My Experiment: Change in Zonation Patterns and Attempts to Look at Juvenile Resettlement


Experimental Design


Discussion of Results

Why my Experiment Didn't Work

How to Improve Experiment

Mussel's Motivation for Movement




The movement of mussels in the intertidal is quite intriguing because they are known as sessile organisms. The first experiment I did was to look at the zonation patterns of the mussel Mytilus edulis may change in the rocky intertidal at Nahant, MA. I compared two transects, 6 Sept and 2 Nov, and found no significant difference in the percent cover Mytilus between the two dates in the low or mid intertidal. The second study attempted to look at the distances traveled during the resettlement of Mytilus juveniles. Reasons are discussed as to why the experiment failed. I concluded that the mussel's motivation for movement, primarily as young juveniles, is for survival. Young mussel's settling away from adults can grow without the competition from adults or threat of being eaten before primary settlement, while association with adults is beneficial for protection against things like desiccation and wave stress.


Experimental Design

Zonation of Mytilus edulis

The first time the class went to Nahant, 6 Sept, everyone did a transect, setting down quadrats at regular intervals. My group places a 25 x 25cm quadrate every 2 m along a 15 m transect vertically across the intertidal from high to low intertidal. Mussels were recorded based on percent cover, estimated based on the visual division of the quadrat into 9 equal parts and the number of parts covered, rounded to the nearest five percent. From this data I was able to develop a picture of the musselís zonation. See Figure 1A

I was also able to get everyone elseís data from 6 Sept and do a rough conversion to percent cover that I felt was comparable to the data I collected so that I could make a further comparison with transects done on 2 Nov.

On Nov 2, the third trip to Nahant, I did four transects, each 15 m long. A quadrat 25 x 25 cm was placed every three meters along the transect line. Random integers between 1 and 10 were generated from a TI-83 calculator to determine how many steps to take away from the transect line, alternating left and right, before setting down the quadrat. Figure 1B shows the zonation of Mytilus on 2 Nov.

To compare the number of mussels in the low tide on 6 Sept with the number in the low tide on 2 Nov I performed a two-tail, unpaired t-test and found no significant difference. The null hypothesis being that there was no difference between the mean percent cover of Mytilus in the particular section of the intertidal. These results are summarized in Table 1. The same statistical test was used to compare the numbers found in the mid-intertidal on both dates, again no significant difference was detected, see Table 2.  See discussion of results below.


Click on "video clip" to see an in field description of the transect process (you need quick timeģ to view this)

  Video Clip


Juvenile Resettlement

The trip to Nahant on 4 Oct I set up an experiment to observe the secondary settlement of Mytilus edulis juveniles. Approximately 100 small mussels were marked in each of four different mussel beds. Two of the beds (A and B) were somewhat wave-exposed and approximately 20 x 30 cm in dimension. Mussel bed A contained mostly very small Mytilus while mussel bed B contained larger Mytilus. Mussel bed C was slightly smaller, more wave-protected and was in the vicinity of 2 Asterias at the time of the marking. Mussel bed D was much larger than the others, fairly wave-protected and only a small portion of the bed with very small Mytilus was marked. The mussels in each bed were marked with a different color nail polish. The proposed plan was to return on 2 Nov, relocate the beds and count the marked mussels still remaining. The second part of the plan was to search for the marked mussels that had moved and measure the distance the mussels had moved from the original bed. Doing this would give me an idea of how far on average the mussels moved in their resettlement. There were several flaws and failures in the design and implementation of this experiment which are covered in the discussion section.






Figure 1 shows the percent cover for each quadrat on a transect line moving away from the low tide from Sept.6 (A) and November 2 (B). The transect for Nov. 2 is an average of 4 transects.



Low Intertidal 6-Sep 2-Nov
Mean 19.05 0
Variance 1047.776667 0
Observations 4 4
Pooled Variance 523.8883333  
Hypothesized Mean Difference 0  
df 6  
t Stat 1.177038353  
P(T<=t) two-tail 0.283753495  
t Critical two-tail 2.446913641  

Table 1A Shows the results of a two-sample, two-tail t-test (p < .05) comparing the percent cover of Mytilus edulis at Nahant in the low intertidal between 6 Sept and 2 Nov. The results show no significant difference between the two.


Mid Intertidal 6-Sep 2-Nov
Mean 42.5 12.5
Variance 784.8333333 106.8333
Observations 4 4
Pooled Variance 445.8333333  
Hypothesized Mean Difference 0  
df 6  
t Stat 2.00932406  
P(T<=t) two-tail 0.09123992  
t Critical two-tail 2.446913641  

Table 1B Shows the results of a two-sample, two-tail t-test (p < .05) comparing the percent cover of Mytilus edulis at Nahant in the mid intertidal between 6 Sept and 2 Nov. The results show no significant difference between the two.

Have a look at the raw data

Discussion of Results

The results of the t-test suggest that there was no difference in the distribution of Mytilus edulis between the 6 Sept and 2 Nov in either the low intertidal or the mid-intertidal. The graphs show there may perhaps be a slight shift in the zonation pattern, the peak in numbers occurs lower in the intertidal and with less percent cover. Interestingly enough, my observations were in conflict with the results of the graph or even of the statistical test. Unfortunately, due to my attempts to set up a different experiment on 4 Oct, I was unable to do transects during that visit to Nahant. On 6 Sept and even more so on 4 Oct I noticed an abundance of juvenile Mytilus in the lower intertidal, lower than their normal distribution.  There were also a number of Asterias forbsei, sea stars, on the first and second visit and I was initially very interested to see if we would see a decline in the number of Mytilus in the low intertidal on the third visit. Indeed, no Mytilus were found in any of the quadrats in the lower intertidal on 2 Nov, even though this was not significantly different from the number found in the quadrats done on 6 Sept. Also, on 2 Nov I found several broken Mytilus shell pieces, unmistakable for their blue color, and I had not observed this quantity of dead Mytilus pieces in previous visits. It would have been very interesting to have done four transects on 4 Oct, I had no trouble finding mussel beds in the low intertidal to mark for my attempt at a second experiment. Also, this would have given me better data to compare to the more controlled transects done 2 Nov which were not as comparable as would be desired to the 6 Sept. transects. What would have been especially desirable would be to have done four transects on 4 Oct in the same place that they were done on 2 Nov. This would have given me a paired comparison, and would be a more accurate picture of change, especially since the limited amount of time I had at Nahant only allowed for four transects to be completed each visit.


Why My Experiment Didnít Work


There are numerous reasons why my experiment didnít work. The first major problem was lack of information. My experiment was based on reading a short textbook summary of the settlement process of Mytilus juveniles. Had I done more research beforehand I would have discovered that the initial juvenile settlement usually occurs in the summer months (Gilg and Hilbish 2000, Bayne 1964). I also would have had a better understanding of the size of the early and late plantigrades, <500μm and >500μm to 1000μm shell length, respectively (Gilg and Hilbish 2000). The mussels I was marking in October were most likely juveniles that had already undergone secondary settlement in August and were already about 5-15mm in shell length.

            Another serious problem was that my marking method was very ineffective. The 4 Oct was a relatively rainy day and other people in the class using the same marking method (nail polish) went back the next day only to find their markings had completely washed away. The size of the mussels also would have made it nearly impossible to find any marked ones had they actually moved. In fact, on 2 Nov., I was only able to relocate one the mussels beds (Mussel Bed 2) that I had marked. Part of this was due to the serious decline in mussels in the lower intertidal on the second visit, presumably due to Asterias predation, based on the observation of Asterias abundance and Mytilus shell pieces. There were also a number of Carcinus maenus, the green crab, also a known predator of Mytilus edulis, in the intertidal at Nahant.



How to Improve the Experiment


This experiment would require much more time at the intertidal than I had this semester. It would also need to be done during the summer, when Mytilus larval settlement is at its peak. The experiment could take place within a month. It would be extremely difficult to mark and follow early plantigrades due to their tiny size as well as there being thousands of them settling and present within only 5 square meters of the intertidal. I could not find any estimates as to the distance the plantigrades travel during their second settlement. I think the best place to start would be to find the vertical distribution of early plantigrades and adult mussels within the intertidal by doing several transects and collecting and measuring sample groups under a dissecting scope. Then after two weeks I would return to the same transect lines and look at the distribution of late plantigrades using the same method. This would at least give an idea of how the distribution may or may not change. Gilg and Hilbish (2000) found that the mussels did not show any evidence of differential settlement during either primary or secondary settlement and suggest that it is natural selection which leads to the zonation pattern seen in adult Mytilus, inhabiting primarily the mid-intertidal. 


The Mussel's motivation for movement


The musselís motivation for larval movement can be thought of as their adaptive mechanism for survival, both as larvae and as adults. Larvae, as described above in the juvenile resettlement section, settle away from adults so that they can grow to a reasonable size without competition with adults for food and oxygen. Also, adults can take in their own larvae through their inhalent tube. One prediction suggests that an adult, in normal filtration practices, may filter water containing up to 100,000 invertebrate larvae in 24hr. Thus settling away from adults initially promotes the survival of the species. So why move again? It is also suggested that mussel beds help reduce wave stress and prevent desiccation for the members of the bed. Living in close association is also promotes fertilization success, since, as mentioned above, their fertilization occurs externally.


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Zonation of Mytilus in Intertidal

Juvenile Resettlement