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 three 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 as it could provide an easy means of identification.
I have not seen A. porcata in my area, so the first photo below is of specimens I collected in the Sydney area, in N.S.W, way back in the 1970’s. The second photo shows A. constricta from Sulphur Creek near Burnie, Tasmania. The thirds is, I believe, A. brevis from Don Heads near Devonport.
In A. constricta the shell is a unicoloured off white. It has pronounced spiral ribbing with two to three ribs on the penultimate (second last) whorl and six to eight on the body whorl. Here on the Tasmanian north-west coast this is the dominant species.
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 uniform right across the surface of the shell. The spiral ribbing is weak with three or four ribs on the penultimate whorl and eight to twelve on the body whorl.
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 two to four ribs on the penultimate whorl and three to eleven ribs on the body whorl. Specimens from one location were unicoloured on the dorsal surface of the body whorl.
That brings me to the last photo. I believe it to be A. brevis and have labelled it so in my collection. It certainly doesn't have the uniform colour of A. constricta and it doesn't have the uniform pigmentation of A. porcata. The problem is, it doesn't quite looks like other speciemens of A. brevis that I have seen. There is always a possibility it it a hybrid, or else an unusual form of A. constricta, but it's the only one I've ever seen like this and I am pretty certain it is A. brevis.
Update: Just before posting this I received an email reply from Dr. Simon Grove regarding the identity of this shell. After summarising some of the taxonomic confusion among the genus he stated, "... I would veer towards A. brevis, reserving A. constricta for shells that conform to the standard unbroken longitudinal ribs and lack of purplish-black coloration." So it was nice to have my identification confirmed and also good to know the experts find them confusing too. He also suggested that more work need to be done on the genus "with the benefit of current molecular approaches". So, what name will be given to this gastropod in a few years time is anyone's guess.
…..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 🙂
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)
Shapes of nature are so amazing. Saw many authors of patents saying that they came to their invention just by watching nature. If we were not so disconnected from nature,we will see many inspirations in nature more often.
Yes very true. Thanks for your comment.
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nice collection i upvote
Thanks. Glad you like it.
beautiful shells.
Thank you
wow love what am seeing nice post
and am new here please teach me more about steemit
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Wow! I admire you for those details and patience of yours over the many years of studying animals. Well done!
Thanks. Glad you enjoyed the post.
Happy you followed my advice :) Loved the post! Btw, would you mind sharing with my how you the gignature with the thumbnails ? Is there a tool or something ? ^__^
That bit with the four previous posts is just a HTML table. I dodn't know if you know HTML. If not, a good place to start would be at this link.
Good post it is a nice collection !
Thanks. Glad you enjoyed it.
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STOPNice posting...
Thank you