Boundaries in the ocean and the cons on having a long larval life

Large scale pattern of coastal currents for the east coast of North America. Currents are represented as the annual average of current speed (m/s) in grids of 100km, the data was extracted from Lumpkin and Garrafo 2005 (http://www.aoml.noaa.gov/phod/dac/drifter_climatology.html). Two recognized biogeographic boundaries, Cape Hatteras and Cape Cod, are indicated in the map.

by Paula Pappalardo

On land, it is easy to imagine that some species cannot reach some places. A mountain, a desert, a river—depending on the organism, these or other landscape features can act as barriers to movement. But what stops organisms in the ocean? Although ocean waters appear to connect all marine habitats, different regions have water masses with different properties, and the associated variation in temperature, salinity or nutrients may affect species distribution. Traditionally, temperature has been viewed as the most important factor to explain distribution of species in the ocean, but recent studies suggest that currents also play an important role.

Ocean currents can transport spores, eggs and larvae of marine species. Many marine invertebrates have a larval stage in their life cycle, which can last from days to months, during which they drift or swim in the ocean waters. It is generally accepted that more time in the water can be associated with greater dispersal distance. But more time in the water also means more time to be affected by currents. Depending on their direction and pattern of circulation, currents can carry the tiny larvae far from suitable habitat, and thus act as a boundary between ocean areas. If currents are important to explain boundaries in the coastal ocean, we would expect more boundaries for those species with larvae that stay in the water for a longer period, and we would expect the distribution of species boundaries in the ocean to be related to the pattern of circulation.

To explore these ideas and understand better what determines species boundaries in the coastal ocean we analyzed the distributions of almost 1800 marine invertebrates (crustaceans, mollusks, annelids, echinoderms and cnidarians) along the eastern coast of North America. We gathered information on depth distribution of the species—that is, whether they inhabit shallow or deep waters—and we also classified species by “short” or “long” larval duration, depending on the time that larvae spend in the water to complete development.

Examples of larvae with long life in the plankton. On the left, a pluteus echinoderm larva (photo credits: Bruno C. Vellutini / CC-BY-SA-3.0), on the right, a barnacle nauplius larva (photo credits: http://planktonnet.awi.de/index.php?contenttype=image_details&itemid=16952#content, CC-BY-SA-3.0). 

Did all species have the same chance of being affected by a boundary? No. Species with different characteristics showed different locations of boundaries. Some locations along the coast had more boundaries for shallow species; some locations were more important barriers for deep species. When comparing similar locations, we found more boundaries blocking movement from south to north than from north to south. Taking a closer look at the northern boundaries we found, as expected, more boundaries in species with “long” larval duration.

Overall, our results suggest that the pattern or circulation – that is, the direction of currents and regions of slow transport – can affect the location of range boundaries, particularly in species with long larval duration. The consistency of these current patterns and their strong effect on larvae can help to explain why species boundaries of so many different species are concentrated in narrow portions of the coast. And it may affect how climate change influences species distributions, which may depend not only on gradual changes in ocean temperature, but also on the changes in ocean circulation.

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