Why so one-sided?

As a marine ecologist I've spent a lot of time at the beach looking at marine life. Here are some bits and pieces of information/observations/wild speculation I've (often literally) picked up over the years...

I thought I would start with the one-sided case of the small-ish bivalve; Tellinota edgari (Iredale, 1915).

This bivalve (clam) is endemic to New Zealand, and according to Powell (1979, p416) is found: "North Island and northern part of the South Island in shallow water off ocean beaches; uncommon." This quote is pretty much the sum total of the ecological knowledge regarding this species and is over thirty years old. Tellinota edgari has a white shell which often has a yellowish tinge towards the umbos (where the hinge between the valves is located). The shell can reach ~60 mm maximum dimensions.

I believe that one of the reasons this species is so little known is that it is sympatric (found in the same place) with another tellinid bivalve that is more common, and very similar; Peronaea gaimardi (Iredale, 1915). As a consequence, they are hard to tell apart. Tellinota edgari has a more central hinge than P. gaimardi, but even seen together, it's not obvious (see figure below).

Anyway, I often find T. edgari on Auckland west coast beaches. The thing is, I virtually always only find one valve, and it's always the same side. These one-sided finds puzzled me a little, but I didn't think all that much of it, as the species is rare and these occurrences were probably due to chance. Then one day out on the beach at north Piha in 2012 I found dozens of T. edgari washed up around the stream-mouth. Again, it was all the same valve. Some were quite fresh, and one even had an intact hinge and a fragment of the other valve attached, but alas, there were no whole animals to be found. I spent about half an hour walking up and down the tide line looking for the other valve, but didn't find any. Not terribly scientific I know, but I was carrying a surfboard at the time.

Since finding that chance wash-up of T. edgari valves I have wondered both how they died, and why only one valve was washed up. Here are some speculative thoughts...

Only one valve makes to the shore because on the orientation of the shell in the sediment. They are similar to Macomona liliana (Iredale, 1915) (another more common, well known New Zealand tellinid bivalve) and perhaps also sit horizontal buried in the sediment, with the left valve (the one that seems to wash up) upper-most (Morton & Miller 1968, p487, see image below). However, when M. lilliana dies you often see both valves, so perhaps something else is going on here?

How they died? Maybe they were killed by an influx of freshwater from the stream (at north Piha), and the change in salinity is what did for them. I remember that there was a lot of rain before this particular wash-up. Since they live in a comparatively high salinity environment, I suspect that they might be vulnerable to rapid changes in salinity?

They were killed by something else. That finding 30ish left-hand valves, while seemingly unlikely is due to chance. Perhaps their washing up was the end result of some predation? This seems unlikely for a couple of reasons. Firstly the single valve (apart from the ones I broke) is intact, which precludes any kind of fish predation (in my experience you tend to get either whole animals, or munched up fragments - you would also get both sides of the shell). Also, I know of no fish dietary studies that include this species as prey. However, it would take a molluscan expert to reliably discern this species from P. gaimardi (especially if they were chewed up). Even so, something probably eats them. Also, predation doesn't explain why only one valve washes ashore.

In summary, I don't know why only one valve washes up and it may be a challenge to answer this question. These are not likely to be easy animals to work on due to their habitat; they live on open, exposed beaches, in shallow water (the surf zone, most likely). For example, the Auckland west coast beaches are very rarely calm enough for sampling in the surf zone. You would be looking at a few days a year when sampling could be achieved safely, maybe. It is possible that this species could be obtained as bycatch in surf clam fisheries?

This small bivalve is one of many marine species in New Zealand waters where ecological knowledge is lacking. We don't really know where it lives, or how many there are. We don't what it eats, or what eats it. Sadly, this is the norm, rather than the exception for most small invertebrate species in New Zealand waters.

Cheers
Dr. Matthew Jones


References

Morton, J., Miller, M.C., 1968. The New Zealand Sea Shore. Collins, Auckland.
Powell, A.W.B. 1979. New Zealand Mollusca. Collins, Auckland.

A wee update… (October 2014). There was another wash-up in the same place as mentioned earlier and (again) a preponderance of left-valves. However, there was one whole one! see below. Perhaps it is just a numbers game after all?