Thursday, 21 August 2008

Shells on the strandline

Off to Portarlington and Indented Head tomorrow. I was supposed to go today but the rain and wind and hail changed my plans. Don't get me wrong — I love rain when I'm indoors and rugged up. But when I'm digging around for bivalves on a muddy beach …

I've already been out once this week. On Tuesday, I went to Limeburners Bay, near Geelong.

I didn't have time to walk the path that runs along the western bank of Hovell's Creek. That's another item to go on my ever-increasing list of the Things To Do. Instead, it was quick survey of the beach for bivalves (the primary reason for the trip) and a scan of the water for birds (entirely incidental).

A short spit runs out into the bay. To the north, the beach is shallow sand and mud overlying anoxic sediment packed with oyster and Anadara shells. White-faced herons drop in here to search for crustaceans and fish among the sea grass.

At first glance, it looks as though there's not much happening. But there are all sorts of goodies down in the mud, judging by the recently dead shells stranded on the spit. The mussel Xenostrobus inconstans lives in sediments on the upper shore, along with the operculum-bearing pulmonate snails Salinator fragilis. Empty Xenostrobus shells are abundant. Live Salinator even more so.

Lower down are laternulids (top) and tellinids (bottom) and even the introduced bag mussel Musculista senhousia. Musculista lives on the sea grass beds, anchoring itself with a net of byssal threads. Clumps of mussels provide a solid surface in an otherwise soft and sloppy substrate.

The tip of the spit is composed entirely of gastropod shells, the most conspicuous of which are the chunky mud whelks Velacumantus (= Batillaria) australis. Although I encountered a few live individuals on the upper shore, the majority were at or below the low tide line. And by majority I mean there were freakin' millions of them.

Mud whelks feed by grazing on surface deposits and by collecting suspended particles. This second activity contributes to the transfer of organic material from the water column to the sediment. (Don't make me draw a picture of how that happens.) Where the whelks gather in such high numbers, that transfer has gotta be significant.

There are a million stories in the strandline. It's amazing what you can find out when you stick your nose in the mud. Not my target bivalves, as it happens, but lots of other interesting stuff ...


Tsun-Thai Chai said...

I like the last photo.



>operculum-bearing pulmonate snails Salinator fragilis

Huh? Pulmonate as in a member of the Pulmonata?

Snail said...

Thanks, Chai :)

Certainly is, Aydin! Salinator belongs to Amphibolidae, a small Indo-Pac family of archaeopulmonate snails that have an operculum as adults. Golding et al. revised the family last year. The whole paper is available by subscription at Zootaxa. (We don't subscribe, unfortunately.) Here's the abstract.


Aha! I was under the impression that Amphibola was the sole operculated pulmonate genus. I have to get a copy of that paper. Thanks!

Dave Coulter said...

"...a million stories in the strandline..." What a great line! :)

Snail said...

Stolen, I'm afraid, Dave!