Why here and then?

This blog post is another regarding a small New Zealand invertebrate (in this case a small marine snail) that not all that much is known about. It's also about finding things when you didn't expect to, and then finding them again in the same place (and at the same corresponding time) nearly thirty years later. There is also some speculation regarding how and why this might have occurred.

Often, when you are looking for marine animals you will find the remains of dead ones before you find living examples. This is almost universally true when it comes to shell collecting. Thanatocoenosis is a term to describe these death assemblages (it’s more of a paleontological term, but I just wanted to use a really big word). Also, in these assemblages you will tend to find a variety of sizes, from small juveniles to adults. Anyway, this brings me back to the topic of this post, which is when the opposite occurred; I found live ones before I found dead ones.

Margin shells are from the family Marginellidae, and in New Zealand waters we have quite a few endemic species. They live from the intertidal down to quite deep water. For example, I've taken one from the intestines of a white rattail that was caught in 901 metres of water (Jones 2008). As a family, they are characterised by having very small, quite polished shells. Many species, although small, are quite pretty (if you are that way inclined). 

Mesoginella koma (Marshall, 2004) is arguably the most common New Zealand margin shell. It is a small (6 mm) gastropod (snail) found throughout New Zealand in shallow water (Powell, 1979). It does not have a common name (to my knowledge) and was formally more well known as Marginella pygmaea Sowerby, 1846. Anyway, this is a species I’ve never found at Mt. Maunganui, well, I should say I’ve never found a dead one. So, going way back to the 21^st of August 1982, I was collecting shells on the beach that is on the Matakana Island side of the Mount (see pictures below). It was a low tide in the afternoon and I noticed that the surface of the sand was being broken by something coming up through it. It turned out to be this small snail. The interesting thing was that these were all live, and all adult. I know this area well, and often came here at other times of the year in similar conditions, but never found them (dead or alive).

So if we leap forward to the 11^th of August 2010, I was again looking in the same place, in the afternoon, at low tide and I found them again, coming up through the sand. This time I took pictures:

This is the side of Mount Maunganui, looking north and showing the sample area.

Looking south across Tauranga Harbour showing the rocks, between which I found the M. koma.

A close-up of Mesoginella koma (~ 6 mm).

Mesoginella koma coming out of the sand.

Another picture of a M. koma coming out of the sand.

Another close-up of M. koma. 

I thought that these occurrences raised some (possibly only to me) interesting questions. For instance, why only live adults? Why only here? Why this time of year?

So here are some speculative thoughts…

As to why only in that location, I wonder if there is a small population of this species in the Tauranga Harbour entrance, and they live in slightly deeper water, just beyond the intertidal zone? I’ve never found them anywhere else in the Mt. Maunganui area. This species is really common in the north, so perhaps this is a small isolated population?

As to why only adults and only at this time of year is probably easier to speculate over. My best guess is that they are coming in to shallower water to breed or spawn. They could be doing this in the evening during winter, as the hours of daylight are less and presumably they would have less chance of being eating or drying out, especially if they live most of their lives below low tide. This would explain both their adult size and the timing. I do recall hearing somewhere that they might be nocturnal, which would make sense. However, this doesn’t explain why I never seen dead ones of any size. The harbour channel rapidly descends to about 30 m depth in the middle, so perhaps the severe depth gradient and high current environment washes away smaller and/or dead shells into deep water?

There is always is the possibility that since these are chance observations, that I am just seeing what I want to see here. However, I used to come to this part of the beach a lot in the 1980's (not a lot for a teenager to do in the Mount back then) and I never found dead examples of this species; something I would have expected if it were living in the intertidal or just offshore there. So, more questions to be answered here...  I would suspect that since this species is far more common further north that it wouldn't be that hard to work on. It would be interesting to understand more about the role this species plays in the sub-tidal ecosystem.

Cheers

Dr. Matthew Jones

References

Jones, M.R.L., 2008. Dietary analysis of Coryphaenoides serrulatus, C. subserrulatus and several other species of macrourid fish (Pisces: Macrouridae) from northeastern Chatham Rise, New Zealand. New Zealand Journal of Marine and Freshwater Research 42: 73–84.

Marshall, B.A., 2004. New names for four common Marginellidae (Mollusca: Gastropoda) from northern New Zealand. Molluscan Research 24: 7–20.

Powell, A.W.B., 1979. NZ Mollusca.

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?