five Vertical Height Preference/ Species Interaction experiments. Six Pachygrapsus crassipes and six Hemigrapsus oregonensis were initially placed into the cage. The position of each crab was recorded 24 hours later at the next low tide. 56
• ' • • ' •
I I I I 57
Figure 10. Tolerance to desiccation of Hemigrapsus oregonensis (n = 15) and Pachygrapsus crassipes (n 16). Difference in survival time was not significant, P >0.50, Student's t-test (Zar, 1974). (\J ~
(.0 ::0 oJ (T) ....., w L 1 L + 0 CSl L-..1 (T) ~. Q) I ...... E ~ 4J ('\j
I co ......
(\J J..--1
CSl (.O~~mN ...... CSlrnro~~~~mN..--~~~ ...... 59
Figure 11. Regression of desiccation survival time of Hemigrapsus oregonensis against crab size as determined by carapace width. R '1' 2 = coefficient of determination, Syx = standard error of estimate and n =number of crabs. Broken lines are the 95 percent confidence belts for the regression line. 60 / / I 54 / I I I I 48 I I ,...., I (J) I L 42 + I I L I + I 1.-J I I 36 .f. I ([) / II ...... E I ,, I I +> 30 I I ..--1 I / 0 I Intercept = -L f2157e 01 > 24 +I I '+* ·rl I I Slope ::::: 2.005e 00 / I > I' L y il Rt2 = 0.860 :J 18 I (J) / / +I Syx = 4. 125 I I N = 15 12 I I / I / I / I 6 / I / I I ; .I 0 0 6 12 18 24 30 36 42 48 54 60 carapace width [mm] 61
Figure 12. Regression of desiccation survival time of Pachygrapsus crassipes against crab size as determined by carapace width. R '1' 2 = coefficient of determination, Syx = standard error of estimate and n =number of crabs. Broken lines are the 95 percent confidence belts for the regression line.
l 63
Figure 13. Tolerance to silty-clay water. Pairs of Hemigrapsus oregonensis and Pachygrapsus crassipes matched for size and placed in 500 ml jars. Ten jars contained aerated seawater (Control) and 10 contained aerated seawater with 50 ml unconsolidated lower bank mud (Silty-clay water). Dotted lines indicate no change in status until experiment was terminated at 156 hours when the last crab died in the experimental jar. 64
Silty-clay water
Cl) > ...... -tll • ~c.. 0 <+- 0
& 0 ::z
1
......
. + - li. gregonens i a 0 ::z o-~ ocassipes
1 Control
a~~~~--_. __ _.__ ~--~------~--~~~~--~ ra s 12 1a 24 ~ 36 42 48 54 59 66 12 time Chrs]