Dive Dry with Dr. Bill

046: Food Chains and Webs

Water conditions this past weekend were not good enough to coax me in for a dive or ten. Visibility was about as poor as it gets here, and the June gloom and subnormal air temperatures suggested surface intervals would be colder than being underwater. So, I aborted a whole weekend of diving. Of course I was lost, not knowing what to do with my "found" time once I got completely caught up on editing my video footage. With no new footage, I was forced to look back for material to write about this weekend. Even with some 62,000+ still images archived from my diving, it didn't take along to find an appropriate subject.

Not long ago I was down at 75 feet videotaping an obscure sabellid worm at the base of an elk kelp holdfast. For some reason I looked up to see a number of fish feeding on scraps of flesh scattered in the water column. I started looking around for the source, and found it "buried" under a thick carpet of huge elk kelp blades. It was the carcass of a bat ray that had been worked over by the fish and a sheep crab. I pulled the carcass out from under the "forest" and placed it out in the open on the sandy bottom where it was easier to film.

On the next day's dives I relocated the carcass and found there was a large sheep crab sitting on top of it. The crab was slowly feeding on the carcass in two different ways: tearing flesh off with its large claw and transferring it to its mouth, or just lowering the mouth to the carcass and munching like your little brother might do at the table when Mom's not looking. Fish including kelp bass, a barred sand bass, sheephead and senoritas surrounded the carcass at respectful distances from the crab's long claws. Every once in a while one of the fish would dart in and steal food from the claw, or pieces that had been torn loose. The crab would simply try to brush them off with one of its eight legs, keeping the two claws busy feeding.

Bat rays are not a normal food source for the sheep crab or the fish. This is a good example of an "opportunistic" meal for all... kind of like when you're invited to a friend's home for a new ethnic meal. It's not part of your normal diet, but you don't have to hunt it or cook it yourself. Most animals like such opportunities to add a new menu item to their mix, especially when little energy is required to obtain it. Sheep crabs are normally carnivores, tearing apart starfish, clams and octopus but they do also scavenge on dead organisms like the bat ray. And to make things more interesting, living bat rays like to feed on crabs so this one turned the tables a bit!

Sheephead eat crabs, clams, mussels, snails, sea urchins, octopus and worms. Kelp bass usually feed on fish, octopus, squid, crabs and shrimp. Barred sand bass eat fish, crabs and octopus. This makes them all competitors for several foods that the sheep crab and each fish likes to eat. If all species focused on a common food like octopus, there would be intense competition for that food. Instead, most species have a varied diet allowing to feed on whatever is common or easiest to capture. A species with a very narrow diet, say a nudibranch specializing on only one species of bryozoan, would probably have fewer competitors but would starve if that type of brozoan didn't exist within its habitat.

Diversity in the diets of species within a biological community is often important for the stability of that community. If each species had a specialized diet, there is more chance for disruption. An example of a simple linear food chain would be sea urchins, sheephead which eat urchins, and harbor seals which eat sheephead. If an ecological disturbance like higher water temperatures caused the urchins to die off, both the sheephead which feed on them and the harbor seal which feeds on the sheephead would be seriously impacted by the loss of this primary food resource.

In the real world, most communities have a number of different food webs where species have a much wider range of food. For example, the sheephead can switch to alternate foods such as crabs, clams, mussels, or snails. If the urchins die off, there are other menu choices for the sheephead. Of course for each of these food sources, other predators compete with the sheephead for them. And if the sheephead numbers declined, the harbor seal could switch to blacksmith or other seafood delicacies. For many ecological communities to be stable it is important that their food webs be complex and therefore flexible rather than simple food chains which are more easily disrupted by environmental changes (including extensive commercial fishing).

I am one individual who biologists would classify as an "omnivore." I eat anything (I have to given my lack of culinary skills!). Were I a finicky eater like my little sisters (who only liked macaroni and cheese for years), life might have been more difficult. Of course those of us living on Catalina have to be flexible. When something is not on the shelves at Vons, we have to improvise. If the barge doesn't arrive one winter night, there may be no milk, bread or eggs and we may have to settle for ice cream or... brussel sprouts!

© 2003 Dr. Bill Bushing. Watch the "Dive Dry with Dr. Bill" underwater videos on Catalina Cable TV channel 49, 10:00 AM and 5:00 PM weekdays.

Image caption: Bat ray carcass inspected by kelp bass; sheep crab as the "king" of the food chain;
sheep crab feeding on carcass; barred sand bass held at bay by the crab's leg

This document maintained by Dr. Bill Bushing.
Material © 2003 Star Thrower Educational Multimedia