NEW CONCHOLOGICAL INFORMATION

ON BLASICRURA TERES-LIKE SHELLS OF THE HAWAIIAN WATERS

by E.L. Heiman*)

Abstract: New conchological information in “The Visual Database of Cypraea teres-related shells from O’ahu, Hawai’i”

published recently throws additional light on intraspecific variation in Blasicrura teres. Based on this information statistical shell characteristics are calculated for several batches of teres-like shells forming together one huge batch of 815 shells.

Numerous images presented in “The Visual Database” can be separated into three groups: 625 shells of one group can be identified as B. teres; 91 shells of the second group (11%) can be identified as Blasicrura rashleighana; 99 shells of the third group (12%) have shell characters of intermediate form between the first and the second group.

Analysis of shell characteristics of these three groups and their comparison with other available conchological material showed that the two extreme formae—B. teres and B. rashleighana— are connected by the intermediate forma and are not separable conchologically on a specific level. The resulting preliminary assumption is that all known teres-like taxa of the Hawaii Is. are different formae of a single species Blasicrura teres.

Key words: Mollusca, Gastropoda, Cypraeidae, Blasicrura teres, Blasicrura rashleighana, variability.

Blasicrura teres (Gmelin, 1791) is a variable species widely distributed in the Indo-Pacific region. In the waters bordering the Hawaii Islands several cowry taxa form a group with shells superficially similar to B. teres: Blasicrura alisonae (Burgess, 1983), Blasicrura burgessi (Kay, 1981), Blasicrura latior (Melvill, 1888) and Blasicrura rashleighana (Melvill, 1888); they are called teres-like taxa or the teres complex. A description of B. alisonae and B. burgessi is based mostly on the animal’s anatomy. Clarifying the taxonomic identity of these taxa is difficult due to the scarcity of conchological material.

New conchological information is now available after Dayle (2004) published “The Visual Database of Cypraea teres-related shells from O’ahu, Hawai’i” (below named “the Database”). This electronic book on a CD contains conchological data of 885 teres-related shells. There are six pictures of each shell (different views) on the Database, totally more than 5300 pictures. All the imaged shells are from the island of Oahu, Hawaii. The conchological data for each shell includes its lengh, width, height, weight, the number of labial and columellar teeth, and number of marginal spots. The Database has additional information, which may be useful to researchers.

Working with the Database

I applied conchological methods used for comparing cowry populations after “selection” of certain specimens pictured on the Database. 70 of 885 illustrated shells are subadult, very eroded, broken, and/or completely without the dorsal pattern; they can be used for measurements but are not suitable for counting qualitative shell characters hence they are excluded from analyzing. The remaining 815 shells are labeled below “teres-like” in a broad sense (sensu lato).

The shell images of the Database are used for counting the percentage of shells with a given shell character in the population. Six shell characters, which may be used as diagnostic for the teres complex, are chosen: the shell shape elliptical or subpyriform, the presence or absence of the columellar and labial ridges, the presence of callosity on the hind top of the inner lip and the presence of basal spots.

Four groups of shells can be distinguished in the huge batch of 815 teres-like s.l. shells:

a) rashleighana-like shells; 91 of 815 shells can be easily separated from other specimens of the database by the left side distinctly margined and angled and the pyriform shape—Figs. 1-4.

1.-4. rashleighana-like shells

b) teres-like in a strict sense (s.s.)—shells, which can be identified as B. teres; there are 724 such shells, which conform to the diagnostic characters of B. teres on the Database (as mentioned above, B alisonae and B. burgessi are practically not separable conchologically from B. teres). The majority of these shells has an elliptical to subcylindrical shape—Figs. 5.-8.

5.-8. teres-like s.s. shells

c) 99 shells can be distinguished as forma intermediate (Figs. 9.-12.); some of them have elliptical broad shape but the others are too wide and their shape is more close to subpyriform than to elliptical.

It is impossible to specify the exact, distinct border between f. intermediate and the remaining 625 teres shells and a decision in this case is arbitrary. Shells with the width to length ratio more that 0.6 are recognized here as f. intermediate and shells with this ratio lesser than 0.6 are related to teres in a strict sense (teres s..s.)—Figs. 5.-8.

9. –12. f. intermediate shells

The results of analyzing the conchological data of these four batches of shells are given in Table 2 below.

The Hawaiian teres-like shells as a whole seems to be smaller and broader than shells of many other populations of the species—Heiman & Mienis (2002)—although a number of normalized labial and columellar teeth is practically the same.

This corresponds with the data regarding other teres-like populations examined by the present author. The formula for 83 teres-like shells collected by Mrs. Avril Bourquin (Canada) on beaches of Maui and Kauai Islands is 23.57.22.22; the formula for 22 shells of B. rashleighana collected in the same place is 21.69.16.21; the formula for 42 teres-like shells from Oahu I. collected by K. Zeilinger (USA) is 27.59.22.20. Apropos, cowry collectors think that shells of all cowries from the Hawaii Islands are larger than shells of the same species in other areas of the Indo-Pacific region. This is not always true and teres-like shells of the Hawaii Islands are an example of such an exception.

It is possible that there are perhaps more shells with the dorsal blotch than reflected in the table because some of the studied shells are partly eroded and it was necessary to make an arbitrary decision in these cases; if any trace of the blotch was visible, the decision was positive. However not all shells in the analyzed batches from the Hawaii Is. have the dorsal blotch so in any case this shell character is not of a specific value.

The dorsal blotch is absent in all rashleighana-like shells but it is also absent in many shells of other batches.

Table 1

Characteristics of different batches of teres-like shells

groups of shells	number of shells	formula	standard deviation				the percentage of
			L, mm	W/L, %	LTR	CTR	dorsal blotch	basal spots	columellar ridge	labral callus or ridge	hind top callous	subpyriform shape
teres-like	815	21.58.23.24	4.7	6	2	2	53	6	13	100	87	26
teres s.s.	625	21.56.24.25	5	2	2	2	62	1	<1	100	100	7
f. intermediate	99	23.62.23.24	4.5	2	2	2	41	8	8	100	85	75
rashleighana-like	91	19.71.20.19	2.6	4	1	2	0	37	99	100	2	100

Basal spots and a distinct columellar ridge are found to be very rare in the studied shells of teres s.s. The labral ridge is present in all studied shells although its form may be different.

It follows from Table 1 that it is not possible to separate conchologically on a specific level the compared batches of shells. These batches cannot represent subspecies as well because all these shell were collected from the same area.

Discussion

A working hypothesis can be formed that conchologically all the studied batches of teres-like shells belong to a single species Blasicrura teres and consequently B. rashleighana is an extreme forma of B. teres (f. rashleighana) connected with the typical shells of the species by intermediate forms.

The fact that one can easily distinguish between f. rashleighana and typical shells of B. teres is not sufficient evidence that this form deserves a specific rank. Formae are known in populations of many cowry species, which can be easily separated from typical shells of the relevant species: rostrated and melanotic shells, shells of Erosaria nebrites (Melvill, 1888) f. dilatata, Erosaria turdus pardalina (Dunker, 1852) f. dilatata, several ecotypes of Monetaria moneta (Linnaeus, 1758) and many others. Once these formae are described, illustrated and known to cowry collectors there is no problem to diagnose them. Such formae in their extreme can sometimes be found in large numbers and taken alone may induce one to describe them as subspecies or even species.

B. rashleighana always was and still is a rare taxon. Schilder & Schilder (1952) examined only four shells of this taxon in the Dautzenberg collection and those shells are not from the Hawaii Is. Rashleighana-like shells of the Database are collected over a long period of time. Taken alone this large batch can give the impression of a good separable species and this view is widely accepted (including Heiman, 2004). But now that the Database allows us a more extensive idea about intraspecific variability of B. teres in the Hawaiian waters, the specific rank of B. rashleighana is made doubtful.

It should also be remembered that clear diagnostic characters are absent in Melvill’s description of B. rashleighana (as Cypraea rashleighana of unknown origin): “C. testâ ovatâ, anticé subprolongatâ, dorso convexiusculo, lilacino, tribus brunneis fasciis decorato, fasciâ centrali distinctiori et latiore, lateribus albis parcipunctatis, extremitalibus immaculatis, dentibus parvulis, albis, basi albâ nitente. Long. 18 mm., lat. 11 mm. Habitat?”

“the disposition of the brown bands on the lilac ground is a little like the arrangement in C. sanguinolenta (Gmelin), the shape and upper surface slightly recall C. macula (Adams), though the underside has a perfectly different disposition of teeth, C. macula being allied more to the fimbriata section of the genus.”

Although a few shells were available for study to researchers, new diagnostic shell characters are added to Melvill’s description in Schilder & Schilder (1938), Burgess (1969, 1970, 1985), Gay (1971) and Lorenz & Hubert (1993, 2000). A subspecific rank of B. rashleighana was taken for granted until the new conchological information discussed above was published.

Acknowledgements

I would like to thank Robert C. Dayle (USA) for sending a copy of the Database, sharing other conchological information and commenting this article, H.K.Mienis (National Mollusc Collection at the Department of Evolution, Systematics & Ecology, Hebrew University of Jerusalem and Zoological Museum, Tel-Aviv University, Israel) for providing malacological information and literature for this work and Solly Singer, the English Editor of TRITON, for reading and discussing the manuscript.

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*) E.L. Heiman E-mail: heimel@netvision.net.il