Centennial Beach Intertidal Exploration – Boundary Bay Regional Park

Centennial Beach Intertidal Exploration – Boundary Bay Regional Park

Photos and Trip Report by Sheila Byers

A small but keen group of explorers joined me on Thursday, June 6, 2019 to walk the two kilometre distance to the subtidal eelgrass meadows at Centennial Beach. Round trip for the exploration was close to 5 km; an indication of the very low Spring tide at 0.5 m elevation. It was a cool, breezy day but luckily no rain. 

Centennial Beach Eelgrass Meadows

We found most of the “usual suspects” that inhabit the sandy mudflats, including the lugworms. We dug up a small red/orange individual (~10 cm length) but saw no sign of their balloon-like egg sacs. Perhaps it is too early in the season. 

To our combined delight and dismay, we found two Bay small ghost shrimp (Neotrypaea californiensis) crabs that were lying on the surface of the sediments. What a surprise! These tunnel-digging shrimp should have been well below the surface in their burrows thus seldom seen ‘in the flesh’. One was a female with several clusters of eggs attached to her pleopods or swimmerets on the underside of the abdomen. She was working hard to move water over the eggs to keep them aerated. The second shrimp was a male with the right cheliped or claw twice as long as the left one. Because of their small size (5.5 cm compared to 12 cm  adults) their burrows were likely near to the surface of the mudflat and I suspect that the intense heat from the sun and the extra low Spring tides probably caused them to move out of their burrow to acquire more oxygen. We took our photographs and buried them in the sediments again, not too deep and close to a channel, in the hopes that the incoming tide would bring cooler water and fresh oxygen to help them survive. What a great find!!

Bay Ghost Shrimp

In the eelgrass meadows we saw the red-eyed medusae (Polyorchis penicillatus) and several dead (spawned-out?) Pacific sand lances. We saw lots of the white bubble shell (Haminoea vesicula). For some reason I wanted to call the bubble shell a Taylor’s sea hare or Zebra leafslug (Phyllaplysia taylori). How crazy was that considering that the latter is distinctively green with white stripes while the former has a blackish mantle covering the white internal shell. Nonetheless, both are quite small and detailed observation of the eelgrass blades is required to find them.

Red-eyed Medusa
Pacific Sand Lance

Who knows what surprises lie in wait for our return trip to Centennial Beach on July 6 for another good low Spring tide. See you there!

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