Opisthobranchia

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Opisthobranchia
Temporal range: Carboniferous–Recent[1]
Notodoris minor
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Mollusca
Class:
Gastropoda
Subclass: Heterobranchia
Informal group: Opisthobranchia
Milne-Edwards, 1848
Included groups

clade Cephalaspidea
clade

Thecosomata

clade
Gymnosomata

clade
Aplysiomorpha

group Acochlidiacea
clade Sacoglossa
group
Cylindrobullida

clade
Umbraculida

clade Nudipleura

Opisthobranchs (/əˈpɪsθəˌbræŋks, -θ-/[2]) is a now informal name for a large and diverse group of specialized complex gastropods which used to be united in the subclass Opisthobranchia. That taxon is no longer considered to represent a monophyletic grouping.[3]

Euopisthobranchia is a taxon containing a revised collection of opisthobranchs, and that taxon is considered monophyletic. Euopisthobranchia does not include some "traditional" opisthobranchs such as the Sacoglossa and the Acochlidiacea.[4] The subclass Heterobranchia now contains all the species which used to be assigned to Opisthobranchia, plus all the species in the Pulmonata.

The subclass Opisthobranchia included species in the order

Nudibranchia.[3]

Opisthobranch means "gills behind" (and to the right) of the heart. In contrast, Prosobranch means gills in front (of the heart). Opisthobranchs are characterized by two pairs of tentacles and a single gill behind and to the right of the heart. With the lack of a heavily mineralized shell, there has been very little fossil record of the group. However, molecular clock studies have suggested that Opisthobranchia emerged as early as the Carboniferous.[5][6]

Bullina lineata

Taxonomy

Under the 1931 classification system drawn up by Johannes Thiele, the class Gastropoda was divided into three subclasses; Prosobranchia, Pulmonata and Opisthobranchia. The latter two were later combined into a single order.[7]

The current classification of the gastropods is more nuanced, with the following subclasses:[8]

  • Subclass Caenogastropoda
  • Subclass Cocculiniformia
  • Subclass Heterobranchia
  • Subclass Neomphalina
  • Subclass Neritimorpha
  • Subclass Patellogastropoda
  • Subclass Vetigastropoda
  • Subclass Gastropoda incertae sedis

As a non-monophyletic taxon

sea hare
.

It was speculated as far back as 1985 that the Opisthobranchia were

paraphyletic, based on morphological evidence, and had given rise to the Pulmonata, a group also of subclass rank.[9]

Because the Pulmonata are a sibling group to an opisthobranch taxon, some authors argued that the Opisthobranchia are therefore not a monophyletic group and can no longer be accepted as a valid taxon.[10] The opisthobranchs are now included within the subclass Heterobranchia, although many manuals and websites still use the old classification.

A phylogenetic study published in November 2004,[11] gave new definitions of the seven main lineages of the Opisthobranchia.

However, in 2005, a study of

rRNA gene sequences could not resolve monophyly versus paraphyly of the Opisthobranchia [12]
Subsequent taxonomic classification in 2005 overturned the Opisthobranchia as a valid clade, reclassifying it as an informal group within the Heterobranchia.[3] Accordingly, articles no longer use the term Opisthobranchia, replacing it with Heterobranchia to emphasise a different concept.[4]

By 2011, the "Opisthobranchia" were declared artificial and obsolete and were replaced by new phylogenetic hypotheses.

Linnean taxonomy

Order Opisthobranchia Milne-Edwards, 1848 – sea slugs

Description

The reduction or loss of the shell, the elaboration of the head, foot or mantle, and the acquisition of chemical defences are evolutionary trends shared by most opisthobranch taxa.[15]

The loss of shell in the group is an example of parallel evolution and has occurred on multiple independent occasions.[16]

Opisthobranchs have undergone detorsion, an evolutionary reversal of the half revolution

streptoneurous
, the more common condition among gastropods, in which these ganglia form a distinct twist within the animal's body).

There is no marked distinction between head and mantle. The

Gymnosomata
, these parapodia are used to move in a swimming motion.

Their eyes are simple

lens and cornea capable of detecting light and the passage of shadows but not of producing a coherent image.[17]

Ecology

Opisthobranchia represents a morphologically diverse group of gastropods occupying a great variety of

Defense

Principally soft-bodied marine creatures with a reduced or absent

toxicity they have few predators. However, some use warning colouration. Animals that do predate opisthobranchs include other opisthobranchs and toxin-resistant predators like sea spiders
.

Opisthobranchs secrete irritants such as

strong acids or accumulate toxins from their food. Aeolidioidea pirate the stinging cells from their cnidarian prey and use them for their own defense.[19][16]

Diet

Oxynoe olivacea

Opisthobranchs may be

sedentary prey. They may eat bryozoans, Cnidaria, or sponges, absorbing the sponge toxin for defensive purposes. Opisthobranchs may maintain the zooxanthellae of their coral prey and use their metabolic products for themselves. Some herbivorous slugs of the sub-order Sacoglossa do the same with the chloroplasts of the algae they eat.[20]

Communication

Pleurobranchus mamillatus

Like most lifeforms, they use chemical cues for much of their life cycle. The

conspecifics
.

Reproduction

Winged snail Clione limacina, a sea angel.

Opisthobranchs are hermaphrodites and have complex reproductive strategies, typically involving reciprocal sperm transfer and storage until the eggs are ready for fertilisation.[21] Eggs are commonly laid in ribbons of varying structure. The egg ribbons are usually unique to each species and in some cases are the only means of differentiating them.[22]

See also

The dictionary definition of opisthobranchia at Wiktionary

References

This article incorporates CC-BY-2.0 text from reference[15]

  1. S2CID 30138345
    .
  2. ^ "Opisthobranch". Lexico UK English Dictionary. Oxford University Press. Archived from the original on 26 August 2022.
  3. ^
    ISSN 0076-2997
    .
  4. ^ .
  5. .
  6. .
  7. .
  8. ^ Gofas, S. (2014). Gastropoda. Accessed through: World Register of Marine Species at http://www.marinespecies.org/aphia.php?p=taxdetails&id=101 on 2015-02-12
  9. .
  10. ^ Schroedl, Michael; Jörger, Katharina; Klussmann-Kolb, Annette; Wilson, Nerida (1 July 2011). "Bye bye 'Opisthobranchia'! A review on the contribution of Mesopsammic sea slugs to euthyneuran systematics". Thalassas. 27: 101–112.
  11. PMID 15336672
    .
  12. .
  13. ^ Schrödl M; Jörger KM; Klussmann-Kolb A; Wilson NG. "Bye bye "Opisthobranchia"! A review on the contribution of mesopsammic sea slugs to euthyneuran systematics" (PDF). Thalassas. 27 (2): 101–112.
  14. ^ MolluscaBase eds. (2020). MolluscaBase. Opisthobranchia. Accessed through: World Register of Marine Species at: http://marinespecies.org/aphia.php?p=taxdetails&id=382226 on 2020-12-23
  15. ^
    PMID 20205753
    .
  16. ^ .
  17. .
  18. , retrieved 15 November 2021
  19. ^ Debelius, Helmut (2001) Nudibranchs and Sea Snails Indo-Pacific Field Guide p. 7 Ikan, Frankfurt

Further reading

External links