Wave velocity and its effect on colony density and ...

[Anonymous]

Wave velocity and its effect on colony density and secondary habitat selection of a temperate sea anemone, Anthopleura elegantissima (Brandt)

Abstract

Fourteen colonies of the sea anemone Anthopleura elegantissima were found to have significantly higher densities (P=.06) and individuals were found to have larger average sizes (P=.08 ) in areas of lower wave velocities. Colonies of A. elegantissima are found in relatively sheltered areas in the intertidal and may be more fit at less exposed areas. Individuals of A. elegantissima (n=46) were found to be more motile at differing water velocities (P<.001), although there was no direction of preferred movement with respect to current direction. The ability to detect different wave velocities is probably a factor in the secondary habitat selection of A. elegantissima.

 

[Anonymous]

Introduction
Wave action is one of the most important factors in determining the community structure present in marine organisms. Water flow has impacts on allelopathy, competition, larval and spore dispersal, phytoplankton and zooplankton availability, and sedimentation rates, among many other factors (Dennison and Barnes, 2001). There is evidence that species diversity increases with increasing wave action as well (Bell and Denny, 1994). Besides affecting organisms at a community level, wave action can have profound physiological effects on a species. Scleratinian corals have widely varying colony densities and morphological structures based on the intensity of wave action (Veron, 1995). Likewise, certain macroalgae have differing morphologies within species depending on the intensity of the wave action they are exposed to (Koehl & Alberte, 1988). Rates of photosynthesis, respiration, food capture, waste removal, and nutrient uptake are all positively affected by wave action (Olson et al, 1991).
There are two aspects of wave action that should be distinguished--average velocity and maximum velocity. Average velocity can affect any process that requires rapid diffusion such as chemoreception, fertilization, and waste removal. (Denny and Gaines, 1990; Bell and Denny, 1994). Maximum wave velocity is a vector measure of the highest force produced by waves near an organism. The maximum wave velocity that an organism is exposed to can therefore increase mechanical stress, the probability of dislodgement, and even inhibit filter feeding. Thus, maximum wave velocity can exert a powerful influence on an organism’s growth form. In areas of high maximum velocity, organisms tend to have more compressed body forms in order to reduce drag and friction from wave action (Veron, 1995; Vogel, 1994).
Natural History
Anthopleura elegantissima (Brandt) is a temperate water actinarian that reproduces asexually by binary fission to form colonies of aggregated clones. Individuals of sufficient size (> 2.0 cm) undergo sexual reproduction once annually. Adult colonies are common in the intertidal zone of the northern Pacific Ocean and are usually found in surge channels that are relatively protected from direct wave impact (pers. obs.). Individuals' size can vary on average between and within colonies but is generally within 1.0 - 5.0 cm (Sebens, 1982, pers. obs.). Primary recruitment of the planula larvae of A. elegantissima usually occurs in mussel beds; juveniles are capable of subsequently moving to nearby tidepools and surge channels by pedal locomotion. Individuals may remain in mussel beds for several years, but are typically smaller than reproductive size in these areas. Larger individuals (4.0 - 5.0 cm) are typically only found in areas of less intense wave action than mussel beds are exposed to (pers. obs.). The movement of juveniles to different areas of the intertidal to find a more suitable habitat for reproduction is an example of secondary habitat selection. Secondary habitat selection is an ongoing process that can occur any time during the life of an organism if the suitability of a habitat changes. It appears to be the primary purpose of motility in sea anemones (Sebens, 1982). Anthopleura elegantissima is capable of moving in response to light levels (Pearse, 1974) and aggression from other sea anemones (Francis, 1973). No study to my knowledge has been conducted to investigate wave intensity as a potential factor for secondary habitat selection.

 

[Anonymous]

Materials and Methods
Density and average diameter
I measured the density and individual average size of 14 colonies of A. elegantissima from Horseshoe Cove and the area directly west of the Bodega Marine Laboratory in Bodega Bay, CA. All colonies selected were exposed to air at low tide.
Colony density was measured by randomly placing a 10 cm2 quadrat over colonies and counting all anemones visible that had their mouth or 50% of their oral disk within the quadrat. This was repeated 5 times for all colonies. The quadrat was always placed in an area of complete anemone coverage; there were no bare areas of rock or other large organisms occupying the quadrat. Individual average diameter was measured by placing a 1 m transect over the colony and measuring the diameter in cm of all individuals touching the transect. The diameter measured was defined as the shortest possible line that passed through the individual’s mouth.

Average wave intensity
Certain parameters of water motion such as velocity in laminar flow are fairly straightforward to measure quantitatively. Measuring mass flux or average water velocity quantitatively is confounded by stationary objects that create turbulence and change the flow locally. (Vogel, 1994). Fortunately a qualitative method for measuring average water velocity is readily available. Any substance that dissolves in saltwater will lose mass faster in areas of greater average water velocity or turbulence (Bell and Denny, 1994).
I used stationary blocks of Plaster of Paris as indicators of average water velocity in the vicinity of colonies of A. elegantissima. Plaster blocks were cast in small paper cups to create a near cylindrical shape. The blocks were baked to remove all water, weighed to determine initial mass (Mi), then placed in protective netting to prevent abrasion by large particles or animals and cemented to a rock surface. The blocks were placed for 4 days near all 14 colonies. There were two control blocks placed in still seawater. The blocks were then removed, baked again to remove all water, and weighed to determine final mass (Mf). Flow index (FI) was calculated with the following equation:
FI = (Mi – Mf)/Mi
The elevations of the blocks varied slightly between colonies but were all approximately +1 – 1.5 m above mean lower low water level. I assumed that any difference in time submerged between the blocks would not significantly impact their dissolution rate.

Maximum wave intensity
I attempted to measure maximum wave intensity with a spring scale placed in waves near the 14 colonies. The scale is based on the maximum force recorder described in Bell and Denny, 1994. The recorder is a simple tool that measures the largest force imparted to a spring by drag, thus giving a quantitative measure of the maximum wave intensity over a given time period. I measured the maximum force by placing the spring scale at the end of a length of PVC pipe and holding it near the colony for five minutes. The spring used measured forces in the approximate range of 0 to 10 N.

Habitat selection
There is currently no evidence that motility in A. elegantissima is directed to locate areas of differing wave intensity. I investigated whether A. elegantissima was more motile at differing wave intensities by tracking individual anemones in aquaria of differing flow intensity. Two clear glass containers (approximately 25 cm wide X 35 cm long X 5 cm tall) were set up with flow provided from a Carlson surge device (Carlson, 1996). This device provided a brief but powerful surge within the containers to attempt to replicate natural water flow. I used different diameter piping and flow restrictors to adjust the intensity of flow to each container. It should be noted that this did not affect flow duration or period between containers. One container was designated high flow and had an average flow velocity, as measured by the dissolution rate of Plaster of Paris blocks, approximately 4 times that of the low flow container. One cm2 grids were placed beneath the containers to show position of the anemones.

Individuals from 12 colonies were collected from the field. I placed three anemones at a time in both containers and allowed approximately one hour for their foot to attach to the glass. Draining all the water out of the containers aided the anemones’ attachment to the glass. I allowed water to flow into the containers and recorded individual’s positions every other hour for a total of 8 hours. I calculated average speed and direction of motion from these data.

 

[Anonymous]

Results
The data showed that larger individuals, on average, and less dense colonies of A. elegantissima were found in areas of lower wave action. Similarly, smaller individuals, on average, and denser colonies of A. elegantissima were found in areas of relatively high wave action. The average density of anemones per 10 cm2 increased with respect to the flow index according to the linear regression equation:
Average density = 6.502 + (6.97 x Flow index); (F1, 12 = 4.257, P = .061, R2 = .26).
It should be noted that in all cases the substrate was at or near 100% coverage by anemones; thus, density should be interpreted as an alternate measure of individual sizes within the colony rather than colony success or fitness.
The average diameter of individuals decreased with respect to the flow index according to the linear regression equation:
Average diameter = 3.302 – (1.147 x Flow index); (F1, 12 = 3.719, P = .078, R2 = .24).
Only two of the 14 colonies had average diameters smaller than 2.0 cm (1.91 and 1.67 cm), the approximate size at which individual anemones can sexually reproduce. It is interesting to note that these two colonies were located in the area of the highest average wave velocities measured (FI = 0.93 and 0.97, respectively) on the south side of Horseshoe Cove. Local topography and prevailing wind direction make this the area most directly hit by ocean currents of all the areas studied.

This area was, perhaps not surprisingly, one of the few locations that any readings were obtained on the maximum wave velocity recorders. The majority of locations tested showed a zero reading on the spring scale. The only colonies that had waves intense enough to register maximum wave readings had flow indexes in the range of 0.80 to 0.97. These locations imparted a drag equal to approximately 2-3 N or less. It should be noted that readings in more exposed areas (mussel beds) ranged as high as 15 N. Although the maximum wave recorders were not accurate enough to provide quantitative data, they did confirm two findings: Colonies of A. elegantissima are found in areas relatively sheltered from wave action and, assuming average and maximum wave velocity are positively correlated, that the plaster blocks were a good indicator of average water velocity.

Habitat Selection
Individuals of A. elegantissima showed significant differences (P <0.001) in average speed between the high and low flow treatments in the lab. The mean average speed (0.483 ± .045 SE cm/h) of anemones in the high treatment was over twice that of mean average speed (0.204 ± .035 cm/h) in the low treatment (Fig.6). The flow indexes of the high and low flow treatments were calculated as 0.52 and 0.16, respectively, using the same methods that were used to calculate flow indexes in the field. These flow indexes should only be used comparatively between the two treatments and not to the field data. Several factors such as exposure to air, differing temperature, grazing organisms, and abrasive sediments confound any comparison between the two.
Average direction of anemone movement was also recorded and compared between treatments. The extra force imparted to the anemones from the increased flow could explain the above data. That is, the increased flow could simply be “pushing” the anemones at a higher average speed across the glass. If this were the case, I would expect to see a significant difference in movement away from the flow output. Instead, anemones in both treatments showed no significant preference to move in any particular direction. Although anemones responded to differing average flow velocities, there appeared to be no ability to detect what direction the flow was originating from.

 

[Anonymous]

Discussion
Growing to a large adult size is likely to be advantageous to A. elegantissima for a number of reasons, including a lower chance of predation, a larger area for food capture, and an increased ability to deal with some abiotic stresses such as heat and desiccation because of a smaller surface area per volume ratio (Denny, 1988). The most important advantage, though, may be the increased reproductive output that larger size confers. Larger individuals can dedicate a larger volume of their body to gonadal tissue and produce more gametes, thus increasing their fitness. The assumption made here is that all larvae are equally likely to survive the planktonic stage; it is possible that smaller individuals produce a small number of more robust larvae that are more likely to survive the planktonic stage. It would be impossible, however, to track the thousands of larvae produced by a single anemone to their eventual recruitment site and estimate true fitness.
This study showed that larger individuals of A. elegantissima are not found as often in areas of more intense wave action. Intense wave action thus appears to decrease fitness in A. elegantissima. One possible reason for this is that larger individuals experience more mechanical stress from intense wave action due to drag. It is unlikely that an anemone firmly secured to the substrate would be dislodged by intense wave action, but it may tear or be ripped by strong waves and debris. Wave velocity may also play an important role in rate of food capture in A. elegantissima. The diet of A. elegantissima is largely comprised of small zooplankton such as copepods and amphipods that are washed away from surrounding areas (Sebens, 1982). Although an increase in average water velocity may increase zooplankton availability to A. elegantissima, this does not necessarily mean rates of food capture are higher. Food capture rates are actually inhibited in abnormally high flow velocities in a wide range (mussels, Cucumaria, Pseudocolochirus, crinoideans) of filter feeders (Ackerman, 1999, Denny, 1988, Toonen, pers. comm.). Among these animals there is a fairly narrow range of flow velocities at which food capture is maximized. This may be the case with A. elegantissima as well. Sebens (1980) showed that asexual reproduction was inhibited in A. elegantissima if they were fed continuously. From this we can assume that individuals would be less likely to asexually divide, and be larger on average, in areas where food was being captured at higher rates. This would help explain why larger individuals are found primarily in relatively low flow environments.
Intense wave action is beneficial to sessile organisms such as A. elegantissima for a number of reasons; increasing wave action will positively influence oxygen uptake, waste removal, and spread of gametes. However, as described by Denny (1988), there is a “saturation point” for an organism at which any increase in wave velocity will not significantly affect these types of processes. As shown, there is a decrease in fitness above this “saturation point” for A. elegantissima. Thus, there is a narrow range of wave velocities that A. elegantissima is most fit in. In order to maximize fitness, it would suit A. elegantissima well to be able to detect differing wave velocities and respond by selecting a habitat that had wave velocities within this narrow range.
The results of my laboratory study show that individuals of A. elegantissima are indeed capable of detecting differing water velocities and respond by becoming more motile. Denny (1988) pointed out that organisms smaller than about 0.5 cm are largely unaffected by drag of wave action in the same way larger organisms are. This may help to explain why planula larvae of A. elegantissima are able to colonize mussel beds. At some size, the habitat of the mussel bed becomes unsuitable and juvenile anemones must colonize areas of the intertidal that are less exposed. There are several possible factors that might direct this movement including chemoreception, substrate texture, and perhaps wave velocities. Adult anemones may move as well to find areas of more suitable flow if local conditions change. Individuals appear to be unable to detect the direction that flow is coming from; they may rely on a temporal, rather than spatial, “sense” to direct movement. Simply put, an individual may keep moving until it finds a flow regime that it finds suitable. Future studies could focus on the ability of A. elegantissima to detect flow velocities and directions, and measures of fitness of entire colonies of A. elegantissima in the field relative to wave velocity.

 

[Anonymous]

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[Anonymous]

If you read all that, congratulations!

This is how science works. It takes pages and pages of explanation to say what most reefkeepers already know: "Simply put, an individual (anemone) may keep moving until it finds a flow regime that it finds suitable."

Cheers!
Matt