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.