Just before the weather turned ugly, we made is out to the shoreline below the cliffs at Cape Lazo. We were trying to make sense of some of the feeding relationships along the shoreline. As we get more experience and exposure, we’ll be able to figure out more of these complex intertidal food webs, where everything seems to eat everything. I’ll break down the experience into (1) Observations (2) Follow-up points and (3) Links for further consideration.
We noted the primary producers in various states of life and decay. Living seaweeds, algal crusts and phytoplankton (seen under the microscope in water samples taken from Cape Lazo) are the photosynthetic organisms that pull carbon (and nitrogen, phosphorous et al.) out of the water to create the organic matter that ultimately fuel the rest of the ecosystem. Their productivity is governed by nutrient availability, sunlight and how clear the water is- low light penetration is a limiting factor, particularly for the seaweeds.
We saw a number of organisms who feed directly on the seaweeds and algae– small limpets (Lottia sp.), snails (periwinkles– Littorina sp.) and shore crabs (Hemigrapsus sp.). We looked at the modified legs of the shorecrab, whose diet consists mostly of diatoms, green algae (eg. Ulva) and red algae. A potpourri of indiscriminate filter feeders/suspension feeders (mussels, oysters, bryozoans, sponges) consume the phytoplankton, zooplankton (including larvae) and detritus that happens to go there way as water is passes through.
Higher up the shoreline, we saw a melange of decaying/fermenting seaweeds. There are organisms that feed quite specifically on this decaying layer. We say Sand fleas and an indeterminate annelid worm. These organisms are in turn fed on by other invertebrates (eg. the fast moving wolf-spiders common on the shoreline) as well as larger birds. Many of you have seen the gulls poking at the sand above the high-tide line. They are foraging for sand fleas, among others.
Further down the shore, we saw swathes of small acorn barnacles completely shattered and removed from their rocky substrate. After some speculation, we noted a star-shaped pattern to this activity matching the shape of the purple sea star. This unsuspecting predator pulls apart the plates and valves of its prey and then inserts its stomach to digest the contents held within. By this means, it preys on barnacles, mussels, snails, chitons, limpets and even urchins. While we didn’t witness it on Thursday, we did trade horror stories about watching large gulls consume whole sea stars
We saw a variety of other feeding relationships– the circular borings of the molluscan radula into shells- likely the work of a gastropod. We tried to figure out what some of the gulls, harlequin ducks, cormorants and scoters were eating, as all were numerous. Unclear for now.
(2) Follow up points
We tried three methods for observing feeding relationships
-direct observation (works well for small creatures who don’t feed exposed)
-spotting interactions with binoculars then visiting the scene of the crime (will work with birds with a little patience)
-fecal analysis. The scat, frass, pellets and plops of the intertidal can usually be matched to the organism that left them. Some organisms (herons, crabs, minks, otters) leave very clear records of their diet in their scat. Others (gulls), not so much.
A fourth means is to actually take a live or recently dead sample and look at the gut contents. This may be ethically uncomfortable for some.
(3) Links for Further Consideration
Shirley brought a hard copy of Kozloff’s Marine Invertebrates of the Pacific Northwest
. This is the most comprehensive key to shoreline (and all other marine) life in our region. The paperback version can be had for about $50, but the keys themselves are all available online
, courtesy of Dave Cowles.
FEEDING RELATIONSHIPS: Shoreline Group Member David I. passed along a link to a very comprehensive, evidence-based site from Thomas Carefoot, an emeritus professor of biology from UBC. A Snail’s Odyssey
looks at the major invertebrate groups of the shoreline and breaks down their feeding, reproduction, predators, physiology, locomotion and more based on study data that he has been curating for decades. Most of the data and syntheses are very local.
Another email incoming about our next outing.
See you soon,