The effect of large-scale oceanographic fluctuations and small-scale
microhabitat differences on the Pacific coast intertidal sea urchin, Strongylocentrotus purpuratus
Michael Russell
Department of Biology,
The
purple sea urchin is one of the most intensively-studied marine
invertebrates. It plays a pivotal role
in community dynamics in subtidal as well as
intertidal marine systems. Both
historical data collected over the last six decades and more recent population-level
surveys from sites ranging from Vancouver Island, Canada to Punta Baja, Mexico,
reveal that intertidal populations are sensitive to the influences of the
relative strengths of El Niño and La Niña events. Reanalysis of a long-term data set of monthly
dissections from 1952 - 1964 at a single northern California site revealed that
reproduction (assessed as percent body weight of gonad tissue) showed a strong
correlation with the Multivariate ENSO Index (MEI) – an indicator of
large-scale ocean productivity. Our
recent work further quantifies this association on a broader geographic scale
as well as the influence of substratum and microhabitat differences on growth,
behavior, and morphology. The effect sea
urchins have on the rock substratum has been recognized for over 100 years and
specifically in purple sea urchins at least since the original publication of Between Pacific Tides in 1939. Through their grazing activity purple sea
urchins act as bioeroders and form cavities or pits of varying depths. The depth and extent of these pits is
associated with the friability of the rock substratum – in “softer” rocks like
mudstone, urchins excavate deeper pits than in less friable substrates like
granite. These pits in turn affect a
variety of biologically relevant and demographically important metrics like
growth, allometry, and behavior.
Caption: Purple sea urchins, Strongylocentrotus purpuratus, exposed at low tide (Cape Arago, Oregon). In areas where the rock is friable like this sandstone, the scraping of spines and rasping of teeth form cavities, or pits. Sea urchins favor these microhabitats when available over flat substrata and individuals found in pits have significantly greater height : diameter ratios in the test. Occupying pits also affects food consumption rates and the allometric growth patterns of Aristotle’s lantern.