Cephalaspidea

Headshield slugs
A live individual of Chelidonura varians in the family Aglajidae, head end towards the lower left
Scientific classification
Kingdom:	Animalia
Phylum:	Mollusca
Class:	Gastropoda
(unranked):	clade Heterobranchia clade Euthyneura clade Euopisthobranchia clade Cephalaspidea
Superfamilies
See text

The suborder Cephalaspidea, also known as the headshield slugs and bubble snails, is a clade of sea slugs and bubble snails, marine gastropod mollusks within the larger clade Euopisthobranchia.^[1]. Bubble shells is another common name for these families of marine gastropods, some of which have thin bubble-like shells.^[2]

Members of this worldwide clade appear to be the most primitive of the opisthobranchs.^{[citation needed]} Headshield slugs are the most morphologically diverse group of all the opisthobranchs.^{[citation needed]}

Anatomy

The vast majority possess a shell, although it may be reduced or internal. They have a well-developed headshield, a characteristic broadening at the head, which is used to plow beneath the surface of the sand. This headshield prevents the sand entering the mantle cavity. There is a muscular foot with or without parapodia (fleshy wing-like flaps).

Life habits and related anatomical structures

Headshield slugs often live just beneath the surface of the sand and can also be seen crawling on rocks. They have well-developed sensory structures to detect prey, which may be other opisthobranchs, polychaetes or bristleworms and foraminiferans. Several species are voracious carnivores.

Members of the brightly colored genus Chelidonura also have well-developed eyes on the anterior end of the head and bundles of sensory cilia around the mouth. With these cilia they are able to track their prey by following the victim’s mucous trail.

The Hancock's organ is a chemosensory organ situated between the foot and the headshield. It plays a role in olfactory and sensory detection. It is visible as a dark brown pit at the base of the right rhinophore.

Taxonomy

The taxonomy of the shelled cephalaspideans, the bubble snails, like that of many shelled mollusks, used to be based very simply on shell characteristics. But because there are some similarities in shell morphology throughout this group, more recently taxonomists have taken other anatomical characteristics into consideration, such as the radula, gizzard, penis, and Hancock’s organ.

Linnaean taxonomy

• Suborder Cephalaspidea P. Fischer, 1883
  • Superfamily Acteonoidea D'Orbigny, 1835
  • Superfamily Bulloidea Lamarck, 1801
  • Superfamily Cylindrobulloidea Thiele, 1931 - These are now included in the suborder Sacoglossa
  • Superfamily Diaphanoidea Odhner, 1914
  • Superfamily Haminoeoidea Pilsbry, 1895
  • Superfamily Philinoidea J.E. Gray, 1850
  • Superfamily Ringiculoidea Philippi, 1853

2005 taxonomy

In the taxonomy of Bouchet & Rocroi (2005), the clade Cephalaspidea is arranged as follows:

• Superfamily Bulloidea: family Bullidae
• Superfamily Diaphanoidea: families Diaphanidae and Notodiaphanidae,
• Superfamily Haminoeoidea: families Haminoeidae, Bullactidae and Smaragdinellidae
• Superfamily Philinoidea: families Philinidae, Aglajidae, Cylichnidae, Gastropteridae, Philinoglossidae, Plusculidae and Retusidae
• Superfamily Runcinoidea: families Runcinidae and Ilbiidae

The superfamily Acteonoidea has been included into the new Informal Group "Lower Heterobranchia" and the superfamily Cylindrobulloidea becomes part of the Group Cylindrobullida.

2009 taxonomy

Malaquias et al. (2009)^[3] have rearranged taxonomy of Cephalaspidea sensu lato:

• reinstated Architectibranchia^[3]
• reinstated Runcinacea as a valid name outside Cephalaspidea.^[3]
• reinstated Scaphandridae as a valid family.^[3]
• they did not use superfamilies in the classification scheme.

The taxonomy of Cephalaspidea sensu lato by Malaquias et al. (2009)^[3] is arranged as follows (there are listed genera under molecular analysis; not analyzed families are under "incertae sedis"):

Architectibranchia Haszprunar, 1985

• Family Acteonidae d’Orbigny, 1843 - Acteon, Mexacteon, Pupa
• Family Aplustridae Gray, 1847 - Hydatina, Micromelo
• Family Bullinidae Gray, 1850 incertae sedis
• Family Ringiculidae Philippi, 1853 incertae sedis
• Family Notodiaphanidae Thiele, 1931 incertae sedis

Runcinacea Burn, 1963

• Family Runcinidae H. Adams & A. Adams, 1854 - Runcina
• Family Ilbiidae Burn, 1963 incertae sedis

Cephalaspidea Fischer, 1887 - This means Cephalaspidea sensu stricto

• Family Diaphanidae Odhner, 1914 - Diaphana, genus Colpodaspis is incertae sedis
• Family Cylichnidae H. Adams & A. Adams, 1854 - Cylichna
• Family Scaphandridae G. O Sars, 1878 - Scaphander
• Family Retusidae Thiele, 1925 - Retusa, Pyrunculus
• Family Rhizoridae Dell, 1952 - Volvulella
• Family Bullidae Gray, 1827 - Bulla
• Family Philinidae Gray, 1850 - Philine
• Family Aglajidae Pilsbry, 1895–96 - Aglaja, Chelidonura, Navanax, Odontoglaja, Philinopsis
• Family Philinoglossidae Hertling, 1932 - Philinoglossa
• Family Gastropteridae Swainson, 1840 - Gastropteron, Sagaminopteron, Siphopteron
• Family Plusculidae Odhner, 1968 incertae sedis
• Family Haminoeidae Pilsbry, 1893 - Atys, Haminoea, Phanerophthalmus, Smaragdinella, Ventomnestia incertae sedis
• Family Bullactidae Thiele, 1826 incertae sedis

2010 taxonomy

Subsequently Malaquias (2010)^[4] moved Bullacta exarata (formerly the only member of Bullactidae) into the family Haminoeidae.^[4]

Jörger et al. (2010)^[1] moved Cephalaspidea sensu stricto and Runcinacea into the Euopisthobranchia and they confirmed the placement of Acteonoidea within the Lower Heterobranchia.^[1] All families of Architectibranchia were already within the Lower Heterobranchia in the taxonomy of Bouchet & Rocroi, except for the Notodiaphanidae, which has been placed in the Lower Heterobranchia since 2010, in order that the Architectibranchia can be considered to be monophyletic.

References

1. ^ ^{a b c} Jörger K. M., Stöger I., Kano Y., Fukuda H., Knebelsberger T. & Schrödl M. (2010). "On the origin of Acochlidia and other enigmatic euthyneuran gastropods, with implications for the systematics of Heterobranchia". BMC Evolutionary Biology 10: 323. doi:10.1186/1471-2148-10-323.
2. ^ Dance S. P. (1996). Shells. The visual guide to over 500 species of seashell from around the world. Dorling Kindersley. London, New York, Stuttgart.256 pp. pages 200-203. ISBN 0-86318-811-7.
3. ^ ^{a b c d e} Malaquias M. A. E., Dodds J. M., Bouchet P. & Reid D. G. (2009). "A molecular phylogeny of the Cephalaspidea sensu lato (Gastropoda: Euthyneura): Architectibranchia redefined and Runcinacea reinstated". Zoologica Scripta 38(1): 23-41. doi:10.1111/j.1463-6409.2008.00354.x.
4. ^ ^{a b} Malaquias M. A. E. (2010). "Systematics, phylogeny, and natural history of Bullacta exarata (Philippi, 1849): an endemic cephalaspidean gastropod from the China Sea". Journal of Natural History 44(33 & 34): 2015-2029. doi:10.1080/00222933.2010.487574.

External links