Helmets...

Some observations on sea-shells...

Helmet shells are attractive orange and brown sea-shells and belong in the family Cassidae. They are found in temperate and tropical waters worldwide. People mostly know of them from cameo broaches. Cameos are often made from tropical helmet shells; the designs being carved out the external shell layers.

In New Zealand the most commonly seen helmet shell doesn't really have a common name, apart from well, helmet shell, which makes it difficult when there are actually more than one species... Semicassis pyrum is the New Zealand species most people are likely to encounter and are frequently washed up on exposed sandy beaches (Figure 1). In the north the much scarcer S. labiata is also found. There are several other species of helmets, but most are rare, and only found in the north or in deep water (see Powell 1979; Spencer et al., 2009). Semicassis labiata is generally narrower than S. pyrum, and can often be a much darker colour, but they can often be quite similar-looking (see figures 1 and 2).

Figure 1. Helmet shell (Semicassis pyrum). Pilot Bay, Mt. Maunganui, August 2015.

Figure 2. Helmet shell (Semicassis labiata). Pilot Bay, Mt. Maunganui, August 2015.

Their ecology
Both species have quite a broad distribution, with S. pyrum also found in southern Australia, while forms of S. labiata are found in both southern Australia and South Africa. It is thought that this wide distribution is due to a relatively long planktonic larval stage (Beu 1976) (the larvae swim around in the sea for ages and this enables them to be spread far and wide).

The feeding ecology (what they eat) of these two species is not that well known, although it is thought that they feed on echinoderms, particularly heart urchins (Echinocardium spp.). Oddly enough, I've not been able to find any published data confirming this, just anecdotal information, so at the moment it seems to be assumed. Interestingly, I did find some speculation in a 1967 paper that suggested that S. pyrum could be responsible for the predation of offshore toheroa (Paphies ventricosa) populations off the west coast of the North Island (Waugh & Greenaway, 1967). However, other members of this family are known to feed on heart urchins, so this is something that could use further exploration.

Typically these two helmet shell species turn up in wash-ups on sandy beaches, after storms, so it was with much surprise that I recently discovered both species live in the intertidal zone of Mt. Maunganui’s Pilot Bay. To find both species, virtually together, and poking up through the sand was quite unusual. It was something I had not seen in my many years of fossicking around the base of the Mount. Here are some pictures...

Semicassis pyrum

Figure 3. Semicassis pyrum. Pilot Bay, Mt. Maunganui, August 2015.

Figure 4. Semicassis pyrum. Pilot Bay, Mt. Maunganui, August 2015.
My foot for scale.

Figure 5. Semicassis pyrum Pilot Bay, Mt. Maunganui, August 2015.
Showing the area of Pilot Bay where this shell was found.
The arrow points to the location of the half-buried shell

Figure 6. Semicassis pyrum). Pilot Bay,
Mt. Maunganui, August 2015.
This is a different shell taken a day later in a another part of Pilot Bay.

Semicassis labiata

Figure 7. Semicassis labiata Pilot Bay, Mt. Maunganui, August 2015.
This is the shell in-situ. 

Figure 8. Semicassis labiata Pilot Bay, Mt. Maunganui, August 2015.

Figure 9. Semicassis labiata. Pilot Bay, Mt. Maunganui, August 2015. Same day as Fig. 6.

What does all this mean?

What were they doing there? As mentioned above, I'd not seen them here before and S. labiata is never common. I remember being told in the 1980’s that they could be found around the side of the Mount. However, I didn't see one live until August 1995. This could mean that they don't do this very often, perhaps it only happens in winter, and not every winter. Perhaps this could be a case where viewing the occurrence is rarer than the occurrence itself.

This first thing that occurred to me was that maybe they had come into the shallows to breed. However, I didn't see more than one shell at any one time, so this seems unlikely. More likely would be coming up the beach to lay eggs. Laying eggs in winter would make sense, as then the eggs could develop and hatch in spring, which would enable the larvae to take advantage of springtime plankton blooms. Cutting them open to see what sex they were would have answered this question, as all of them were large and probably adult, but I wasn't going to kill them just to satisfy my curiosity. On further recollection, the one I found in 1995, I did collect, and on inspection was found to be male. This is contrary to expectations, so maybe there's another reason…

What I do know is that I didn't see enough of them to draw any firm conclusions, so I'll have to make sure that I'm there next year to see if they turn up again...



References

Beu, AG 1976. Arrival of Semicassis pyrum (Lamarck) and other tonnacean gastropods in the    Southern Ocean during pleistocene time. Journal of the Royal Society of New Zealand, 6(4) 417.

Powell AWB 1979. NZ Mollusca. Collins.

Spencer, HG, Willan RC, Marshall B, & Murray TJ.  2014.
   Checklist of the Recent Mollusca recorded from the New Zealand Exclusive Economic Zone.

Waugh, GD, Greenaway JP 1967. Further evidence for the existence of sublittoral populations of    toheroa, Amphidesma ventricosum Gray (Eulamellibranchiata), off the west coast of New Zealand.    New Zealand Journal of Marine and Freshwater Research 1 (4): 407–411.