Aplysia fasciata
| Aplysia fasciata | ||||||||||||
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Aplysia fasciata |
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| Poiret , 1789 |
Aplysia fasciata is a species of sea hare from the family Aplysiidae and the order Opisthobranchia found in the Mediterranean region. The brown to black colored animals can be observed between February and September. They are characterized by a reddish parapodic border and deeply bifurcated parapodic lobes. These make Aplysia fasciata very good swimmersin contrast to other species of the Aplysiidae family.
Aplysia fasciata is a herbivore and prefers to feed on seaweed. Reproduction often takes place in the hermaphrodite animals in large dynamic groups in different formations. It happens that an animal mates as a male and a female at the same time. Aplysia fasciata , like many Aplysiidea , is able to expel a purple cloud of ink in defense.
Anatomy and appearance
Aplysia fasciata has an elongated head with paired rolled up mouth tentacles as well as paired rolled up rhinophores , which have given the animal the name sea hare due to the resemblance to rabbit ears. The pheromones released during mating and oviposition are perceived via the chemosensory rhinophores . These pheromones play an important role in mating, laying eggs, and ingesting food.
The eyes of Aplysia fasciata lie behind the rhinophores. The head goes over into the neck area and further into the abdomen with the pair of parapodic flaps. The parapodia are lateral wing-like extensions of the foot and are used for swimming in Aplysia fasciata . In the abdomen, in addition to the hermaphroditic gland and vital organs such as the heart and gills, there is the greatly reduced inner shell. This is up to 7 cm long, very fragile and pale amber in color. It is visible from the outside between the parapodial lobes under the thin dorsal skin. The breathing movement in Aplysia fasciata takes place via a coordinated contraction of the gills and siphon, the inhibition of the heart and the closing of the parapodial flaps.
The coloration of Aplysia fasciata varies from brown to black depending on the individual. Characteristic of the appearance of Aplysia fasciata are the parapodial lobes, which are surrounded by a reddish mantle hem and deeply forked at the back. These make Aplysia fasciata a good swimmer. When swimming, the parapodia are struck with a wave-like movement from front to back. A complete parapodic stroke takes about 3 seconds and moves the animal forward by about a body length. In addition to the parapodial lobes, the white male sexual organ of Aplysia fasciata is another distinguishing feature from other Aplysiidae . Adult individuals of the species Aplysia fasciata are up to 40 cm long and weigh about 1 kg.
Distribution area and habitat
The distribution area of Aplysia fasciata extends from the Atlantic coast of France across the Mediterranean to West Africa. The animals also occur sporadically on the coasts of Great Britain. According to Wild Travel magazine , there were increased numbers of Aplysia fasciata sightings in the UK in 2007 .
Aplysia fasciata is one of the species within the sea hare family known to be swimming. They usually only swim a few meters before they attach themselves to their target, for example food, partner or underground. If necessary, however, the animals can swim long distances over a period of up to 35 minutes. In contrast to the benthic Aplysiidea, Aplysia fasciata occurs mainly in shallow and calm waters that are protected from strong waves. When the water is rough, the animals hide in caves, cracks or under rocks to avoid stranding or colliding with the coast.
nutrition
Aplysia fasciata is a herbivore and feeds on a wide range of seaweed, including various brown, red and green algae. The sea lettuce Ulva lactuca is particularly preferred . Populations of Aplysia fasciata occur mainly in areas with high levels of seaweed, and seaweed is believed to contribute to the metamorphosis from larval to juvenile stages. The sea hare eats food both in motion and at a standstill.
Reproduction
Aplysia fasciata belongs to the hermaphrodites and has a hermaphroditic duct ( English large hermaphroditic duct , short: LHD ). The own sperm is released through this LHD and the foreign sperm is absorbed. The eggs are also deposited on the LHD. So that mating and oviposition take place in a synchronized manner within a population, pheromones are released from the LHD during both mating and oviposition. The synchronization of these processes ensures the genetic variability of the offspring of a population. In a population, mating is usually initiated by a single active individual through the release of pheromones. The active animal takes on the role of the male. It moves its head shaking, crawling or swimming towards the inactive animal, the female. The roles of Aplysia fasciata can be changed several times before mating. As a male, the individual ultimately mates with the stronger sex drive. During mating, the more active male-acting animal places its head between the parapodia of the female-acting animal and inserts the penis into the LHC. After mating, the animal previously mated as a female begins to actively wiggle its head and continues to mate as a male.
In Aplysia fasciata, reproduction usually takes place in large dynamic groups with changing partners. Such a mating group includes up to 20 individuals and can take different forms: clumped, linear or in chains. In a mating chain, the animals can mate as males and females at the same time; the rearmost animal takes on the role of the male, the frontmost animal the role of the female. All animals in between mate simultaneously as males and females. Proximity during mating is important to ensure that each individual in the population is stimulated by pheromones and that reproduction is synchronized. The pheromones released during mating are absorbed by the rhinophores and activate a positive feedback loop, which makes it easier for other animals to take part in the mating. Aplysia fasciata changes sex and partner continuously during reproduction. This promiscuity offers the snails advantages in adaptation and distribution in their variable environment.
Aplysia fasciata becomes sexually mature as a very young adult, depending on the water temperature and the influence of light, and mates several times during its life.
Eggs and larvae
The egg-laying time for Aplysia fasciata begins , as with most species within the Aplysiidae, in spring, but, unlike other sea hares, does not end until autumn. Aplysia fasciata lays eggs as a very young adult animal. These are bright orange to yellow and are deposited from the LHD in long spawning cords on the underside of ledges as well as on seagrass and stones. The estimated number of eggs per clutch of Aplysia fasciata is approx. 26 million eggs. The oviposition takes place synchronized. The plankton-like Veliger larvae hatch after about 15 days. The larval stage in marine invertebrates varies in length depending on temperature, food availability and salt content and has not yet been adequately researched in sea hares.
Life cycle
Aplysia fasciata has a one year life cycle. This starts at the beginning of February and ends in September. With the beginning of kelp growth in early February, the metamorphosis of the plankton-like Veliger larva into juvenile animals begins. During the juvenile phase, Aplysia fasciata grows, gains weight and forms the gonads . In the subsequent adult phase, mating and oviposition take place several times. The adults die in September at the latest. The lifespan of an adult Aplysia fasciata is thus shorter than one season and is estimated at four months.
In Aplysia fasciata, a population always includes animals of different age groups, stages of development and sizes. Young animals join the population in the first six months of the eight-month season, almost the entire season. The later a juvenile animal joins the population, the smaller and lighter the animal, as more energy is invested in the development of the gonads.
The seasonal occurrence of Aplysia fasciata depends, among other things, on the water temperature and the availability of food. The metamorphosis of the Veliger larva is linked to the occurrence of seaweed and seaweed. The increased seagrass growth at the beginning of the season stimulates the metamorphosis of Veliger larvae to juvenile animals in Aplysia fasciata . Animals of different sizes and from different stages of development occur in the thalli of the sea grass.
Aplysia fasciata is nocturnal. Due to its low metabolism rate, it spends comparatively little time ingesting food and therefore more time mating and laying eggs. Mating is generally a slow and lengthy process in gastropods that can take several hours.
Defense behavior
When threatened, Aplysia fasciata emits a purple cloud of ink for defense. The cloud of ink serves as a visual protection from the threat and allows Aplysia fasciata to escape. The purple color of the ink comes from the red algae ingested with food. The ink is made in special glands called Blochman's glands or ink glands. It is non-toxic and mainly consists of water and other volatile organic and mineral substances. However, the ink ejection takes place very rarely and has hardly been observed in Aplysia fasciata in their natural environment. One reason for this is that there are no known predators of the adult Aplysia fasciata .
The ink has other functions that have not yet been fully clarified: It is believed that Aplysia can excrete undesirable food components through the ink. Furthermore, the ink could serve as a warning signal for possible enemies or as a signal for fellow species.
swell
- ↑ a b M. Levy, M. Blumberg, AJ sweet wines (1997): The rhinophores sense pheromones regulating multiple behaviors in Aplysia fasciata. Neuroscience Letters, Issue 2, 225, pp. 113-116
- ↑ a b c d e T. H. Carefoot (1987): Aplysia: Its Biologie and Ecology. Oceanography and Marine Biology. 25, pp. 159-220
- ↑ J. Koester, E. Mayeri, G. Liebeswar, ER Kandel. (1974): Neural control of circulation in Aplysia. II. Interneurons. Journal Neurophysiology, 37, pp. 476-496
- ↑ a b c d e f g Y. Achituv, S. Gev, AJ Susswein, S. Markovich (1984): Behavioral patterns of Aplysia fasciata along the Mediterranean Coast of Israel. Behavioral and Neural Biology 41, pp. 7-22
- ↑ a b Y. Achituv, AJ Susswein (1985): Habitat selection by two Mediterranean species of Aplysia: A. fasciata poiret and a. depilans Gmelin (Mollusca: Opisthobranchia). Journal of Experimental Marine Biology and Ecology, Issue 2, 85, pp. 113-122
- ↑ AJ sweet wines, M. Benny (1985): Sexual behavior in Aplysia fasciata induced by homogenates of the distal large hermaphroditic duct. Neuroscience Letters, 59, pp. 325-330.
- ^ I. Ziv, S. Markovich, C. Lustig, AJ Susswein (1991): Effects of food and mates on time budget in Aplysia fasciata: Integration of feeding, reproduction, and locomotion. Behavioral and Neural Biology, Issue 1, Volume 55, pp. 68-85
- ↑ S. Gev, Y. Achituv, AJ sweet wines (1984): Seasonal determinants of life cycle in two species of Aplysia found along the Mediterranean coast of Israel. Journal of Experimental Marine Biology and Ecology, 74, pp. 67-83
- ^ RD Beeman (1977): Gastropoda: Opisthobranchia. Reproduction of Marine Invertebrates, Vol. IV Academic Press, pp. 115-179
- ^ F. Flury (1915): About the Aplysiengift. Arch. Exp. Pathol. Pharmakol., 79, pp. 250-263
- ↑ a b T. H. Carefoot et al. (1999): A test of novel function (s) for the ink of sea hares. Journal of Experimental Marine Biology and Ecology, Issue 2, 234, pp. 185-197
