DISCUSSION
Seastar Biology
Taxonomy
Life History
Echinoderms
Rocky Intertidal
The Study
Introduction
Methods
Results
Discussion
Picture Gallery
Bibliography and Weblinks
Glossary
Temperature and Distribution
Data analysis shows the findings in this study in agreement with other comparable abundance and distribution studies. The northern seastar Asterias vulgaris is found in moderate densities while the ambient air temperature remains fairly warm but tends to migrate into deeper waters of the subtidal region once temperatures become colder. Water temperatures fluctuate less dramatically than air temperatures, and during winter months remain much warmer than air temperatures as well. With A. vulgaris preferring temperatures ranging between 6º and 17ºC, it is clear why they would prefer to remain underwater and not suffer from exposure to harsher environmental conditions that occur during the winter.
In some
cases, seastars were found at much higher elevations within the intertidal
than was expected for later months. They showed no clear pattern in their
distribution and were scattered quite randomly throughout the intertidal,
existing in a wide range of habitats. One slight trend, however, was
noticed. All seastars sampled during the months of October and November
located in the mid or high intertidal zones were found within tide pools.
Two more seastars at the low intertidal were also found residing in tide
pools. This trend further suggests that ambient air temperatures during
these months were perhaps testing the lower temperature tolerance of the
seastars, which found refuge in the slightly warmer tide pool waters. Water
temperatures were shown to be warmer than air temperatures in three of the
four months measurements were recorded. The temperature data gathered
during the first trip in August shows that tide pool temperature was also
shown to fluctuate significantly less than air temperatures. In each of the
four categories measuring water temperatures – mid intertidal exposed, mid
intertidal sheltered, low intertidal tide pool, and very low intertidal with
seawater inflow – water temperatures remained stable, fluctuating only 0.5ºC
for a very short period of time.
Foraging effects
The continual presence of Mytilus edulis mussel beds during the later months samples were taken suggest that food is not a predominant factor in determining seastar abundance and distribution. If the mussel beds had become scarce leading into the winter months with the abundance of seastars also decreasing, then we might consider food abundance to affect seastar distribution. Surveys of the intertidal showed that mussel beds persisted at all tidal heights, indicating that temperature proves to be a greater factor in determining seastar distribution than does food abundance. Presumably, there are other sources of food for A. vulgaris to consume in the subtidal range other than the mussel Mytilus edulis, which allows the seastar to remain submerged in the subtidal and avoid unnecessary migration to the intertidal where weather conditions are more marginal.
