Crack crabs

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Crack crabs
Alpheus distinguendus

Alpheus distinguendus

Systematics
Sub-stem : Crustaceans (Crustacea)
Class : Higher crabs (Malacostraca)
Order : Decapods (decapoda)
Subordination : Pleocyemata
Partial order : Caridea
Family : Crack crabs
Scientific name
Alpheidae
Rafinesque , 1815

The crack shrimp (Alpheidae), also pistol shrimp , are a very species-rich shrimp family from the partial order of the Caridea . They are predominantly distributed in the tropics and subtropics, with a particularly large number of species living in coral reefs , but with some species penetrate into moderate latitudes. Few species also live in brackish water , four species even in fresh water.

All Alpheidae are ground-living ( benthic ) and bad swimmers. They come in a wide variety of coastal ( littoral ) habitats , such as B. Mangrove forests , up to the deep sea. Many species live as single tenants ( equilines ) or symbiotically in or on large organisms such as B. sponges , starfish , polyps of cnidarians , others live in community with fish ( gobies ) and other crabs.

anatomy

The Alpheidae resemble other shrimp-like Decapoda in their general body shape. Their torso is encased in a carapace , which is extended forward into a thorn-like appendage, the rostrum (in rare cases it may be absent). A special feature of the Alpheidae are two other lateral appendages of the carapace, which protect the stalked eyes. These orbital covers are very important for differentiating between species. The abdomen, like most Decapoda, is very muscular and curved downwards. At the end there is a tail fan. If their abdomen suddenly hunched over, the animals can catapult themselves backwards when they approach predators (the so-called escape reaction ). At the head of the Alpheidae there are two scissors ( chela ), which consist of a solid base and a smaller scissor finger on it. The scissors are small and symmetrical in some species. In many species, mostly only in the male sex ( sexual dimorphism ), one of the claws is greatly enlarged. These scissors can be so large that their mass almost equals that of the rest of the body. The enlarged scissors are used to fight conspecifics and to ward off predators, the second, smaller scissors are mainly used for nutrition. Some species also use the enlarged scissors for digging to build burrows, excavating both soft mud and hard coral limestone. Whether the right or left scissors are enlarged does not seem to be fixed, both forms appear side by side. But there are also species with two claws that are not so extremely enlarged. In some species the claws are enlarged in both sexes. In life, depending on the species, the scissors are either carried forward or struck down; in some species the scissors are rotated (the scissor finger on the stomach side). Only in the case of the genera Alpheus and Synalpheus , which are both very species-rich, are the scissors modified in such a way that the characteristic popping noises can be generated.

In the Alpheidae, especially in the genus Alpheus, there are species with a strongly calcified integument , which thereby acquire a lobster-like appearance. However, many species have a very delicate, often completely transparent body shell that allows internal organs and eggs to shine through. Many species are brightly colored, sometimes multicolored, and are therefore sought after in the aquarium hobby.

Bang generation

The German name is derived from the fact that many species can make a very loud noise with one of their two claws. The basis of the mechanism is a tooth on the scissors finger (dactylus), which is inserted into a suitable cavity in the base member (pollex) when the scissors are closed. At Alpheus, movement is also influenced by a locking mechanism. Round plates with a very smooth surface sit on both scissor links. These hit each other when the scissors are open. The scissors are fixed in the open state by adhesive forces and can only be closed with considerable effort. The energy released when the locking mechanism is opened results in an extremely fast, almost explosive movement that massively amplifies the generation of sound. When closing the pop scissors, the crabs quickly emit a jet of water that forms a cavitation bubble that implodes with a very loud bang. According to model calculations, a toroidal cavitation bubble forms at the interface between the water jet and the surrounding water, and when it collapses, a pressure pulse of 10 bar (80 bar in the immediate vicinity) occurs. This leads to a sonoluminescence phenomenon ( called shrimp poluminescence by Detlef Lohse and co-authors ), the creation of a flash of light by implosion of the cavitation bubbles, with temperatures of over 5000 Kelvin being measured. They use this weapon as a warning, in combat with conspecifics, when catching prey or for intra-species communication. The crabs became famous during World War II when their popping noises disrupted military sonar tracking . Small crabs, worms and small fish can be stunned by the pressure.

To date, it has not been possible to find an organ of sound perception in the crabs. Possibly the point of the mechanism is to be looked for more in the production of the water jet, and the sound production itself is only a side effect. The water jets are also aimed specifically at conspecifics and have been proven to also serve for intra-species communication. A similar behavior is also known in mantis shrimp (Stomatopoda).

Reproduction

Most Alpheidae carry their eggs with them glued to the pleopods until they hatch, a common behavior pattern in the decapods. In most groups, as is usual in the kin group, floating (planktonic) larval stages hatch, which after metamorphosis into young crabs with adult morphology change to the benthic way of life. Some groups, especially within the genus Synalpheus, have direct development instead. Here the larval stages are passed through within the egg shell, after which young crabs hatch immediately with a morphology corresponding to that of adults. The Alpheidae are separate sexes, parthenogenetic reproduction does not seem to occur. The females have no storage organs for sperm, so each egg packet has to be fertilized directly. At least some species, and possibly many, are facultative or obligate protandric hermaphrodites. This means that some or all of the individuals are males at first, but transform into females when they molt later. These relationships are not unusual in Decapoda and have been documented by numerous kinship groups. What is more unusual is that in some species it seems that the largest individuals can revert to males.

Many species of the Alpheidae live together in a permanent, monogamous couple relationship of a female and a male in a common hiding place.

Behavior, sociality

Some members of this family live in symbiosis with gobies , sea ​​anemones or sea ​​cucumbers . They are therefore also called symbiotic shrimp. Synalpheus carinatus and Synalpheus stimpsoni and some exclusively Indo-Pacific species live with hairy stars , the latter of which have several color variants, adapted to the color of the host.

In 1996, the American marine biologist J. Emmett Duffy of the Virginia Institute of Marine Science discovered that the popcorn Synalpheus regalis is a state-building eusocial animal, and social behavior has now also been found in a number of related species, all of which live on sponges in the Caribbean . Up to 350 individual animals live inside sponges . The sponges also form the food source of the species, which can therefore be classified as a parasite . It has not yet been clarified whether the host organism will suffer significant damage as a result. Many species are host specific to a single sponge species.

Each colony consists of a single reproductive pair ( queen and king ) and their offspring and colonizes a sponge organism. The sponge is defended against invading conspecifics or members of related species. The offspring will likely develop into males (this is difficult to decide, as Synalpheus males have no external signs of sexual maturity). They do not mate further within the colony. From DNA studies it is clear that the inbreeding rate within the colonies is very low, this excludes direct mating of the offspring. The young develop directly and remain in the sponge organism. How the spread and fertilization takes place has not yet been clarified, but the males are probably the medium of spread. However, wild animals are extremely rare in the habitat. As protandric hermaphrodites, males can later transform into females. Some of the animals within the colony transform into a form with particularly large chelae , these are characterized by particular aggressiveness towards intruders. Analogous to social insects such as ants and termites , they are called soldiers . Synalpheus lacks a working class in the true sense of the word ; all animals feed independently on the sponge tissue, there is no mutual feeding.

The colonies of Synalpheus are the first and so far only known example of state formation in an animal living in the sea and to this day the only eusocial crabs. Unlike the social hymenoptera, but like the termites, the animals are diploid in both sexes. The emergence of sociality obviously deviates here from the familiar mechanism in Hymenoptera states, where it is mostly explained by the particularly close relationship between the females, which increases their inclusive fitness when raising sisters. Similar to socially living aphids and fringed- winged birds, a special model for evolution-derived social behavior has been set up that is based on the high value of a defensible resource (here a sponge organism) that can be monopolized as a fortress against conspecifics and related species. It could have been beneficial for the evolution of social behavior that large groups are preferred to smaller colonies in direct confrontation, which has even led to the establishment of common colony of unrelated individuals ( pleometrosis ) in other species . This mutualism hypothesis emphasizes the value of cooperative behavior even in the absence of peculiarities of relative selection. The direct development of the crustaceans was probably also of importance, since related individuals remain close to one another while they are widely dispersed during planktonic larval development.

Partner goby with pop cancer
Sea anemone , next to which you can see the claws and antennae of symbiotic fire shrimp. A partner shrimp on the front of the finger .

Systematics

Both morphological and DNA studies have confirmed that the Alpheidae form a monophyletic community of descent. Sister group within the superfamily Alpheoidea is probably the family Ogyrigidae . Some morphologically primitive genera such as Yagerocaris , Potamalpheops and Stenalpheops do not have enlarged claws , they are reminiscent of Hippolytidae (cleaner shrimp). The higher Alpheidae, the genera Alpheus and Synalpheus , are sister genera . This suggests that the characteristic bang mechanism evolved only once.

The Alpheidae family comprises more than 600 described species in 36 genera. However, this number of species is only an approximate, lower estimate, because new species are still described every year. In addition, cryptic species have become known in numerous genera that are difficult to distinguish morphologically or not at all , some of which can be differentiated on the basis of their behavior or subtle differences in color. Others have so far only been differentiable on the basis of their DNA sequences.

Genera

Incertae sedis

literature

  • Arthur Anker, Shane T. Ahyong, Pierre Y. Noel, A. Richard Palmer: Morphological phylogeny of alpheid shrimps: parallel preadaptation and the origin of a key morphological innovation, the snapping claw. In: evolution. Vol. 60, No. 12, 2006, ISSN  0014-3820 , pp. 2507-2528, doi : 10.1111 / j.0014-3820.2006.tb01886.x .
  • J. Emmett Duffy: The ecology and evolution of eusociality in sponge-dwelling shrimp. In: Tomonori Kikuchi, Noriko Azuma, Seigo Higashi (eds.): Genes, Behaviors and Evolution of Social Insects (= Proceedings of the Congress of the International Union for the Study of Social Insects. 14). University of Hokkaido Press, Sapporo 2003, ISBN 4-8329-0317-9 , pp. 217-252, ( PDF; 182 kB ).
  • Knallkrebse (= coral. Marine aquarium specialist magazine. No. 17, October / November 2002, ISSN  1439-779X ). Natur und Tier Verlag, Münster 2002.
  • Martin W. Johnson, F. Alton Everest, Robert W. Young: The Role of Snapping Shrimp (Crangon and Synalpheus) in the Production of Underwater Noise in the Sea. In: The Biological Bulletin. Vol. 93, No. 2, 1947, ISSN  0006-3185 , pp. 122-138, doi : 10.2307 / 1538284 .

Individual evidence

  1. ^ S. De Grave, Y. Cai, A. Anker (2008): Global diversity of shrimps (Crustacea: Decapoda: Caridea) in freshwater. Hydrobiologia 595: 287-293. doi : 10.1007 / s10750-007-9024-2
  2. ↑ Phoevos Koukouvinis, Christoph Bruecker, Manolis Gavaises (2017): Unveiling the physical mechanism behind pistol shrimp cavitation. Scientific Reports 7, Article number: 13994. doi: 10.1038 / s41598-017-14312-0 (open access).
  3. ^ Peter OK Krehl: History of Shock Waves, Explosions and Impact: A Chronological and Biographical Reference. Springer, 2008, page 822; limited preview in Google Book search
  4. ↑ D. Lohse, B. Schmitz, M. Versluis: Snapping shrimp make flashing bubbles. In: Nature. 2001 Oct 4; 413 (6855): 477-478.
  5. ↑ Kristin Leutwyler: Snapping Shrimp. In: Scientific American. September 22, 2000.
  6. ↑ EJ Duffy: Eusociality in a coral-reef shrimp. In: Nature. 1996 Jun 6; Vol 381 512: 514.
  7. ^ EJ Duffy, CL Morrison, KS Macdonald: Colony defense and behavioral differentiation in the eusocial shrimp Synalpheus regalis doi: 10.1007 / s00265-002-0455-5
  8. ↑ David C. Queller & Joan E. Strassmann (1998): Kin Selection and Social Insects. In: BioScience Vol. 48, No. 3: 165-175.
  9. ^ Norman Lin & Charles D. Michener (1972): Evolution of sociality in insects. In: Quarterly Review of Biology 47 (2): 131-159.

Web links

Commons : Knallkrebse (Alpheidae)  - Collection of images, videos and audio files