Intertidal Exploration at Centennial Beach

Intertidal Exploration at Centennial Beach

Report by Cathy Walker

On July 2, 2022, I participated in the Marine Biology field trip, led by Sheila Byers, to the intertidal zone at Centennial Beach in Delta. 

Sheila focused primarily on eelgrass, lugworms and bivalves. I learned a lot. The native eelgrass, Zostera marina, has the wider, longer leaves while the introduced species, Zostera japonica, has the narrower, shorter leaves. There is a lot of Z. japonica on the sandy mudflats of Centennial Beach. The native eelgrass prefers to be immersed in water, below the low tideline or submerged in tidal channels, while the introduced eelgrass is more tolerant of exposure to the elements in the mid intertidal areas. I also learned that these seagrasses are actually a flowering plant, not a seaweed.

Three of the eight participants were also members of Delta Naturalists Society. The Delta Naturalists have produced a first-rate brochure, Intertidal Life in Delta, with excellent photographs and short descriptions of eelgrasses, seaweeds and a lot of animals. Some of the participants were responsible for the photographs and developing the contents of this brochure, as well as several of the other 10 brochures published by The Delta Naturalists Society – available from the link below:

The first animal of note was the Pacific Lugworm (Abarenicola pacifica). Their poop piles were everywhere. These coiled piles are produced by the lugworms that have an interesting method of both ingestion and defecation by peristaltic movements of the body within the burrow: ingestion of sediments for food such as nematodes (roundworms) and bacteria create the funnel-shaped end of their burrow; defecation of sediments through the other end is evident by its poop piles. The Delta Naturalists brochure says: “Lugworms live out of sight in J-shaped burrows, in sandy mudflats. Buried head-first, the worm excretes its waste from the tail-end into distinctive coiled castings visible on the surface at low tide. Worms can grow to 15 cm.”  Sheila explained that they are important ‘gardeners’, tilling the sediment much like earthworms in gardens, while also providing food for other animals.

The exact method of reproduction of this lugworm is still a bit of a mystery other than the formation of gelatinous translucent, balloon-shaped sacs containing hundreds of fertilized eggs that are found on the surface of the sediments at low tide. The egg sacs remain attached to the end of the burrow until the tiny lugworms develop and disperse. I had previously thought these sacs were jellyfish!

Sheila was also very knowledgeable about bivalves, explaining the anatomy and physiology of the species that we encountered – some native, some introduced. These animals have two hard, hinged shells and filter the water for food. As we came across clams, cockles and gapers, Sheila explained them to us. 

Two species of interest were the Manila Clam and the Pacific Littleneck Clam. The Manila Clam (Ruditapes philippinarum), “an accidentally introduced species also known as the Japanese Littleneck Clam which grows to 8 cm and has thick, elongated, oval shells with a latticed sculpture of ribs and ridges. It varies in colour from cream to grey to brown with streaked patterns. The interior of the Manila Clam shell is distinctive, with blue-purple markings on the edge” (Delta Naturalists brochure). By contrast, the Pacific Littleneck Clam (Leukoma staminea), “A traditional food of First Nations, has a chalky, round-to-oval shell, rarely exceeding 8 cm wide, with a latticed sculpture of ribs and ridges. The shell is white to brown, with dark patches. It lives 10 cm deep” (Delta Naturalists brochure).

A very interesting adaptation of the native Nuttall’s Heart Cockles (Clinocardium nuttallii) is their protective interlocking shells. However, the Red Rock Crabs have a special technique of repeatedly squeezing the same part of the cockle with its pincers, over 200 times (yes, Sheila told us a researcher actually counted the number of hits). This is a stressful period, while the cockle of course tries to hold its shells tightly together with its muscles. But the poor cockle eventually gets tired and relaxes its fatigued muscles. Then the crab grabs the opportunity and pries the cockle apart to gain and eat the large amount of food inside.

The only downside to the field trip was the rather cool breeze. I thought I would wear my rubber boots and warm wool socks to help keep me warm but some of the tidal channels were deep enough that the water went over my boots. Fortunately, the water was warm, so it was fine.  But it was quite funny to hear me squishing and squelching as we trekked out to the low tideline and back again.

Additional Comments by Sheila Byers

The July 2, 2022 spring tide low of 0.9 m elevation occurred at 14:06.

Several other species encountered on the sandy mudflats on our way to the low tideline included the introduced Purple Mahogany Clam (Nuttallia obscurata), the Mudflat Snail (Battillaria attramentaria), the Soft-shelled Clam (Mya arenaria); native species included the Butter Clam (Saxidomus gigantea), Fat Gaper (Tresus capax) along with the Eccentric Sand Dollar (Dendraster excentricus) and some recently molted Dungeness Crab (Cancer magister) exoskeletons (and a few smelly dead crabs!). 

There were several birders in the group who identified a special treat to see on the sandy mudflats – Bonaparte’s Gull (Larus philadelphia)!

The July 11, 2021 spring tide low of 0.4 m elevation at 12:41 after the Heat Dome: observations of effects on the intertidal organisms: 

  • Many dead Heart Cockle shells covered the mudflats: there were newly-opened shells, some of which had the soft bodies intact as opposed to the more commonly dug-up ones that are partially eaten by birds. The numbers of dead cockles reached as high as 7 per square metre, far more than ever encountered in past surveys. The Heart Cockle lives barely beneath the surface of the mudflats, making them very vulnerable to the intense heat during a lengthy low tide exposure.
  • The native Zostera marina eelgrass meadows near the low tideline: sections of the beds exposed on the mudflats (adjacent to the channels) were dried out with black leaves and appeared dead. Were they completely dead or just with burnt leaves that could potentially affect their photosynthesis and growth over the remainder of the summer? The native eelgrass is a perennial species with extensive rhizome structures well-established within the sediments.

On July 2, 2022 at low tideline: entire beds of eelgrass appeared stringy, less dense with almost bare, pale yellow/green stems but few long green leaves. However, those green leaves that were observed were in reproductive mode with seed packets very evident towards the leaf tips. Although appearing damaged and tattered (likely due to the exposure to extreme sun and heat, and wave action), we can only hope that the eelgrass rhizomes survived and will continue to grow new roots and leaves in the coming years. That is, assuming that there isn’t a repeat of the intensive, lengthy heat wave that was experienced in 2021.

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