Aplysia californica, sea slug, sea hare

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Taxonomy

cellular organisms - Eukaryota - Fungi/Metazoa group - Metazoa - Eumetazoa - Bilateria - Coelomata - Protostomia - Mollusca - Gastropoda - Orthogastropoda - Apogastropoda - Heterobranchia - Euthyneura - Opisthobranchia - Aplysiomorpha - Aplysioidea - Aplysiidae - Aplysia - Aplysia californica

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Brief facts

• Aplysia is an opisthobranch (characterized by gills, a shell that is reduced or absent, and two pairs of tentacles) mollusk of the order Anaspidea. It is used frequently in studies of nervous system development because of its large identifiable neurons. Aplysiatoxin and its derivatives are not biosynthesized by Aplysia, but acquired by ingestion of Lyngbya (seaweed) species.
• Adult animal reaches up to 50-60 cm in length and weighs up to 3.5 kg (permanent growth during the life cycle).
• Aplysia californica inhabit coastal regions thick with vegetation and have few natural predators. Among them are the giant green anemone, Anthopleura xanthogrammica and the spiny lobster Panulirus.

Aplysia as a neurobiological model

• Aplysia californica Cooper, 1863, is a classic example for neurobiological investigations, involving behavior. E.R. Kandel performed many of his investigations on learning and memory of this animal. He created the basic understanding of nerve functioning and learning in humans and was awarded the Noble prize of medicine in 2000 for his life time research.
• Aplysia californica nervous system has a relatively small number of nerve cells.
• Many of these cells are very large (up to 1 mm in diameter).
• Hundreds of neurons have been uniquely identified at the single cell level and have been linked to the animal's behavior.
• These neurons can be isolated and cultured in vitro and they form circuits which can be explored.

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Life cycle

Under ideal laboratory conditions generation time of can be as short as 19 weeks.

• Fertilized
  egg MeSH Aplysia is a hermaphrodite; the animal recieves sperm from another individual (allosperm) during copulation; the sperm is stored until egg-laying; mature eggs are ferilized in fertilization chamber inside the animal and then released from the reproductive system and deposited on the substrate by the weaving head movements. Each animal lays more than 10⁵ eggs.
• Embryo Embryonic development (from fertilization to egg hatching) lasts about 10 days (at 22℃) Movements of the embryos inside the egg capsules can be seen 4-5 days after laying and spontaneous hatching occurs 5-6 days later.
• Post-hatching
  • Larval Planktonic stage of post-hatching development; also called veliger. Newly hatched veligers measure approximately 125 μm across the maximum diameter of their shell. At room temperature this stage takes a minimum 34 days. Veliger swims by means of the coordinated beating of long velar cilia. Veliger growth reaches plateau after approximately 25 days and measure approx. 400 μm. At the end of this phase, the larvae settle preferably (debated) on the seaweed Laurencia pacifica and metamorphose.
  • Metamorphosis
    MeSH Metamorphosis lasts about 3 days. At this stage major developmental and behavioral changes occur. Transition from swimming to crawling (with first appearance of foot extention, or propodium) and shedding of the velar cilia marks the onset of metamorphosis. The change from ciliary to radular feeding signals the end of metamorphosis. Swallowing and biting responses appear after metamorphosis is complete.
  • Juvenile Animals develop the mantle shelf which grows over the shell, parapodia, siphon, gill, rhinophores and anterior tentacles. Young juveniles aquire a red coloring which later becomes a mottled brown. Logarithmic growth continues upto 90 days after hatching, then it tapers and sexual maturity is reached at approximately 120 days after hatching.
• Adult Reproductive maturity is reached at approximately 120 days after hatching.

Tissues

• Parapodia Bilateral extensions of the mantle, winglike flaps that are used for swimming in some species of Aplysia (for example, A. brasiliana and A. fasciata); Aplysia californica is unable to swim and escape danger by "galloping away" and also, secreting brightly colored ink and opaline that are deemed to have different effects on different predators.
• Nervous system
  • Buccal ganglion The symmetrical buccal ganglia are formed by a limited number of cells, some of which are of a large size; furthermore, they are involved in several definitive functions, i.e., the control of the buccal mass and of the first part of the digestive system posterior to the mouth.
  • Cerebral ganglion The cerebral ganglia of Aplysia californica play an essential integrative role for receiving, transmitting, and processing information originating from the pedal, pleural, buccal ganglia, and peripheral sensory sites such as rhinophores and anterior tenta.
  • Pedal ganglion Pedal ganglia motor neurons involved in pedal wave generation in the foot of Aplysia .
  • Pleural ganglion Pleural ganglia contain sensory neurons which innervate tail.
  • Abdominal ganglion Abdominal ganglia are fused in A californica (Kuenzi FM, 1994).

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Appendix 1: other species of sea slugs

Wägele H, Klussmann-Kolb A. Opisthobranchia (Mollusca, Gastropoda) - more than just slimy slugs. Shell reduction and its implications on defence and foraging. Front Zool. 2005 Feb 16;2(1):3.

Examples of opisthobranch species.
A) Micromelo undata (Bruguière, 1792) (Acteonoidea) – Queensland, Australia,
B) Scaphander lignarius (Linné, 1758) (Cephalaspidea) – Northern Sea,
C) Chelidonura pallida Risbec, 1951 (Cephalaspidea) – Queensland, Australia,
D) Elysiella pusilla Bergh, 1872 (Sacoglossa) from the Indo Pacific, feeding on the green alga Halimeda; due to incorporation of chloroplasts, Elysiella has the same colour as the algae,
E) Aplysia punctata (Cuvier, 1803) (Anaspidea) – Mediterranean Sea,
F) Tylodina perversa (Gmelin, 1791) (Tylodinoidea) – Mediterranean Sea,
G) Pleurobranchaea meckelii Meckel in Leue, 1813 Mediterranean Sea.

Examples of nudibranch species.
A) Armina neapolitana (Delle Chiaje, 1824) (Cladobranchia) – Mediterranean Sea,
B) Risbecia tryoni (Garrett, 1873) (Anthobranchia) – Queensland, Australia,
C)Glossodoris cruenta Rudman, 1986 (Anthobranchia) – Queensland, Australia,
D) Bornella stellifer (Adams & Reeve in Adams, 1848) (Cladobranchia) – Queensland, Australia,
E) Flabellina exoptata Gosliner & Willan, 1991 (Cladobranchia) – Queensland, Australia,
F) Ceratosoma magnifica (Van Hasselt, 1824) (Anthobranchia) – New South Wales, Australia,
G) Spurilla major (Eliot, 1903) (Cladobranchia) – Queensland, Australia,
H) Cuthona sibogae Bergh, 1905 (Cladobranchia) – Queensland, Australia

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Appendix 2: sea slugs (Sacoglossa) that use photosinthesis

Händeler K, Grzymbowski YP, Krug PJ, Wägele H. Functional chloroplasts in metazoan cells - a unique evolutionary strategy in animal life. Front Zool. 2009; 6: 28.

Photosynthetic ability varies widely among sacoglossans; some species have no plastid retention while others survive for months solely on photosynthesis.

Representative sacoglossans showing habitus typical of major plakobranchacean clades.
Approximation of length is given.
(A) Cyerce nigricans on Chlorodesmis fastigiata (Lizard Island, Great Barrier Reef; 4 cm),
(B) Polybranchia orientalis (Lizard Island, Great Barrier Reef; 2 cm),
(C) Elysia ornata (Lizard Island, Great Barrier Reef; 2 cm),
(D) Elysia crispata on sediment (Dominican Republic; 4 cm),
(E) Plakobranchus ocellatus (Lizard Island, Great Barrier Reef; 4 cm),
(F) Elysia pusilla (Maldives; 1 cm),
(G) Elysia tomentosa (Lizard Island, Great Barrier Reef; 1.5 cm)
(H) Thuridilla carlsoni (Lizard Island, Great Barrier Reef; 2 cm).
(I) Elysia viridis (Mediterranean Sea, animal placed on brown algae; 1,5 cm).
(J) Thuridilla hopei (Mediterranean Sea, 2 cm).

References

• Capo TR et al. Larval growth, development, and survival of laboratory-reared Aplysia californica: Effects of diet and veliger density. Comp Biochem Physiol C Toxicol Pharmacol. 2008 Oct 30
• Kicklighter CE et al. Sea hares use novel antipredatory chemical defenses. Curr Biol. 2005 Mar 29
• Kuenzi FM, Carew TJ. Head waving in Aplysia californica. III. Interganglionic pathways underlying the coordination and control of searching movements. J Exp Biol. 1994 Oct (Article (pdf))
• Kriegstein AR. Stages in the post-hatching development of Aplysia californica. J Exp Zool. 1977 Feb;199(2):275-88.
• Kriegstein AR. Development of the nervous system of Aplysia californica. Proc Natl Acad Sci U S A. 1977 Jan;74(1):375-8.
• Kriegstein AR et al. Metamorphosis of Aplysia californica in laboratory culture. Proc Natl Acad Sci U S A. 1974 Sep;71(9):3654-8.

Websites and other references

• Animal Diversity Web: Aplysia californica
• Sequencing the AplysiaGenome: white paper (pdf)
• Wikipedia: California sea slug
• Pawlik JR. Larvae of the sea hare Aplysia californica settle and metamorphose on an assortment of macroalgal species. MARINE ECOLOGY PROGRESS SERIES (1989)
• Donovan DA et al. Swimming in the sea hare Aplysia brasiliana: Cost of transport, parapodial morphometry, and swimming behavior. Journal of Experimental Marine Biology and Ecology 328 (2006) 76–86
• Intertidal Invertebrates of California by Robert Harding Morris. Stanford University Press (November 2002) ISBN-10: 0804710457