Cryptogemma

Cryptogemma quentinensis, from the Smithsonian Institute.

Belongs within: Turridae.

Deep sea divergence
Published 19 February 2024

I have often had cause before to remark on the taxonomic morass that is represented by the assemblage of gastropods historically referred to as the ‘Turridae’. Recent decades have seen this family undergo multiple revisions, as more modern methods have confirmed what was long suspected, that the ‘turrids’ represented a paraphyletic assemblage to the related Conidae and Terebridae. As a result, the historical Turridae was dispersed among several families, reducing the actual Turridae to a residual core. Among those genera remaining among the true turrids was Cryptogemma.

Cryptogemma timorensis, from the Smithsonian Institute.

Cryptogemma species are found in deep waters of the tropical and subtropical Indo-Pacific and western Atlantic, at depths between 200 and 3000 metres (Zaharias et al. 2020). Defining the genus on morphological grounds has proven tricky, but in general species have a more or less fusiform shell with a well-marked anal sinus on the shell aperture. The length of the siphonal canal varies between species. Ornament-wise, whorls typically bear at least one spiral cord bearing tubercles or gemmae; some may bear multiple such cords. The protoconch is multispiral with four to five-and-a-quarter whorls. The radula bears duplex marginal teeth and a unicuspid central formation.

Relationships within Cryptogemma were assessed using molecular data by Zaharias et al. (2020), leading them to identify a remarkable degree of intra-specific variation. Specimens included in the genus (as recognised by Zaharias et al.) had previously been assigned to 23 different species; Zaharias et al. reduced that number to eight. Specimens of the Indo-western Pacific species C. aethiopica varied in the number of gemmate cords on the shell (from three to none), with specimens in the western part of the species’ range being smoother than those in the east. The similarly ranged C. praesignis became the first known case of sexual dimorphism in turrids, with female shells having extra notches in the shell aperture. Males of this species were found to possess a particularly large, muscular penis, suggesting that the notches were to allow its entry. Multiple species exhibited variation in shell colour; for the western Pacific C. tessellata, darkness of the shell appeared to correlate with the darkness of the substrate on which the animal lived*.

*Which raises further questions: what is the significance of correlation between substrate and animal colour for an animal that lives below the level of general light penetration?

Cryptogemma phymatias, from the Natural History Museum, London.

Perhaps the most unexpected case of synonymisation, however, involved the genus’ effective type species Cryptogemma phymatias. This species is the deepest-dwelling of all Cryptogemma, being the only species found under 1400 metres (and not being found above this depth). Surprisingly, molecular data established that C. phymatias was found across the Pacific basin and in the western Atlantic. While such a distribution has previously been found in surface-dwelling marine species, C. phymatias may be the only known case of a deep-sea gastropod being found on both sides of the Isthmus of Panama. How could such a range have originated? Did founders somehow make the crossing during the planktonic larval stage? Were they somehow carried over by human activity (as has been suggested for other benthic invertebrates showing this distribution, such as some annelids)? Or is evolution slow enough among turrids that this species existed before the isthmus’ formation, and the divided populations have still not diverged enough to be regarded as distinct?

Systematics of Cryptogemma
<==Cryptogemma Dall 1918P66
    |--*C. benthina (Dall 1908) [=Gemmula benthina]P66
    |--C. antigone Dall 1919D19
    |--C. calypso Dall 1919P66
    |--C. chrysothemis Dall 1919O27
    |--C. corneus (Okutani 1966)BK11
    |--C. cymothoe Dall 1919O27
    |--C. eidola Dall 1919O27
    |--C. exulans (Dall 1889)P66
    |--C. herilda (Dall 1908) [=Gemmula herilda]O27
    |--C. oregonensis Dall 1919O27
    |--C. pernodata (Dall 1908) [=Gemmula esuriens var. pernodata]O27
    |--C. quentinensis Dall 1919P66
    `--C. serilla (Dall 1908) [=Gemmula serilla]P66

*Type species of generic name indicated

References

[BK11] Bouchet, P., Y. I. Kantor, A. Sysoev & N. Puillandre. 2011. A new operational classification of the Conoidea (Gastropoda). Journal of Molluscan Studies 77: 273–308.

[D19] Dall, W. H. 1919. Descriptions of new species of mollusks of the family Turritidae from the west coast of America and adjacent regions. Proceedings of the United States National Museum 56 (2288): 1–86, pls 1–24.

[O27] Oldroyd, I. S. 1927. The Marine Shells of the West Coast of North America vol. 2 pt 1. Stanford University Press: Stanford University (California).

[P66] Powell, A. W. B. 1966. The molluscan families Speightiidae and Turridae: an evaluation of the valid taxa, both Recent and fossil, with lists of characteristic species. Bulletin of the Auckland Institute and Museum 5: 1–184, pls 1–23.

Zaharias, P., Y. I. Kantor, A. E. Fedosov, F. Criscione, A. Hallan, Y. Kano, J. Bardin & N. Puillandre. 2020. Just the once will not hurt: DNA suggests species lumping over two oceans in deep-sea snails (Cryptogemma). Zoological Journal of the Linnean Society 190: 532–557.

Leave a comment

Your email address will not be published. Required fields are marked *