INTRODUCTION

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Seastar Biology
Taxonomy
Life History
Echinoderms
Rocky Intertidal

The Study
Introduction
Methods
Results
Discussion

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Glossary

Geographic Distribution

         The northern seastar, Asterias vulgaris, is a predominant predator along the rocky intertidal coast of the Gulf of Maine.  Conspecific with A. rubens, found in Europe and the British Isles, the two species are morphologically indistinguishable but could perhaps be ecologically and genetically isolated from one another (Franz et al. 1981).  For this reason, American Atlantic populations of the northern seastar are known as A. vulgaris.  The geographic range of A.vulgaris extends southward to Cape Hatteras off the coast of the Carolinas all the way northward to Labrador (Bousfield 1960), corresponding with a preferred temperature range between 6º-17ºC (Franz et al. 1981).  Northern seastars are found throughout both the intertidal and subtidal regions and have been seen at depths exceeding 300 meters.  Vertical distributions show juvenile seastars and small adults generally prefer the shallow water and intertidal areas whereas large adults (>20 meters) are mainly found on soft bottoms at greater depths (Himmelman & Dutil 1991). 

Seastars as a keystone predator

Seastars are voracious intertidal predators and have been documented as a determining factor in patterns of distribution, abundance and diversity within low intertidal habitats (Lubchenco & Menge 1978).  A. vulgaris plays an important role as a keystone species in the North Atlantic rocky intertidal by restricting the growth and distribution of the mussel, Mytilus edulis, the dominant competitor in the Atlantic rocky intertidal zone.  In the absence of A. vulgaris mussels outcompete all other organisms in the lower intertidal and subtidal regions and the lower edge of the mussel zone is extended into deeper waters.  The presence of A. vulgaris limits the mussel zone to higher in the intertidal, at heights where the seastars cannot feed due to prolonged emersion.  By removing mussels, seastars encourage biodiversity in the lower intertidal by allowing other organisms, such as barnacles and seaweeds, to settle in areas otherwise occupied by mussel beds.

Seastar migration patterns

Seastar abundance fluctuates seasonally due to inshore migration in summer to feed on mussels and offshore migration during winter months, presumably to avoid severe wave conditions in shallow waters of the subtidal region (Menge 1979).  In a comparative study between the seastars A. vulgaris and Leptasterias polaris, another seastar found in the Gulf of Maine, it was found that following harsh winter conditions numerous individuals of L. polaris had scars on the aboral surface, possibly caused by ice abrasion, whereas A. vulgaris observed at the same time and depth had no damage.  This evidence along with a noted seasonal migration in one year of study suggests that A. vulgaris tends to migrate to greater depths during winter months (Gaymer et al. 2001) in order to avoid the harsh environmental factors that occur in winter.  Environmental conditions moreso than predation are thought to be the limiting factor in the abundance and distribution patterns of A. vulgaris.  Predation on A. vulgaris is unlikely to be a limiting factor since predators are uncommon and probably limited to other seastars (Himmelman 1991). 

Study objectives

            The objective of this study was to survey the rocky intertidal zone at Nahant to better understand the abundance and distribution patterns of the northern seastar, Asterias vulgaris.  It seems evident from the literature that seastars occur in low numbers across the rocky intertidal (Gaymer et al. 2001) and tend to migrate into deeper waters with the onset of winter to avoid environmental stress.  This study sought to explore this paradigm by recording seastar abundances and distributions over a three-month period (September to November) during monthly visits to the rocky intertidal.  As previous studies have suggested, I expected that the abundance of seastars in the intertidal would decrease in numbers as the months passed.  I also hypothesized that from the month of September to the month of November, seastars would be more frequently seen in tidal pools and sheltered habitats as opposed to the exposed rocky shore, since water temperatures fluctuate less dramatically and are warmer than the ambient air temperature during winter months.

Abstract                                                                                       Methods

Periwinkle snails
Littorina spp. in a rocky intertidal habitat

Asterias vulgaris
Northern seastar in a tidal pool

Mytilus edulis
Blue mussels attached to a rocky substrate
 

Asterias vulgaris
Seastars surrounded by Irish moss (Chondrus crispus) at low tide