Big spider crab

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Big spider crab
Maja squinado.jpg

Great spider crab ( Maja squinado )

Systematics
Class : Crustaceans (Crustacea)
Order : Decapods (decapoda)
Subordination : Crab (Brachyura)
Family : Triangular crab (Majidae)
Genre : Maja
Type : Big spider crab
Scientific name
Maja squinado
( Autumn , 1788)

The great spider crab ( Maja squinado ) is the largest crab found in the Mediterranean . It lives in kelp forests and on rocky ground to depths of 50 meters. Until 1998, another representative of their Majidae family , Maja brachydactyla , was thought to be the Atlantic variant of the great spider crab, which Volker Neumann refuted in 1998. Maja squinado occurs only in the Mediterranean. It is a popular food crab, which is why the stocks are threatened by overfishing.

Appearance features

The great spider crab reaches a body length of up to 20 cm, whereby it can reach a diameter of up to half a meter due to its relatively long legs. The carapace is slightly convex and longer than wide at the back, which gives the crab a more triangular but rounded shape. It is littered with spines large and small; Particularly interesting are the secondary spines under the strongly pronounced spines along the edge of the carapace, as they are the most important distinguishing feature for their little relative, the little spider crab.

The coloring of Maja squinado ranges from red through all nuances to whitish, there are also spotted animals. The first pair of striding legs, which is provided with the relatively small, equally sized scissors, is as long as the other three, while the fifth is rather short. The legs are studded with bristles and spikes.

The rostrum is clearly divided into two parts and forms two bristle spines that extend clearly above the head. Since the folded-back eyes lie in an orbit , an eye socket, their cornea is not visible from above. This is a characteristic that all species of spider crabs in the Majidae family have.

Detailed description of the body structure

Maja squinado is a species of crab (brachyuran), in turn the sub-root of the crustaceans (crustaceans) and thus the arthropods ( arthropods be assigned). Maja squinado belongs to the decapods , i.e. ten-footed crabs, which is why their body consists of only two units ( tagmata ): the cephalothorax and the pleon . The carapace , the neck shield, is extremely elongated backwards and dorsally fused with all eight thoracomeres, which are all part of the cephalothorax. This is very wide and flat, the typical body shape of the brachyura, the crab. This habit development in the course of evolution is called carcinization.

The pleon is small and narrow, and is permanently folded under the cephalothorax occupied by the carapace. The Carapace thus offers protection to all segments; and so also the gills , which are located in a gap between him and the body wall above the base of the strut legs. The extremities of the first two thoracomers are used for food intake and are therefore called maxillipedes. These have pronounced exopodites, external appendages that help guide food to the mouth. The striding legs, however, have largely reduced the exopodite. On the following segment is the first pair of striding legs, which carry the equally large, rather small scissors. Thoracomers 4–7 carry the remaining four pairs of striding legs. The first three are all the same, and quite long. The last pair of striding legs is a little shorter.

In males, the first two pleopods form a functional unit, a petasma , with which they mate. The pleopods help the females to hold their eggs between the pleon and cephalothorax so that the offspring can develop in a protected manner. A very large mass can be held in this way, which in its entirety can be about half the size of the body of the animal itself.

There are also many extremities on the head. The 2 pairs of antennae are nowhere near as long as those of their relatives, the lower crustaceans (Entomostraca), such as the shrimp. In addition to the eyes, they are used for orientation. In the mouth area are the mandibles, the strongest mouthparts trained to crush the prey. This is followed by the first and second maxilla, which are again mouthparts, which help in chopping up the food and inserting it into the mouth. This mouth field together with the tools is covered by the 3rd maxillipede.

Related species

Four species of the genus Maja have been found in the Mediterranean : Maja crispata Risso, 1827 , also called Maja verrucosa , Maja goltziana D'Oliveira, 1888 , Maja squinado (autumn, 1788) and Maja brachydactyla Balss, 1922 .

Its discoverer Balss named the latter as early as 1922 after morphological investigations, but for a long time it was not regarded as a separate species, but as an Atlantic variant of M. squinado . As a result, many older observations and experiments that were supposedly carried out on M. squinado only apply to M. brachydactyla . Volker Neumann finally refuted this assumption in 1998 through morphological investigations, the findings of which have meanwhile been confirmed by comparing mitochondrial genes of the species. Until a few years ago, M. brachydactyla was considered a species found only in the Atlantic, but this turned out to be wrong in 2011 when it was caught near Málaga . M. squinado, however, is still considered to be native to the Mediterranean.

Like all triangular crabs (Majidae), they have the carapace stretched forward and a "beak-shaped rostrum" in common. They also differ from other crab families in that the rear pair of legs are also located under the torso and to the side, rather than dorsally .

Reproduction

The female's two genital openings, the vulvae , are located in the belly plate (the sternite) of the sixth thoracic segment. The two receptacula seminis , pocket-like structures for storing the sperm until the eggs are fertilized, are extensions of the oviducts ( fallopian tubes ). Males form spermatophores , packets of seeds, in their vasa deferentia , the vas deferens . The eggs usually only mature after a male has mated. Since it belongs to the triangular crab, Maja squinado , like all advanced decapods, experiences internal fertilization.

Females copulate in the still soft state, in which they are after pubertal moult due to the not yet completed hardening of their cuticula , their chitin shell (compare with: moulting and habitat ). Males always look for females who have already passed their pubertal moult, as the vulvae are not yet large enough for mating before that. Immediately after its last moult, a male Maja squinado mounts the female and stays on her for up to a few days. Then he uses his first two pleopods, which act as gonopods in the plasma, to insert his spermatophore into a vulva of the female. During this act, too, the male remains on his partner's back in order to stay there for several days after the work is done.

Mating has also been observed in already hard females, but here the male introduced the spermatophore from a position he had assumed under the female and did not climb her back to remain there for some time. In addition, large concentrations of the spider crab were observed, in the middle of which there were still juvenile males and females. The females were then mated by an adult male immediately after molting. As was probably observed again with Maja brachydactyla , and presumably also applicable to M. squinado, they can be mated by several males, and their sperm can be stored in their receptacula seminis, so that their eggs can mature up to three times a year at a distance of 1 - 4 days to fertilize again for the last spawning (oviposition). Spawning occurs about 6 months after mating. The fertilized eggs are then carried with the pleopods under the rest of the body for protection. The zoe larvae hatch about 9 months after spawning.

ontogenesis

By holding onto the spawn, the mother cares for the brood, which gives the offspring protection and thus a higher chance of survival. It is also the case that spider crabs, unlike most crustaceans, no longer have a free-swimming nauplius larvae . In the egg, the young animal develops so far that a special, different larval form, the Zoëa , hatches, as will be explained below.

Embryonic development

The embryonic development follows the fertilization of the egg cells. This was extensively investigated in 2011 by Durán, Pastor, Grau and Valencia on large spider crabs that were caught off Corsica. However, since the five female animals were already carrying fertilized eggs, their embryonic development could only be observed from the 32nd day before their end. It is divided into four phases.

In the early phase the eggs were not yet pigmented, therefore yellow-orange, and 90% contained yolks. 20.5 ± 3.53 days before the end of embryonic development they were colored red and slightly pigmented; the places where eyes would develop could be guessed at. 9 days before the transition to post-embryonic development, three-quarters of the eggs were pigmented and brown, and movement of the embryos could also be seen. Eventually the pigmentation was complete, resulting in the eggs being black in color, which had four days to embryonic development. During the 28 days up to that point, an average increase in size of 0.89 mm, from 7.41 to 8.3 ± 0.3073 mm, had been measured.

Zdravko Števčić , who first carried out studies on wild individuals in the 1960s and 1970s, observed a similar development.

Larval development

The embryonic development is followed by the postembryonic development, beginning with the larval development.

Like all arthropods , crustaceans must molt in order to grow. This happens more often during larval development. This begins with the Zoëa l larva that hatches from the egg, followed by the Zoëa ll and the megalopalarva. All three stages live in planktonic form , i.e. floating in open water, moving only a little by swimming.

Zoe larvae are typical of decapod crabs. The body segmentation that the adult animal will later have is already completely present in the pleon of the larva, but not yet in the cephalothorax. The development of the pleon, the appendages of which arise later, “overtakes” that of the cephalothorax. This form of segment formation is called irregular anamerism. The cephalothorax is short, the pleon long, narrow and already folded more or less under the rest of the body. The telson has a rather wide tail fork, the furka , which is studded with a few spines. The pleon can also be used for quick recoil in danger, but generally has a protective function, as the spines protrude forward when it is folded. The locomotion is mainly carried out by the two maxillipedas with numerous webbed hairs, which can be moved like propellers. The carapace is powerful and protects the entire body as long as it is in the typical, folded position. Noticeable are a huge dorsal spine that starts in the middle behind the eyes, a rostral spine that points down in the middle in front of them, and lateral spines that protrude sideways above the eyes.

In the following, the further larval development during rearing in the laboratory according to articles by J. Durán (2011) and J. Palma (2008) is discussed. It should be noted that in the case of the latter, female individuals of the species Maja squinado caught in the Algarve are being referred to , which is to be questioned due to the reasons described in the section related species , since this is an Atlantic coast. However, since it is still impossible to differentiate the species in the larval stages without genetic analysis, this article is also used as a source to describe them.

The Zoëa l larva is the one that hatches directly from the egg. It is slightly larger than 1 mm (carapace length) and weighs about 0.12 mg. During this phase, which, depending on the source, lasts 4-8 days, the larva moults numerous times, and finally with morphological changes, which presumably include the further development and increase in size of the cephalothorax (see. Fig. 2: Use fullness of flat bottom tanks on the settlement of spider crab (Maja squinado, Herbst) larvae. J. Palma, M. Correia, JP Andrade (2008)) to enter the second postembryonic phase as Zoea ll larvae. In its further development into the megalopalarve (opós: the eye), which is also closed after approx. 4–8 days, it increases significantly in weight (in Durán's observations by 107% from approx. 0.14 to approx. 29 mg dry weight) and also in size (the same observations showed an increase in carapace length of 58.2%, from approx. 1.27 mm to approx. 2.01 mm).

Post larval development

The larva now undergoes a metamorphosis to the first juvenile stage , which corresponds to the post larva . It changes its planktonic way of life to a benthic one (i.e. living on the sea floor) and its appearance is similar to that of the adult animal. From this stage onwards only growth in height and the development of sexual maturity follow. In the observations under laboratory conditions, 7.13% (Durán) and 13.8% (Palma) of the hatched Zoëal larvae made it to this stage. It should be emphasized here that the same conditions in terms of food, temperature and the like cannot be created in the laboratory. Rather, the young animals developed there under ideal conditions, since the rearing was the first attempt for a commercially viable aquaculture of the popular food animals. (See also section: Significance for humans and endangerment of the species ). Animals in the first juvenile stage perform their first proper moult in the laboratory approx. 21 days after hatching, and thus enter the second juvenile stage. Here there is a considerable increase in length of the carapace by approx. 70.2% to approx. 4.51 mm. The second moult marks the beginning of the third juvenile stage, the animal now has the adult habit with a carapace length of approx. 5.63 mm, but is not yet sexually mature.

Molting and habitat

Here, too, one has to fall back on studies in which Maja squinado is reported, but which were carried out with specimens caught on the Atlantic coast, which must therefore be Maja brachydactyla . Comparable studies must still be carried out on specimens that can be called maja squinado without any doubt .

Juvenile period

Animals in the juvenile period spend about 2 years growing in size, which is always preceded by a molt. At this time they are not yet sexually mature.

The juvenile animals live in shallow water in winter, between rocks in kelp forests near the coast. They spend the summer at a depth of only 4 m on small rocky reefs. After this time they have reached a carapace length of approx. 6-13 cm, whereby no gender-specific differences are noticeable.

Critical molting and adult lifestyle

There are two main periods for the critical moult that follow the roughly two-year growth phase and lead to sexual maturity: the first, the prepubescent, in April, and the second, the pubescent, from July to October. However, it was noted in captive animals that in the case of very large individuals who are in the phase before one of the two moults, one of them can also be omitted entirely, or it can be very late. Three molts were also observed on individual specimens. The average time between the two critical moults was 104 days. Typically, the length of the carapace in animals that are already comparatively large increases less than that of smaller animals after molting, relative to the initial size. This also explains that a smaller increase in length (27%) was observed in pubertal moulting than in prepubertal moulting (36%).

They now spend the summers at a depth of approx. 7 m; for the winter they migrate, mostly in groups, to greater depths of up to 40 m. Adult great spider crabs live for a few years and do this migration every year.

nutrition

Zdravko Števčić examined spider crabs in 1967, which were probably again Maja brachydactyla . However, it can be assumed that there are no major differences in the diet of the two species.

Stevcic found out, through observations in the laboratory and examinations of the stomach contents, that the crabs do not have a very specialized diet. It is important that the prey is on the ocean floor and that it moves little or not at all. It eats echinoderms, molluscs, worms, smaller crustaceans and algae that live on the ground or are sessile. She picks up the food with her scissors, which act as tweezers.

Significance for humans and endangerment of the species

Maja squinado is a much sought-after edible crab, which unfortunately led to its undoing. Most of the stocks in the Mediterranean have disappeared due to overfishing . In the meantime, research is being carried out into ways in which the animals can be successfully raised in the laboratory, for increasing wild stocks and for breeding in aquaculture.

Individual evidence

  1. ↑ a b c Fauna and Flora of the Mediterranean , Verlag Paul Parey, Prof. Dr. Abel, E .; Prof. Dr. Kusel, H .; Prof. Dr. Riedl; R .; v. Salvini-Plawen, L. (1983)
  2. ↑ a b Animals and plants on Mediterranean coasts: in their habitats - from the coastal strip to the open sea , BLV Verlagsgesellschaft mbH, Munich, Identification Book 32, page 162, W. Nachtigall (1983)
  3. ↑ a b c What lives in the Mediterranean? , Franckh'sche Verlagshandlung, W. Keller & Co., Stuttgart, page 218, AC Campbell (1983)
  4. ↑ a b Journal of the Marine Biological Association of the United Kingdom , Volume 75, Number 1, pp. 27-42, AH Hines, TG Wolcott, E. González- Gurriarán, JL González-Escalante, J. Freire (1995): Movement Patterns and Migrations in Crabs: Telemetry of Juvenile and Adult Behavior in Callinectes Sapidus and Maja squinado
  5. ^ A b Journal of Natural history , Volume 32, Numbers 10-11, pp. 1667-1684, V. Neumann (1998): A review of the Maja squinado (Crustaceae: Decapoda: Brachyura) species-complex with a key to the eastern Atlantic and Mediterranean species of the genus.
  6. ^ A b Marine Biodiversity Records , issue 7, e77, P. Abelló, G. Guerao, F. Salmerón, JE García Raso (2014): Maja brachydactyla (Brachyura: Majidae) in the western Mediterranean.
  7. ↑ a b c d e f g h i j k Aquaculture research , edition 43, number 12, pp. 1777–1786, J. Durán, E. Pastor, A. Grau, JM Valencia (2011): First results of embryonic development , spawning and larval rearing of the Mediterranean spider crab Maja squinado (Herbst) under laboratory conditions, a candidate species for a restocking program.
  8. ↑ a b Special Zoology - Part 1: Protozoa and Invertebrates. , Springer Spectrum, 3rd edition, p. 561ff., P. 611ff., Prof. Dr. Westheide, W., Ed. Dr. Rieger, G. (2013)
  9. ↑ Special Zoology - Part 1: Protozoa and Invertebrates. , Springer Spectrum, 3rd edition, pp. 611ff., Prof. Dr. Westheide, W., Ed. Dr. Rieger, G. (2013)
  10. ↑ a b , Scientia marina , issue 75, number 1, pp. 129-134, G. Guerao, KB Andree, C. Froglia, CG Simeó, G. Rotlland (2011)
  11. ^ Science of The Total Environment , Volume 35, Edition 1, pp. 41-51, N. Bihari, R. Batel, B. Kurelec, RK Zahn (1984): Tissue distribution, seasonal variation and induction of benzo [a] pyrene monooxygenase activity in the crab Maja crispata
  12. ↑ Journal of Crustacean Biology , Issue 28, Number 1, pp. 76-81, G. Sotelo, P. Morán, D. Posada (2008): Genetic Identification of the Northeastern Atlantic Spiny Spider Crab as Maja brachydactyla Balss, 1922
  13. ^ Crustaceana , Issue 16, Number 2, pp. 161-181, RG Hartnoll (1969): Mating in the Brachyura
  14. ↑ Journal of the Experimental Marine Biology and Ecology , Issue 220, Number 2, pp. 269-285, E. González-Gurriarán, L. Fernández, J. Freire, R. Muiño (1998): Mating and role of seminal receptacles in the reproductive biology of the spider crab Maja squinado (Decapoda, Majidae).
  15. ↑ Special Zoology - Part 1: Protozoa and Invertebrates. , Springer Spectrum, 3rd edition, page 618, Prof. Dr. Westheide, W., Ed. Dr. Rieger, G. (2013)
  16. ↑ a b c d Special Zoology - Part 1: Protozoa and invertebrates. , Springer Spectrum, 3rd edition, page 568f., Prof. Dr. Westheide, W., Ed. Dr. Rieger, G. (2013)
  17. ↑ a b Science , Issue 3, Number 64, pp. 513-516, HW Conn (1884): Evolution of the Decapod Zoea
  18. ↑ a b c d e f J. Palma, M. Correia, JP Andrade (2008): Usefullness of flat bottom tanks on the settlement of spider crab (Maja squinado, Herbst) larvae. In: Aquaculture research , edition 39, number 9, pp. 1005-1008, doi : 10.1111 / j.1365-2109.2008.01943.x
  19. ^ Dictionary of Zoology , Spectrum, 8th edition, page 291, A. Paululat, G. Purschke (2011)
  20. ^ Dictionary of Zoology , Spectrum, 8th edition, page 87f., A. Paululat, G. Purschke (2011)
  21. ↑ Special Zoology Part 1: Protozoa and Invertebrates. , Springer Spectrum, 3rd edition, page 568 f., Page 616, Prof. Dr. W. Westheide, Ed. G. Rieger (2013)
  22. ↑ a b Journal of the Marine Biological Association of the UK , Issue 83, Number 5, pp. 995-1005, MP. Sampedro, E. González-Gurriarán, J. Freire (2003): Moult cycle and growth of Maja squinado (Decapoda: Majidae) in coastal habitats of Galicia, north-west Spain
  23. ↑ Journal of experimental marine biology and ecology , Issue 189, Numbers 1-2, pp. 183-203, E. González-Gurriarán, J. Freire, J. Parapar, MP. Sampedro (1995): Growth at molt and molting seasonality of the spider crab, Maja-squinado (Herbst) (Decapoda, Majidae) in experimental conditions - implications for juvenile life-history
  24. ↑ Z. Stevcic (1967): The nutritional complex of the spider crab Maja squinado . In: Helgoland scientific marine investigations, issue 15, number 1, pp. 630–636, doi : 10.1007 / BF01618656

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