Thursday, 18 September 2008

Austrocochlea constricta

.....a species complex

The many morphs of Austrocochlea constricta (family: Trochidae) have been a source of confusion among experts for many years so it's little wonder mere amateurs like myself can find them confusing.

These are common shells in the intertidal zone with vernacular names including Ribbed Top Shell and Zebra Top Shell. They are found on and under rocks where they feed on algae. When the tide is out the operculum is retracted quite deeply within the shell. As a result kids will often take them home thinking they are empty only to end up with a putrid mess.

The variation in these shells ranges from plain off white specimens through to dark heavily banded forms. This led to several morphs being given species status over the last two centuries. In the 1970's researchers concluded that the dark banding was primarily a result of environmental factors, it being proportional to the amount of chlorophyll in the food supply. Based on this and the apparent lack of biological differences, the various forms were considered to belong to just one species. Thus, most books of recent decades refer simply to A. constricta with many and varied forms. Others have maintained the name A. porcata for the heavily banded form.

However, according to research carried out in Tasmania (Parsons and Ward, 1994), there are not one, not two, but in fact 3 distinct species. These they say can be separated by genetic differences as well as morphological differences in both shell and animal. These differences hold true in populations where more than one form live side by side (sympatric) as well as where populations are isolated (allopatric). Thus while environmental factors may play some part in the variability of pigmentation within one species, it cannot explain the broad differences in morphology between the three species.

From the point of view of an amateur, the shell morphology is of most interest and provides an easy means of identification.

I have not seen A. porcata in my area so the first photo below is of specimens collected in the Sydney area in N.S.W back in the 70's. The next photo shows A. constricta from Sulphur Creek near Burnie. and third is A. brevis from Don Heads near Devonport.


In A. constricta the shell is a unicoloured off-white. It has pronounced spiral ribbing with 2 to 3 ribs on the penultimate (2nd last) whorl and 6 to 8 on the body whorl. Here on the north-west coast this is the dominant species.


(Click Photo to Enlarge)
Austrocochlea constricta

A. porcata has longitudinal stripes from the spire to the body whorl, alternating between off-white and black-to-red. The intensity of the pigmentation is uniforn right across the surface of the shell. The spiral ribbing is weak with 3 or 4 ribs on the penultimate whorl and 8 to 12 on the body whorl.


(Click Photo to Enlarge)
Austrocochlea porcata


In the new species A. brevis the spire is black to rubiginous. The body whorl is said to have flexuous, undulating, alternate bands of off-white and black-to-red, the pigmentation being most intense on the spiral ribs and faint or disrupted in between. It is weakly spirally ribbed (this can vary due to environment) with 2 to 4 ribs on the penultimate whorl and 3 to 11 ribs on the body whorl. Specimens from one location were unicoloured on the dorsal surface of the body whorl.


(Click Photo to Enlarge)
Austrocochlea brevis - (Top of spire worn)


.....Clear as Mud
It all sounds clear enough does it not? Well of course nothing is ever too simple. In their search through museum specimens they have found some intermediate forms which they suggest may be the result of either natural variation or possible hybridisation. Hmmmm.... well I guess that is to be expected but then this.... In 1996 further research was published on animals from the Abrolhos Islands and Albany in Western Australia. These Western Australian animals were genetically very similar to the Tasmanian A. constricta. However,when they looked at the morphological traits which were species-diagnostic in Tasmania, the Abrolhos animals most closely resembled A. porcata, while the Albany animals appeared intermediate to A. porcata and A. constricta.

Hmmm.... maybe I should take up basket weaving :-)


(A special thanks to Snail )

References:

  • Parsons, KE and Ward, RD (1994). Electrophoretic and morphological examination of Austrocochlea constricta (Gastropoda: Trochidae): A species complex. Australian Journal of Marine and Freshwater Research 45, 1065–1085.
  • Parsons, KE (1996). Discordant patterns of morphological and genetic divergence in the 'Austrocochlea constricta' (Gastropoda: Trochidae) species complex. Marine and Freshwater Research 47, 981–990. (Synopsis only)


4 comments:

  1. Thanks for the guide! I get confused enough without A. brevis.

    Yanno, A. brevis does look a bit like a cross between the other two ...

    ReplyDelete
  2. Thanks Snail - Well if you're confused then I don't have a snails chance in France of figuring them out :-)

    I think if I were in the business I'd more likely be a lumper than a splitter but then what would I know :-)

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  3. Shell be right. Sorry, she'll be right. Taxonomy rules. But put me down as leaning to lumping.

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  4. I like eating scallops and oysters, that's the sum of my shellfish knowledge. :-)

    ReplyDelete