Distribution of sea stars and other sessile macroinvertebrates on pilings at Belcarra Beach, BC

Hanna Jackson

Department of Biological Sciences, Burnaby, BC, Canada

*This report is a class project NOT peer-reviewed science


How are the abundance of sea stars and sessile macroinvertebrate on pilings in the sea at Belcarra beach related?


Sea stars will be present in areas where there are less large sessile invertebrates.


We dropped cameras to the bottom sediments of the ocean while they were recording the adjacent piling. Our data was collected by analyzing footage and recording the number of sea stars and sessile macroinvertebrates present at depths of 1, 2, 3, 4, and 5 (if applicable) meters. In total we observed and analyzed five pilings, and at each depth we averaged the abundances of each organism type across all five pilings.


The abundance of sessile invertebrates was highest on average at a depth of 3 m with 5.4 individuals present, whereas the abundance of sea stars was highest on average at 4 m, with an average of 5.6 individuals present. Both groups were absent from the first meter below the surface. The distribution of sea stars had a similar variance compared to sessile macroinvertebrates, with a standard deviation of 2.4 and 2.34 respectively.

Figure 1 At depths of 1, 2, 3, 4, and 5 meters down pilings, number of sessile invertebrates and starfish visible in the camera frame were counted and averaged across 5 pilings.


Sessile macroinvertebrates had a slightly higher distribution peak than did sea stars. This is potentially explained by the ecological interactions of these two species. Sea stars are top predators (Mauzey, 1966), aggregating in patches at densities up to 20/m2 (Shivji, 1983), and are excellent competitors for space as a result. However, there is also evidence that sea stars avoid areas with sea anemones (Patton, 1991), so our interaction of interest requires further investigation to determine specifically which species of macroinvertebrates and sea stars were present to fully understand the ecology of this system. Regardless, there must be an equilibrium distribution of sea star and invertebrates, perhaps mediated by the periodicity of sea star feeding, with the winter period we sampled in being a known time of decreased feeding (Mauzey, 1966). Future research should focus on the relationship between sessile invertebrates and sea star distributions and their distance to the bottom sediment, to determine if depth below the surface or distance from the ground is the main contributing factor to determining their relative distributions.


We would like to thank Belcarra park for permission to use their land.


Mauzey, K. P. 1966. Feeding Behavior and Reproductive Cycles in Pisaster ochraceus. Biological Bulletin 131:127–144.

Moore Lab. 2020. AquaticBiology Wednesday Lab. Retrieved January 15, 2020, from https://www.youtube.com/watch?v=QJh_la-hyGw&feature=youtu.be

Patton, M. L., S. T. Brown, R. F. Harman, and R. S. Grove. 1991. Effect of the anemone Corynactis californica on subtidal predation by sea stars in the southern California Bight. Bulletin of Marine Science 48:623–634.

Shivji, M., D. Parker, B. Hartwick, M. J. Smith, and N. A. Sloan. 1983. Feeding and distribution study of the sunflower sea star Pycnopodia helianthoides (Brandt, 1835). Pacific Science 37:133–140.