Abstract: For the rocky intertidal at West Quoddy Head, Lubec, Maine, the size distribution of Strongylocentrotus droebachiensis appears to be similar, with almost no significant difference with respect to tidal height. The relationship between the diameter(length) of the shell and total weight of the urchin, is statistically the same at both tidal heights. There does not appear to be a significant difference in the relationship between gonad weight and length across the tidal gradient.
Introduction: For the green sea urchin, Strongylocentrotus droebachiensis, total weight and gonad weight are variable between seasons (Kramer and Nordin, 1978). According to Kramer and Nordin (1978) and Vadas, et. al (unpublished 1989), the reproductive event occurs in December and January. In preparation for this reproductive event S. droebachiensis increases gonadal weight starting in the early fall (Kramer and Nordin, 1978). Much study has been done on algal preference of S. droebachiensis (Vadas, 1977; Larson, et. al., 1980; Prince and LeBlanc, 1992). This research has shown a preference towards Laminaria longicuris, and Chondrus crispus, with a general non preference to Agarum cribosum. Work done by Paine and Vadas, (1969) shows that preference of algae is not necessarily determined by caloric value as both the preferred species and non-preferred species had the same caloric value.
Depth has been analyzed as a determining factor for reproductive output in the subtidal (Keats, et. al.,1984). It was shown that reproductive output was greatest in the shallowest depths. This was also where the preferred species were found.
It has also been proposed that mean urchin size is dependent on tidal pool depth. It has also been stated that mean urchin size is dependent on food availability and quality. Mean urchin gonad weight may also be dependent on the same variables. Kramer and Nordin (1978) observed that the relationship between shell diameter and total weight is linear. The relationship between shell diameter and gonad weight was shown to be dependent on season and variable between individuals and sexes. In this paper we look at these hypothesizes with respect to a sampling study done on September 8, 1994 at West Quoddy Head.
Materials and Methods: Samples were taken from five tide pools near the high and five tide pools near the low tide. A tide pool is a depression in the rocky shore which when the tide goes out is left with standing water. In each pool at least two quadrat samples were taken using a 0.09 m2 wood quadrat, tossed out on top of the pool. Tide pool area was measured by taking the longest distance across a pool and then measuring the greatest perpendicular width. Depth was estimated using a meter stick reading from at least two places within each pool and taking the average. Primary and secondary cover percentage within the quadrats were estimated by a group consensus of 3-4 people. Primary cover was defined as the amount of substrate being occupied. Secondary cover was defined as the amount of material shielding view of the bottom. Strongylocentrotus droebachiensis individuals were collected from within the quadrats and from the entire tide pool. A casual walk along the rocks and in tidal pools not sampled, was done to collect live urchins. Dead urchin shells were collected on this walk also to estimate if a certain size was being preyed upon by gulls. The specimens were taken to UMM where they were placed in a freezer until analysis the following week.
In the laboratory the urchins were measured for shell diameter using Vernier calipers to the nearest 0.1mm, total weight and gonad weight using a Metler balance to the nearest 0.1g. Gonad weight was obtained by shelling the gonad from the urchin, then blotting the gonads with paper towels. PC-SAS was used to do statistical ANCOVA tests.
Results
Distribution and abundance: In the high pools urchins were scarce. In all low pools urchins were found. Due to lack of urchins in most quadrats, urchin counts are for the total in each pool. Abundance is computed as the number of urchins divided by the area of the pool, and is found in Table 1. In general abundance of urchins is greater in the low pools. Dead urchin shells and live urchins were found on the casual walk with a size range for both comparable to the size range of the low pools (Table 1).
| Site | Number | Individ./sq. m | Mean size(mm) +/- 1SE |
|---|---|---|---|
| High 1 | 0 | 0 | 0 |
| High 2 | 0 | 0 | 0 |
| High 3 | 0 | 0 | 0 |
| High 4 | 7 | 0.31 | 25.09 + 6.37 |
| High 5 | 0 | 0 | 0 |
| Low 1 | 11 | 0.11 | 34.12 + 4.5 |
| Low 2 | 45 | 3.44 | 42.42 + 2.07 |
| Low 3 | 7 | 0.93 | 50.23 + 3.97 |
| Low 4 | 44 | 1.74 | 52.20 + 1.88 |
| Low 5 | 15 | 0.63 | 52.44 + 1.36 |
| Casuals | |||
| Alive | 20 | ---- | 48.52 + 1.85 |
| Dead | 23 | ---- | 51.29 + 1.14 |
Morphological Measurements
In Table 2 are the mean total and gonad weights for each pool. The table summarizes the data and gives the standard error of the pool. For most low pools the mean gonad weights and standard errors overlap, but the high pool mean gonad weight only seems to overlap with Low pool 4. Total weight appears to be higher in the low pools with the exception of low pool 1.
| Site (pool#) | Mean Total +/- 1SE | Mean Gonad +/- 1SE |
|---|---|---|
| High 1 | 0 | 0 |
| High 2 | 0 | 0 |
| High 3 | 0 | 0 |
| High 4 | 15.76 + 11.49 | 1.85 + 0.53 |
| High 5 | 0 | 0 |
| Low 1 | 25.29 + 7.41 | 3.18 + 0.31 |
| Low 2 | 41.24 + 4.40 | 5.70 + 0.46 |
| Low 3 | 56.28 + 9.32 | 4.94 + 0.81 |
| Low 4 | 67.29 + 4.59 | 2.43 + 0.08 |
| Low 5 | 46.19 + 8.69 | 3.07 + 0.51 |
An ANOVA test was performed on the relationship between tidal height and shell diameter and showed no significant difference in size between tidal heights at the 95% confidence limit (P=0.0712). There was a significant relationship between total weight and shell diameter (P=0.0001). There was no significant difference in the total weight versus shell diameter relationship between the high and low site (P=0.569). With the same relationship no significant difference can be seen between tide pools in the low (P=0.1663). There was a significant relationship between gonad weight and shell diameter (P=0.0001), but no difference was found between the high and low site (P=0.1646). There was a significant difference between low pools with relation to gonad weight versus length (P=0.0488).
Algal Cover
In high pools in general no dominant algal species was found. The only pool in which there was any kind of a dominant was high pool 4, which is also the only pool where urchins were found. Most lower pools had a dominant of either C. crispus or L. longicuris, two of the preferred algal species. Tables 3 and 4 summarize this data.
| Site (pool#) | Mean primary +/- 1SE | Dominant |
|---|---|---|
| High 1 | 87.5+7.5 | No dominant |
| High 2 | ---- | ---- |
| High 3 | 32.5+17.5 | No dominant |
| High 4 | 19.6+18.2 | Coralline/L. digitata |
| High 5 | 52.5+37.5 | No dominant |
| Low 1 | 16+4.3 | L. longicuris/C. crispus |
| Low 2 | 50+20 | Encrusting/C. crispus |
| Low 3 | 42.5+12.5 | C. crispus/Coralline |
| Low 4 | 30+24.59 | L. longicuris/C. crispus |
| Low 5 | 30+14.45 | No dominant |
| Site (pool#) | Mean secondary +/- 1SE | Dominant |
|---|---|---|
| High 1 | 95+0.00 | No dominant |
| High 2 | ---- | ---- |
| High 3 | 52.5+7.5 | No dominant |
| High 4 | 75+2.88 | L. digitata |
| High 5 | 95+0.00 | No dominant |
| Low 1 | 77+9.7 | L. longicuris/C. crispus |
| Low 2 | 95+5 | C. crispus |
| Low 3 | 95+5 | C. crispus |
| Low 4 | 83+12 | L. longicuris/C. crispus |
| Low 5 | 47+17 | No dominant |
Physical Measurements
In Table 5 are the area, average depth and volume for each pool. High pool 4 has the largest values for area and volume of the high pools. These numbers are also similar to those in the low pools.
| Site (pool#) | Area(m2) | Mean depth +/- 1SE | Volume(m3) |
|---|---|---|---|
| High 1 | 3.1 | 27.5+2.5 | 85.25 |
| High 2 | ---- | ---- | ----- |
| High 3 | 2.9 | 20.0+5 | 58 |
| High 4 | 23.2 | 33.0+6.5 | 765.6 |
| High 5 | 5.4 | 47.5+7.5 | 256.5 |
| Low 1 | 102.5 | 2.9+0.64 | 297.25 |
| Low 2 | 13.1 | 06.7+0.6 | 87.77 |
| Low 3 | 7.5 | 40.0+16 | 300 |
| Low 4 | 25.4 | 32.6+11.6 | 828.04 |
| Low 5 | 24 | 15.0+2.9 | 360 |
Discussion: By looking at the mean shell diameter for the casual pickups it becomes obvious that gulls prefer to eat urchins in the 50-55mm size range. It would be reasonable to assume that the gulls prey more in the high due to greater length of exposure time which would account for the lack of larger urchins in the high pools. Possibly due to low numbers of urchins in the high pools the statistical data doesn't backup the idea that the urchins in the high tidal area are smaller than the low tidal urchins.
The data suggests that urchins of a given shell diameter, no matter where they are located in the intertidal, will have (x) total weight (Figure 1). It also suggests that the relationship between gonad weight and shell diameter is the same throughout the intertidal zone independent of tidal height(Figure 2).
It has been stated that depth was a dependent factor on reproductive output (Keats et al. 1984). That study, while based in the subtidal, does show a correlation between preferred algae species, shallow subtidal water, and larger gonad weights. This relationship can be seen in Table 2 where urchins were only present in those pools with at least one preferred algae species. This can also be seen in Figure 3 as gonad weight increases with total weight in the same line independent of tidal height. It has also been stated that food abundance and quality were important to the reproductive output of urchins (Prince and LeBlanc, 1992). For the two pools with the highest individuals per m2 (Low 2 and Low 4; Table 1), the dominant algal species were C. crispus and L. longicuris. This could be why low pool 2 has the highest mean gonad weights. In both pools, above 80% secondary cover was recorded (Table 4).
The idea of feeding induced aggregation (Vadas et al 1986) may have been the reason for the high number of individuals found in low pool 4. The pool was dominated by L. longicuris and C. crispus, two preferred species of the green sea urchin (Prince and LeBlanc 1992).
An alternate hypothesis that mean gonad weight is dependent on depth of the pool can be seen as not significant in Figure 4. The standard error for low pool 3 overlaps all the other pools and as a result no significant difference can be seen between pool depth and gonad weight.
An alternate hypothesis on urchin shell diameter being dependent on pool depth is shown in Figure 5. It shows the only comparison that could be made was at the 35 cm deep pools. For this depth the mean urchin shell diameter is smaller for the high tide pool than for the low tide pool at the same depth.
Literature CitedFigure Legends
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