Broad-footed snails

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Broad-footed snails
Phyllaplysia taylori

Phyllaplysia taylori

Systematics
Trunk : Molluscs (mollusca)
Class : Snails (gastropoda)
Subclass : Orthogastropoda
Superordinate : Heterobranchia
Order : Hind gill snails (Opisthobranchia)
Subordination : Broad-footed snails
Scientific name
Anaspidea
PH Fischer , 1883

Broad-footed snails or sea ​​hare ( Anaspidea , "Schildlose", also Aplysiomorpha, "sea hare-like") are a subordination of the hindgill snails with around 700 species in 10 genera and two families ( Aplysiidae and Akeridae ). The three names indicate three characteristics: broad lobes growing on the sides of the foot (parapodia), which make standing and locomotion easier, a missing head shield, and an appearance that resembles a sitting person due to large rhinophores, large oral tentacles and body shape in the water Gives rabbits. Characteristic features are the regression of external shells into (partially) coated inner shells or completely missing shells, a vegetarian diet and gizzards.

features

Shuttering . In the evolution of the hind gill, the process of shell regression from existing external shell to varying degrees into an inner shell completely or partially surrounded by the mantle (pallium) up to the complete loss of the shell can be recorded. Indicators for this process are the shape of the shell (curvature, platelets, geometric shape), the shell thickness (degree of calcification, thickness in relation to the specimen size), the embedding (external shell, partially or completely embedded in tissue) and the size (large , small, completely atrophied).
In sea ​​hares , the entire range of evolutionary development can be found between the more primitive Akeroidea (species with an external shell) via the Aplysia (different degrees of embedding) to the. They stand between the rear gills with external scales and rear gills (such as nudibranchs), which basically have no shell.
External shells can be found e.g. B. at the Akera soluta . Its shell is almost the length of its body and covers the internal organs. They are similar in their arched shape the shells of Atys cylindrica ( head shield screw the Haminoeidae family). A moderate amount of calcification provides support and preserves flexibility for your lifestyle.

Syphonota geographica

Inner shells . In some species of the genera Aplysia and Syphonota , the curvature of the shells and thus their enveloping effect has disappeared. The bowls resemble rather flat lids and lie on the back of the body above the mantle cavity with the heart, gills, internal organs. They are partially or completely covered by the outer skin ( coat , pallium ). In Dolabella auricularia , the almost flat shell has the shape of an ear (circle with asymmetrical scraping). It provides access to the inner gills in the very small (regressed) mantle cavity. With the Dolabrifera brazieri only rudimentary shells are left. This is reflected in the relatively small dimensions and the irregular shape of the bowls.
No bowls . In species of the genera Bursatella leachii and Stylocheilus striatus there is no longer a shell: it is completely atrophied.

Aplysia dactylomela : 1 front with throat, 2,3 oral tentacles, 4 eyes (black point on light background), 5,6 rhinophores, 7,8 parapodial lobes, 9 parapodial cavity (with a jacket-covered shell and underlying jacket cavity)

Parapodies . Parapodia are lateral skin processes. Within the hind gill they occur in different groups: in the oar snails , in the head shield snails , in the butterfly snails and in the sea hares. The form of the parapodia differs in form and function depending on the group. In the sea hare, the parapodia grow sideways from the foot. They do not have the wing-like shape like the parapods of the Thecosomata or the Gymnosomata . Instead, the parapodia of the sea hare rather have the shape of a laterally growing edge or lobe of different widths. The flaps have a stabilizing effect . On the floor, they expand the footprint ( foot ). When swimming, they form side wings with which the animals glide in the water or even swim by flapping their wings. Some species can bring the parapodia together over the back. This means that the cloak cavity, which is only insufficiently protected by the recessed shells, can also be covered and hidden (camouflage colors, patterns on the cloth). Species like the Notarchus indicus , Dolabella auricularia go even a step further. With them, the parapodia have even grown together over the back and form the parapodial cavity . Only 1.2 openings ( siphores ) remain . These are used in various ways: as openings through which seawater flows in and out for the gills, in some species ( Notarchus indicus ) even as a reverse current drive .

Mantle cavity . The mantle cavity is often very small due to the evolutionary regression. It contains the heart and the internal gills. A water inflow and outflow is necessary for gas exchange during breathing. This is realized through an opening ( foramen ). This opening either goes directly into the water or leads into the parapodial cavity (see above).

Head . Sea hares have a "normal" head shape, i. H. neither a head shield nor fused tentacles like the cephalaspidea . They have a rasp tongue ( radula ). In the vicinity of the throat there are flat, curled tentacles with chemo- or mechanoreceptors and nerve cells, which extend sideways . The rhinophores , an olfactory sense organ for the perception of substances, are also characteristic . These are located on the top of the animal between the top of the head and the beginning of the parapods. They have the shape of an antenna or rolled surfaces.

Size . The sea hares reach sizes from under 2 cm up to several dozen centimeters. Some specimens of the Petalifera lafonti , Petalifera ramosa and Aplysia pulmonica have a length of 3–5 cm. However, some species in the genus Aplysia grow to be quite large and produce the largest gastropods. The Aplysia punctata are up to 20 cm long , the Aplysia fasciata up to 40 cm long. Specimens of Aplysia vaccaria were even found that were 75 cm long and weighed 4.4 lb (~ 2 kg).

Shape . The sea hares have a more longitudinally symmetrical shape.

Habitats

The sea hares are cosmopolitan and are found in most seas and oceans. Sightings have taken place in the Pacific, the Indian Ocean, the Red Sea, the Caribbean, the Atlantic and the Mediterranean. But they do not only occur in tropical or temperate waters, because sightings have also taken place in South Africa and the North Sea.

Some genera such as Aplysia prefer to colonize shallow water areas on coasts and wave-protected bays with abundant aquatic vegetation. However, other species also occur at depths of 30 m.

Way of life

Diet .

Aplysia dactylomela : throat

Sea hares are herbivores. They often feed on various algae, seaweed, seaweed. The digestive tract can be described using the Aplysiidae species. The digestive tract has six stages: throat, storage chamber, gizzard, stomach, intestines and caecum. The vegetarian food is first gnawed off with the extended rasp tongue and enters the pharynx . At its end, at the transition to a sac-like chamber, there are the outlets of a pair of larger salivary glands . After the inflow of saliva, the pre-processed feed is temporarily stored in the sack-like chamber from the throat . It then enters the gizzard . This begins with an expanding antechamber for intermediate storage again. This is followed by a section that runs through an outer circular muscle. During contractions, this muscle moves chewing plates with pyramid-like chitin attachments inside the stomach. This grinds the vegetable shavings finely and turns them into a thick liquid. The liquid then passes through a narrowing chamber, on the side walls of which a kind of chitin hook filter out larger particles that are still present. Following the gizzard, the liquefied food enters the stomach . Digestive glands can inject secretions through small channels . Behind the canals there are two exits at the end of the stomach: one to the intestine , one to the caecum . The inner walls of the stomach and the caecum are provided with flat brushes made of hair cells. In a complicated process, these separate what is indigestible and what has already been digested from the substances to be digested. Nutrients are extracted in the intestine. In the tubular caecum, the excretions are then shaped into typical spheres.

Locomotion . Sea hares slide on the ground and can climb plants. Some species can swim. The parapodia form airfoils or are used as a drive like wings. The swimming species include a. the
Akera bullata ,
Akera soluta ,
Aplysia brasiliana and
Aplysia fasciata .

Reproduction . Like all hind gills, sea hares are hermaphrodites. They have a hermaphroditic gland with which they can produce male and female germ cells. However, they are not self-fertilizers, but need a partner for fertilization. When mating, there is a mutual mating with roles reversal. z. B. species of Aplysia have packed a penis in pockets under the epidermis. During copulation, this is extended to a long, spiral tube in the direction of the partner's genital opening, through which one's own sperm are transferred. Row farming also occurs in large populations. The eggs are laid in clutches or spawning cords on the ground or attached to plants.

Aplysia californica : color emission

Protection mechanisms . Sea hares are inherently peaceful, herbivorous creatures. Nevertheless, or precisely because of this, they have natural predators. Sea hares do not have offensive weapons with which they injure attackers, but use dazzling and camouflage techniques. In species with external shells, the latter offer limited protection. The effect of the bowls depends heavily on their strength. Other species become largely invisible in their habitats due to an adapted coloring and patterning of the outer skin. In animals with inner shells, the parapodia play a certain role. By covering the back, they offer a certain protection, but this does not come close to solid reinforcement. But the colors and grains on the underside offer additional privacy. Some species (e.g. Aplysia ) emit pigment-containing (white, light or dark purple) liquids ("ink") when threatened or touched ( blending technique ).

Social life . Different species of sea hare are not solitary animals but occur in larger populations. This becomes visible when large numbers of animals wash ashore in unfavorable weather conditions or when fishing. In some places there is an explosive increase: in one single case, a density of 660 animals per m 2 was recorded.
It was also observed that some sea hare species have a roughly two-phase daily routine during their migration, consisting of a grazing and a rest period. During the grazing season, the animals move away from each other. In the resting phase they concentrate and reproduce.
Larger migrations don't seem uncommon either.

etymology

Anaspidea can be derived from ancient Greek : ἀ- = not, none; ἀσπίς = protective shield. Translated it means something like the one without a shield .

Aplysiomorpha can partly also be derived from ancient Greek : μορφο- = -shaped, designed, form of ....

Systematics

The taxonomy of snails is subject to revision and change. Therefore there are different classifications. The one that goes back to Johannes Thiele (1929–1935) is usually regarded as classic . It was recognized until the 1990s. A more modern and the last one established on the basis of purely morphological approaches is that of Ponder & Lindberg (1997). The current system is phylogenetically oriented and goes back to Bouchet & Rocroi (2005).

Systematics according to Bouchet & Rocroi (2005)

With the transition from the systematics according to Ponder & Lindberg (1997) to the systematics of Bouchet & Rocroi (2005), the taxon Anaspidea is replaced by the taxon Aplysiomorpha .

In the system of Bouchet & Rocroi (2005), the subgroup Anaspidea was rearranged (PH Fischer, 1883). The subgroup Anaspidea itself became the group of Aplysiomorpha . While the superfamily Akeroidea was taken over, a new hierarchy level was introduced in the superfamily Aplysioidea . The families Aplysiinae , Dolabellinae , Dolabriferinae , Notarchinae according to Ponder & Lindberg (1997) became subfamilies in the family Aplysiidae . When splitting up, the superfamily Aplysioidea and the family Aplysiidae J.-P. Attributed to Lamarck.

Systematics according to Ponder & Lindberg (1997)

Evolution of the taxon

The taxon Anaspidea was originally introduced by PH Fischer as an unspecified rank above a family. In 1925, J. Thiele specified the rank of subordination . In the system of Bouchet & Rocroi (2005), however, the taxon was replaced by Aplysiomporhpa because it does not come from a family that gave it its name.

Web links

Commons : Anaspidea  - collection of images, videos and audio files

literature

  • A. Kästner: Textbook of Special Zoology Volume I: Invertebrates. Part 3: Mollusca, Sipunculida, Echiurida, Annelida, Onychophora, Tardigrade, Pentastomida. G. Fischer, Jena 1993, ISBN 3-334-60412-8 .

Individual evidence

  1. ^ World Register of Marine Species , Anaspidea Fischer, 1883
  2. ^ WB Rudman: The shell in Sea Hares. In: Sea Slug Forum. Australian Museum, Sydney August 27, 2000. (www.seaslugforum.net)
  3. ^ A b c W. B. Rudman: Towards a phylogeny of gastropod molluscs: an analysis using morphological characters. In: Sea Slug Forum. Australian Museum, Sydney August 27, 2000. (www.seaslugforum.net)
  4. ↑ WB Rudman: Akera soluta. In: Sea Slug Forum. Australian Museum, Sydney September 3, 2000. (www.seaslugforum.net)
  5. ^ WB Rudman: Dolabella auricularia (Lightfoot, 1786). In: Sea Slug Forum. Australian Museum, Sydney April 1, 1999. (www.seaslugforum.net)
  6. ^ WB Rudman: What are Sea Hares? In: Sea Slug Forum. Australian Museum, Sydney April 14, 1998. (www.seaslugforum.net)
  7. ^ RD Barnes: Invertebrate Zoology . Holt-Saunders International, Philadelphia, PA 1982, ISBN 0-03-056747-5 , pp. 376 .
  8. ^ WB Rudman: Aplysioidea - mantle cavity. In: Sea Slug Forum. Australian Museum, Sydney December 22, 2004. (www.seaslugforum.net)
  9. ^ WB Rudman: Anaspidea-gizzard. In: Sea Slug Forum. Australian Museum, Sydney December 22, 2004. (www.seaslugforum.net)
  10. ^ WB Rudman: Comment on 'Teeth' in the gut of Bursatella by Jack Rudloe. In: Sea Slug Forum. Australian Museum, Sydney March 10, 2000. (www.seaslugforum.net)
  11. ^ WB Rudman: Comment on Items from inside Aplysia brasiliana by Jo O'Keefe. In: Sea Slug Forum. Australian Museum, Sydney June 28, 2006. (www.seaslugforum.net)
  12. ↑ PV Hamilton: Migratory molluscs, with emphasis on swimming and orientation in the sea hare, Aplysia. In: Migration: Mechanisms and Adaptive Significance. (= Contributions in Marine Science. Supplement to Vol. 27). 1985, pp. 212-226.
  13. ↑ PV Hamilton: Swimming tracks of sea hares, Aplysia brasiliana, with discussion of the roles of swimming in sea hares. In: The Veliger. 28, 1986, pp. 310-313.
  14. ^ WB Rudman: Aplysioidea - swimming. In: Sea Slug Forum. Australian Museum, Sydney December 22, 2004. (www.seaslugforum.net)
  15. ^ WB Rudman: Comment on Akera bullata swimming by Ian Buzzard. In: Sea Slug Forum. Australian Museum, Sydney June 17, 2006. (www.seaslugforum.net)
  16. ↑ H. Flodrops: Re: Sea hare being eaten by crab - Reunion Is. In: Sea Slug Forum. Australian Museum, Sydney February 9, 2008. (www.seaslugforum.net)
  17. ↑ WB Rudman: Ink glands. In: Sea Slug Forum. Australian Museum, Sydney March 3, 2003. (www.seaslugforum.net)
  18. ^ A b Winston F. Ponder , David R. Lindberg : Towards a phylogeny of gastropod molluscs: an analysis using morphological characters. In: Zoological Journal of the Linnean Society. 119 (2), 1997, pp. 83-265. doi: 10.1111 / j.1096-3642.1997.tb00137.x
  19. ↑ a b P. Bouchet, J.-P. Rocroi: Part 2 - Working classification of the Gastropoda - Bouchet, Fryda, Hausdorf, Ponder, Valdes & Waren. In: Malacologia. Volume 47, No. 1, Ann Arbor 2005, pp. 239-283. ISSN  0076-2997
  20. ↑ P. Bouchet , J.-P. Rocroi (Ed.): Classification and nomenclator of gastropod families. (= Malacologia. Volume 47). ConchBooks, Hackenheim 2005, ISBN 3-925919-72-4 . (vliz.be)
  21. ↑ GT Poppe, SP Tagaro: The New Classification of Gastropods according to Bouchet & Rocroi, 2005. February 23, 2006. ( PDF ( Memento from September 27, 2007 in the Internet Archive ))
  22. ↑ PH Fischer: Manuel de conchyliologie et de paleontologie conchyliologique fasc. 6 . Savy, Paris 1883, p. 513-608 .