Pleurobranchus Areolatus

''Pleurobranchus areolatus'' is a species of pleurobranchid sea slug, a type of marine gastropod mollusc, commonly found in the Caribbean Sea. It is up to 15 cm long and it feeds on ascidians.

Taxonomy

Although there were believed to occur six species of ''Pleurobranchus'' in the Caribbean Sea, the other five (''P. atlanticus'' Abbott, 1949, ''P. evelinae'' Thompson, 1977, ''P. crossei'' Vayssière, 1896, ''Susania gardineri'' White, 1952, ''P. reesi'' White, 1952 and ''P. emys'' Ev. Marcus, 1984) were synonymized with ''P. areolatus'' in 2015, based on molecular and morphological evidence.Goodheart J., Camacho-García Y., Padula V., Schrödl M., Cervera J. L., Gosliner T. M. & Valdés Á. (2015). "Systematics and biogeography of ''Pleurobranchus'' Cuvier, 1804, sea slugs (Heterobranchia: Nudipleura: Pleurobranchidae)". ''Zoological Journal of the Linnean Society'' 174(2): 322–362. . The specific names ''P. areolatus'' and ''P. evelinae'' were also commonly in use in literature, while other names were not.

There is no known type material of ''Pleurobranchus areolatus'', ''Susania gardineri'' and ''P. reesi'', but the anatomy of Mörch's type specimen was illustrated by Rudolph Bergh in 1897. Type material of ''P. atlanticus'' is stored at the National Museum of Natural History (Smithsonian Institution), the Academy of Natural Sciences of Drexel University and the Museum of Comparative Zoology. Those of ''P. crossei'' and ''P. evelinae'' are at the National Museum of Natural History (France) and the Natural History Museum, London, respectively.

The sister species (the closest relative) is ''Pleurobranchus varians'' from the Central Pacific. Those two species split 3.10 million years ago (the Isthmus of Panama formed 3.1–3.4 Mya). Both species have color morphs and for their proper determination the knowledge of their locality is needed.

Distribution

''Pleurobranchus areolatus'' occurs off Mexico, Costa Rica, Venezuela, Brazil, Jamaica, Puerto Rico, St. Thomas, Aruba, St. Maarten/St Martin, Bahamas, Bermuda, Panama, Canary Islands and Madeira. This was the only species of ''Pleurobranchus'' considered to occur in the Caribbean Sea until 2016, when ''Pleurobranchus iouspi'' was reported from Santa Marta, Colombia. The type locality is Saint Thomas, U.S. Virgin Islands.

''P. areolatus'' was also reported from the eastern Pacific, but those records were identified as ''Pleurobranchus digueti'' in 2015.

Description

The shape of the body is oval. The dorsum contains numerous small, polygonal and flat tubercles. The background color ranges from light brown to deep violet, with varying degrees of opaque white pigment on the tubercles. In some cases the opaque white pigment is arranged in a symmetrical pattern across the body. It is up to 150 mm long.

Rhinophores are rolled and fused at the base, with horizontal striations from base to tip. The color pattern of rhinophores is the same as color pattern of the body.Alvim J. & Pimenta A. D. (2016). "Comparative Morphology and Redescription of ''Pleurobranchus'' Species (Gastropoda, Pleurobranchoidea) from Brazil". '' Zoological Studies'' 55(15): .

A well marked pedal gland is visible at the posterior end of the foot in some preserved specimens.

The shell is reduced and internal (as in all other ''Pleurobranchus'' species). The shell itself does not allow proper species identification within the genus ''Pleurobranchus''. The shape of the shell is oval. The protoconch has one whorl and the size of the protoconch is about 400 μm.

The most recent drawing of the reproductive system was published by Goodheart et al. (2015) and by Alvim & Pimenta (2016). The genital cup is just before the gill on the right side of the body. The reproductive system is triaulic (it has two female openings and one male opening). The shape of the prostate is elongated and the frontal part of the prostate is rounded.

The radula has 73 rows, it has no central tooth and it has 115 lateral teeth on both sides (radular formula 73 × 115.0.115), while the radular formula reported from smaller specimens (27 mm) is 66 × 90.0.90.

The gill has the size of 2/3 of the body in living specimens of 27 mm in length. The circulatory system was described by Alvim & Pimenta in detail in 2016, but it is the same as in ''Pleurobranchus reticulatus''. They also described the nervous system, which is very similar to that of ''P. reticulatus''.

Ecology

''Pleurobranchus areolatus'' was found to be abundant in Biscayne Bay, Florida in 1946. Minimum recorded depth is 0 m. Maximum recorded depth is 70 m.Welch J. J. (2010). "The “Island Rule” and Deep-Sea Gastropods: Re-Examining the Evidence". '' PLoS ONE'' 5(1): e8776. doi:10.1371/journal.pone.0008776. This species is found under rocks and coral rubble. For example in Florida it is sometimes found among ''Porites porites'' corals.

They are laying large, translucent and gelatinous egg masses during spring in Florida.

All species in the genus ''Pleurobranchus'' are carnivorous. ''P. areolatus'' probably feeds on ascidians, for example, ''Didemnum'' sp.

Its ectoparasites include copepod '' Anthessius ovalipes''.Walter T. Chad (2015). ''Anthessius ovalipes'' Stock, Humes & Gooding, 1963. In: Walter, T.C. & Boxshall, G. (2015). World of Copepods database. Accessed through: World Register of Marine Species at http://www.marinespecies.org/aphia.php?p=taxdetails&id=348850 on 2016-10-15

Rodriguesic acids and its esters (they are modified diketopiperazines) were isolated from ''P. areolatus'' in 2014.Pereira F. R., Santos M. F., Williams D. E., Andersen R. J., Padula V., Ferreira A. G. & Berlinck R. G. (2014). "Rodriguesic acids, modified diketopiperazines from the gastropod mollusc ''Pleurobranchus areolatus''". ''Journal of the Brazilian Chemical Society'' 25(4): 788–794. . These are the first diketopiperazine derivatives isolated from a mollusc. Similar diketopiperazines were isolated also from ascidians ''Didemnum'' sp. These derivatives may either result from a ''P. areolatus'' symbiont or may come from feeding on the ascidian. These chemical compounds may be used by ''P. areolatus'' as a chemical defense.

Gallery

Pleurobranchus areolatus map.png, Distribution of ''P. areolatus''
Pleurobranchus areolatus shell.png, Drawing of dorsal view of the internal shell.
Pleurobranchus areolatus shell 2.png, Drawing of ventral view of the internal shell including the hyaline sheath.
Pleurobranchus areolatus reproductive system.png, Reproductive system of ''Pleurobranchus areolatus'' by R. Tucker Abbott (1949).
Pleurobranchus crossei.jpg, Left side view.
Pleurobranchus areolatus (Warty Sidegill Slug).jpg, Left side view.
Pleurobranchus areolatus 2.png, Drawing of right side view showing the position of the internal shell, the gill, anus behind the gill, and genital pore in front of the gill.
Pleurobranchus areolatus 3.png, Drawing ventral view shows rhinophores, anterior sinus, velum, foot, gill and posterior pedal gland.

References

This article incorporates Creative Commons (CC-BY-4.0) text from the referenceGoodheart J. A., Ellingson R. A., Vital X. G., Galvão Filho H. C., McCarthy J. B., Medrano S. M., Bhave V. J., García-Méndez K., Jiménez L. M., López G. & Hoover C. A. (2016). "Identification guide to the heterobranch sea slugs (Mollusca: Gastropoda) from Bocas del Toro, Panama". ''Marine Biodiversity Records'' 9(1), p.56. and public domain text from the reference

{{taxonbar, from=Q5048433
Pleurobranchidae
Gastropods described in 1863