Wandering thread snail
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Wandering thread snail ( Cratena peregrina ) on Eudendrium ramosum |
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| Cratena peregrina | ||||||||||||
| ( Gmelin , 1791) |
The migrating thread snail ( Cratena peregrina ) is a sea snail of the genus Cratena from the order of the hind gill snails (Opistobranchia).
description
The most striking features of Cratena peregrina are the slender, milky-white body and the blue-violet appendages on the back. Due to this characteristic color pattern, Cratena peregrina cannot be confused with other thread snails.
Physique and coloring
Cratena peregrina has a slender and elongated body. The basic color is milky-white, which, however, has an orange tinge due to the translucent midgut gland and gonad. Stretched alive, it is almost 50 mm long and a maximum of 3 mm wide and high. However, the height and width of 3 mm is only reached in the region of the pericardium. The pericardium lies 5 mm at the front pole of the animal, between the first and second fifth of the body. Starting from the pericardium, the body becomes increasingly narrower towards the rear and ends in a tapering tail without attachments, which takes up about a third of the body length.
Rhinophores
The rhinophores are sensory organs with olfactory perception. They are smooth, 5 mm long and are located on the front pole. These sense organs touch at the base and are evenly pointed. Both at the base and at the extreme tip, the rhinophores appear transparent. In contrast, the middle third glows orange and the apical section glows orange. The labial or mouth tentacles have a rotating cross-section and taper to a point. They are twice as long as the rhinophores and twice as thick at their base. Furthermore, in contrast to the rhinophores, the labial tentacles are almost entirely milky-white with the exception of the basal part of the body, which is transparent. Between the base of the labial tentacles and the rhinophores there is an elongated orange spot.
Cerata
Along the ridge on both sides, tubular and pointed pistons, called cerata, are placed in nine groups. The groups face each other directly. While the pistons of the first group are arranged in a slightly horseshoe shape, the pistons of the other groups stand in rows and run from one side of the body to the other. When the animal is in motion, the pistons are a little erect and slightly spread apart from the body. With a length of 6 mm, the longest pistons stand dorsally in the first two groups and are thus somewhat longer than the rhinophores. The distance between the pistons between the groups becomes progressively, claudally smaller and the pistons shorter. The distance between the first and second group is 3.5 mm and that between the second and third group is approx. 2.5 mm. The pistons each contain a midgut gland process that ends in a nettle sac (or cnido sac). The midgut gland gives the pistons an orange, sometimes red and apically a purple-violet hue. The epithelium over the purple-violet section is mostly iridescent blue, while the epithelium over the orange region is transparent. In the smallest lateral pistons, the apical and purple-appearing part is largest, while it is proportionally smallest in the long inner pistons.
genitals
The genital opening lies behind and under the first piston group, while the anus is behind the second group, at the level of the third piston. The penis is relatively small and the basal section of the gland and the protective sheath are missing.
jaw
The jaw is longitudinally oval and measures 0.9 × 0.5 mm in an 11 mm specimen. The widest point is found near the vertebra, on the dorsal side it is clearly indented. The purchase extension has a single notched cutting edge. The radula formula of the example mentioned is 20 × 0.1.0. The teeth are horseshoe-shaped, a maximum of 0.12 mm long and a little less wide.
Construction of the cnido sac
The process of the midgut gland extending into the cerata ends blindly in the cnido sac. Its epithelium is monolayer like that of the midgut gland. While the digestive sections of the midgut gland usually have four cell types, the cnidosack contains only two different cells, the nematophages and interstitial cells. The transition region from the midgut gland to the cnido sac consists of undifferentiated cells, which probably serve as a reservoir for the urticaria cells. Both urticaria and midgut gland cells are released from this region. This region is followed distally by the zone of differentiation in which the cnidoid sac cells, also called nematophages or cnidophages, grow to their normal size and shape. The cnidophages are pushed further and further towards the tip by the advancing cells. The nettle capsules are phagocytosed by the cnidophages of the differentiation zone after they have passed the transition point between the midgut gland and the cnidoid sac. The number of nettle capsules per cnidophage can vary greatly and depends not only on the size but also on the shape of the nettle capsule. Furthermore, the arrangement of the nettle capsules within the cnidophages is characteristic of each species and at the same time depends on the shape of the nettle capsule.
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In species that eat Eudendrium racemosum , such as Cratena peregrina , which stores two types of nettle capsules, the haplonemes are not exactly in a certain direction due to their small size of 2 × 5–8 µm. Nevertheless, the discharge poles mostly point in the direction of the urticaria lumen. At 9 × 18 µm, the Euryteles are significantly larger than the Haplonemen but, like these, point towards the lumen. As a rule, no more than two of these large nettle capsules are found in the cnido sac. In addition to the cnidophages, the cnidosack also contains a second cell type, the interstitial cells. These occur sporadically in the lower section of the cnidophage between the cnidophages. In the upper third of the cnido sac, below the differentiation zone, they form a closed epithelial cover. It is possible that interstitial cells are only cnidophages that have not ingested any nettle capsules and have thus remained relatively undifferentiated. Its task, it seems, is to close it quickly after the cnidosack has been opened by the expulsion of the nettle capsules.
Expulsion of the nettle capsule
The nettle capsules are only expelled in the event of an accident. It is important to emphasize that Cratena peregrina does not have a distal, permanent opening of the urticaria. The nettle sac muscles are made up of sloping muscle fibers and are developed to different degrees depending on the type. The urticaria has no continuous muscle sheath and the tip of the cnido sack is free of muscle fibers. Together with the piston muscles, the cnido sac muscles form a functional unit. With a contraction of the muscles and the resulting change in pressure within the cnidosack, the nettle capsules are expelled. The upper third of the urticaria of Cratena peregrina is tubular and, when the muscle contraction begins, is filled with cnidophages from deeper regions of the urticaria, which are also expelled by the strong contraction. Shortly before expulsion, the basal lamina of the midgut gland dissolves in the perforation zone and the interstitial cells give way. Some muscle fibers pull the epidermal cells apart at their base until they tear. The resulting crack is enlarged by the squeezed out cnidophages. As a result, the nettle capsules are expelled along with the surrounding cells. If the connection between the cnidophages does not break, a whole strand of epithelium with interstitial cells is ejected. In the sea water the cnidophages burst and the nettle capsules explode. After the cnidophages have been expelled, the epidermal cells pushed aside migrate back to their original position. The undifferentiated, interstitial cells close the opening in the midgut gland and form a continuous closure of the endodermal tissue.
Inclusion of the nettle capsules
As already mentioned, the nettle capsules are absorbed through food. For a long time it was assumed that these do not damage the aeolidioidea. This assumption only applies to one's own feed polyp, as far as there is an insensitivity to the nettle poison. Aiptasia actinie is an example of this . While Aiptasia can be eaten by the snail Spurilla without any problems , Aiptasia feeds on Cratena peregrina . If there is contact between Aiptasia and Cratena peregrina , however, this can also lead to long-lasting symptoms of paralysis. In order to avoid strong nettle bombardment of its own food polyps, the Aeolidioidea approaches carefully and attacks the polyps in a species-specific manner. Often the polyp heads are bitten off or the stolons are bitten and sucked out. In order to prevent the nettle capsules from coming into contact with the sea water, the buccal cone of the snail, a kind of proboscis, is pressed either over or firmly against the hydroid organ. After ingestion, the nettle capsules enter the midgut gland and are digested or phagocytosed by cnidophages and partially stored, the rest are excreted. Eudendrium racemosum- eating species such as Cratena peregrina , on the other hand, store all existing nettle capsules.
nutrition
Cratena peregrina feeds on hydroid polyps of the species Eudendrium racemosum . It prefers to eat polyps that have only just fished food out of the plankton with their tentacles . The main source of food for the snails is not the hydropolypes, but the plankton, which they cannot prey from the surrounding water. In this context, one speaks of klepto predators , which eat the prey together with the food they have just caught, in contrast to the kleptoparasites , which hunt or steal food from other predators . It is not known which sense organs Cratena peregrina uses to select suitable polyps.
development
Although Cratena peregrina are hermaphrodites, they cannot self-fertilize. The eggs are laid all year round. The salmon-colored spawning cords are wound around the trunks of the hydroid colonies that they graze. 2-6 eggs with a diameter of 0.06 to 0.07 mm adhere to the spawning lines. After 7 to 8 days and a water temperature of 16 ° C, the free-swimming Veliger larvae hatch . The duration of the larval stage depends on the water temperature, food availability and salt content and is still largely unexplored. Metamorphosis turns the Veliger larvae into an adult.
Habitat and Distribution
In Cratena peregrina is an endemic, occurring in the Mediterranean style, which is mainly used in the Spanish through to the Turkish coast. Outside of the Mediterranean, they are also occasionally found on the Portuguese coast, the Andalusian Atlantic coast, the Strait of Gibraltar and the Canary Islands. In a recent paper, it was assumed that Cratena peregrina also occur on the Brazilian coast. The Brazilian species and those found in the Mediterranean were examined and compared with each other under various aspects, as well as molecular-genetic. It was finally concluded that the Brazilian species is not an intraspecific variation, but rather a new species, namely the Cratena minor n. Sp . The habitat of Cratena peregrina extends from shallow water to greater depths, where they can often be found on hydroid colonies.
literature
- Bergbauer Matthias and Bernd Humberg: "What lives in the Mediterranean. An identification book for divers and snorkelers. " Franckh-Kosmos-Verlag, Stuttgart 1999, ISBN 3-440-07733-0 , p. 154 .
- Schmekel Luise and Adolf Portmann: " Opisthobranchia des Mittelmeeres. " Springer-Verlag, 1982, ISBN 3-642-61818-9 , p. 212-214 .
Individual evidence
- ^ A b Aguado, Felipe, and Arnaldo Marin. "Warning coloration associated with nematocyst-based defenses in aeolidiodean nudibranchs." Journal of Molluscan Studies 73.1 (2007): 23-28.
- ↑ a b c Martin, R. "Management of nematocysts in the alimentary tract and in cnidosacs of the aeolid nudibranch gastropod Cratena peregrina." Marine Biology 143.3 (2003): 533-541.
- ↑ a b c Kälker, H., and L. Schmekel. "Structure and function of the cnido sac of the Aeolidoidea (Gastropoda Nudibranchia)." Zoomorphology 86.1 (1976): 41-60.
- ↑ Trevor J. Willis, Kimberly TL Berglöf, Rona AR McGill, Luigi Musco, Stefano Piraino, Claire M. Rumsey, Tomás Vega Fernández, Fabio Badalamenti: Kleptopredation: a mechanism to facilitate planktivory in a benthic mollusc . Biology Letters, 13, 11, The Royal Society, November 2017 ( PDF )
- ↑ Manousis, Thanasis, et al. "New findings of Gastropods in the Hellenic seas with em-phasis on their origin and distribution status." J Biol Res-Thessalon 18 (2012): 249-264.
- ↑ Padula, Vinicius, et al. "Is the Mediterranean nudibranch Cratena peregrina (Gmelin, 1791) present on the Brazilian coast? Integrative species delimitation and description of Cratena minor n. Sp." Journal of Molluscan Studies 80.5 (2014): 575-584.
