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Summary | |
Snapping shrimp, otherwise known as pistol shrimp, are a primarily marine crustacean which get their name from the characteristic snapping sound they make when they close their major chela (large claw). The shrimp belong to the species rich genus Alpheus which has over 300 described species (Cunha et al., 2015).
Throughout this page I will discuss features of two specimens (1 male and 1 female) from the genus Alpheus which most closely resemble the species A. Strenuus Dana. However, I am not confident in identifying them as such, based on the high variation in the genus Alpheus and the fact that the individuals differed slightly in colour and morphology (e.g. right claw of female was large, while left claw of male was large). Although this could simply be sexual dimorphism, or a result of limb regeneration (discussed later), it is also possible that differences between the two specimens are a result of them being two different species.
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Physical Description | |
As previously mentioned, the genus Alpheus is highly diverse, encompassing over 300 species (Cunha et al., 2015). However, the true number is thought to be much higher due to the morphological variability and cryptic nature of some species (Cunha et al.,2015; Williams et al., 2001). While some species have a constant morphology across their global distribution, many others exhibit significant variability within the same reef system (Banner & Banner, 1982). It was this variation that prompted Banner & Banner 1982 to split various species into separate new species or subspecies.
The main regions of the alpheid body plan include the cephalothorax and the abdomen. 5 pairs of pereopods extend out of the ventral surface of the cephalothorax and are used to walk along surfaces. However, the first pair of pereopods (chela) have become large and are use for functions other than walking (prey capture and manipulation, communication, defence, etc…). Pleopods extend out of the first 5 segments of the abdomen (abdomen has 6 segments) and are often used for forward swimming. Females also deposit their eggs and attach them onto the pleopods. The 6th segment of the abdomen is comprised of the telson and uropods which assist by providing a large surface area for propulsion and steering. Some of these features can be seen in figure 1.
The features these specimens share with A. strenuus Dana include similar morphology of the small and large chela, balaeniceps (concentrated rows of setae which share a resemblance with the baleen of whales) on the small chela, many setae on the large chela, and a rostrum which lacks setae. The variations between the two specimens however restricts me from labelling them as A. strenuus Dana.
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| Figure 1 |
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Ecology |
Habitat | |
Snapping shrimp are found across the globe and have colonised a variety of habitats. They are common in tropical waters where they can be found in sand or mud burrows, intertidal pools, under rocks, or simply living among the coral (Williams et al., 2001). Some species are also known to form symbiotic relationships with fish, echinoids, anemones and annelids (Kim & Abele, 1988).
This specimen is from an unknown locality but is likely to have originated from either Moreton Bay in South East Queensland or Heron Island in the Great Barrier Reef.
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Predation | |
The main predators of snapping shrimp are larger fish species, for which the effect of the snapping claw prove useless as a defence. The shrimp have instead developed other strategies to avoid predation. Many species form mutual relationships with gobies, enabling them to receive early information regarding the presence of a potential predator (Yanagisawa, 1984). The gobies benefit from the relationship by using the shrimps burrow as a refuge and nesting site, while the shrimp benefit by receiving early warning signals from the goby (Karplus & Thompson, 2011). This behaviour requires constant antennal contact with the goby whenever the shrimp emerges from its burrow. Quivering from the goby transmits down the antennae of the shrimp and is received as an alarm cue, warning of the presence of a potential predator and triggering a retreat response in the shrimp (Yanagisawa, 1984).
If the shrimp are caught off-guard, and are faced with an immediate threat of predation, they are able to rapidly contract their abdominal muscles causing their abdomen to curl inwards towards the body (Brusca et al., 2016). The large surface area of the tail provides friction and results in rapid bursts of backward swimming (Nauen & Shadwick, 2001). This form of predator avoidance is known as the caridoid escape reaction (Heitler et al., 2000).
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Life History and Behaviour |
Pre-copulation | |
Within the animal kingdom, a male is likely to be disadvantaged (reduced fitness) if he is restricted to monogamous pairing. This is because males typically invest less energy in reproduction than females, and hence would have increased fitness by mating with multiple females (Rahman et al. 2003). Regardless, social monogamy has evolved in many taxa and is often a result of the offspring relying on care from both parents to ensure survival (Rahman et al. 2003).
However, the social monogamy seen in many alpheid shrimp is closely tied to the moult cycle of the female, importance of territory, and the risk of predation outside of their burrow (Knowlton, 1980). Female pistol shrimp are only receptive immediately after they moult and it is important for the male to be present otherwise fertilization cannot occur (Correa & Martin, 2003). Furthermore, time spent mate searching outside of their burrow is extremely risky and increases the chance of predation (Rahman et al. 2003). It is therefore of the best interest of both the male and female shrimp to form monogamous pairs, ensuring that they are together when the female moults and becomes receptive.
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Post-copulation | |
Many crustaceans follow a series of post-hatch developmental changes which result in the formation of a miniature version of an adult (Brusca et al., 2016). In species which produce eggs with a low yolk content, the larvae generally hatch as planktotrophic nauplii (the earliest larval stage) which undergo many developmental changes before reaching a post larval stage (Knowlton, 1973). In contrast, species which produce high yolk content eggs often express prolonged brooding of eggs, causing offspring to develop further and hatch at an advanced larval stage (Knowlton, 1973).
Although few studies have focused on the larvae of alpheid species, it is understood that the larval stage at time of hatching varies greatly between species (Knowlton, 1973). The species which have been most successfully raised in lab environments include those that hatch as advanced zoea larvae or as miniature versions of adults (direct development) (Knowlton, 1973).
Typically, alpheid species skip the nauplius stage and hatch at an early zoea larval stage (Knowlton, 1973). Species which produce relatively small larvae are known to have a longer period of larval development involving up to 9 instars before metamorphosis (9 separate moults before they metamorphose into a miniature version of the adult form) (Spence & Knowlton, 2008). While species which hatch as larger and more developed larvae, progress through as little as 3 instars before metamorphosis (Spence & Knowlton, 2008).
Figure 5 shows the newly hatched larvae of the specimen snapping shrimp. The larvae are seen to be in a zoea stage of development, characterised by the plumose setae on the tips of its telson and lack of abdominal pleopod development (Tracey et al., 2002).
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| Figure 2 |
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| Figure 3 |
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| Figure 4 |
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| Figure 5 |
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Snapping | |
Early research in snapping shrimp lead to the belief that the snapping sound was created by the impact of the dactylus (movable finger) closing onto the pollex (fixed finger) of the large chela (Anker et al. 2006). However more recent research by Versluis et al. (2000) revealed that the noise was actually created by the rapid ejection of water in the socket of the pollex, causing the formation of cavitation bubbles which ultimately implode. It is the implosion of the cavitation bubble which causes the distinctive snapping sound.
The dactylus comes down on the pollex at over 100km/h and creates a water jet that moves at up to 25m/s (Koukouvinis et al., 2017; Brusca et al., 2016). The high-speed movement of water causes a localised drop in pressure which is enough to instantly vaporise a small amount of the water, creating the cavitation bubble (Koukouvinis et al., 2017). Furthermore, the huge change in pressure causes localised temperatures of the cavitation bubble to exceed 4000 degrees Celsius, followed by a burst of light as the bubble implodes (Lohse et al. 2001). The flash of light is not invisible to the naked eye as it is extremely short lived and only emits a low number of photons. Because of this, it is unlikely to have any biological significance and merely serves as an indication of the force produced by the shrimp’s claw (Lohse et al. 2001). In contrast, the resulting shockwave which is emitted from the implosion of the cavitation bubble has a high biological significance and is used in communication, defence from predators, intraspecific competition, and hunting (Hess et al., 2013; Schmitz & Herberholz, 1998). Figure 6 shows the Alpheus species snapping its claw in slow motion. The cavitation bubble is produced too fast to see, even at high frame rates (120fps).
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| Figure 6 |
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Feeding Strategies | |
Little detailed information can be found in literature regarding the feeding behaviour of alpheid shrimp, however most species are understood to be opportunistic omnivorous (Banner & Banner, 1982). The stomach contents of A. euphrosyne richardsoni was examined, revealing that the shrimp had been grazing on algae and seagrass, while also predating upon various polychaetes, fish, molluscs and crustaceans (Banner & Banner, 1982). In some cases, snapping shrimp use the shockwave produced by their large chela to stun or kill prey before consumption (Herberholz & Schmitz 1998).
These particular specimens were housed in an aquarium over the past few months before they were removed for identification. In which time, they had eaten many of the mollusc and crustacean residents.
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Anatomy and Physiology |
External Anatomy | |
All members of the superphylum Arthropoda, to which snapping shrimp belong, are encased in a hard exoskeleton consisting of chitin and minerals (in crustaceans). While this ‘armour’ provides various advantages such as body support and protection against predation and osmotic stress, it also constraints the flexibility and growth of the organism (Brusca et al, 2016; Chen et al., 2008). Arthropods have overcome these challenges by evolving various joints situated in various parts of the body where flexibility in required (Brusca et al., 2016). Furthermore, the problem of restricted growth was resolved by the process of dissolving their exoskeleton when they outgrow it and rebuilding a larger one underneath it (Brusca et al., 2016). This process is known as moulting and is common to members of the phylum Ecdysozoa.
The mobile and predatory lifestyle of alpheid shrimp requires a variety of sensory receptors to interpret information from the surrounding environment. The shrimp have many sensory setae (small hairs) on their body which are especially common on the small and large chela of many species (Brusca et al., 2016). The sensory setae themselves vary in their shape and function, and include long serrulate, plumose, simple short and tubercles setae (Herberholz & Schmitz, 1998). Studies of these structures suggest that their functions range from focusing the water jet of the major chela, to detecting mechanical and chemical information from the environment (Herberholz & Schmitz, 1998). These setae have been known to be important for intraspecific agnostic interactions where the attacking shrimp focusses the water jet at the setae of the receiver (Herberholz & Schmitz, 1998). Some of the mentioned setae can be seen in figures 7 and 8.
Alpheid shrimp also have sensory setae located on their 2 pairs of antennae. The setae on the antennules (first pair) are known to be used in olfaction, while the lesser studied setae on the antennae (second pair) are thought to be chemotactile (chemical and mechanical) and must come into contact with objects to extract information (Bauer & Caskey, 2010; Vickery et al., 2012). It is these mechanoreceptors associated with the antennae which enable the alpheid shrimp do detect early warning cues form mutualistic gobies (Yanagisawa, 1984). Figure 9 shows how the specimen flutters its antennules in the water column, constantly extracting chemosensory information. Figures 10 and 11 also shows electron microscope image of the setae found on the antennae and antennules of the snapping shrimp.
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| Figure 7 |
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| Figure 8 |
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| Figure 9 |
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| Figure 10 |
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| Figure 11 |
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Internal Anatomy | |
Alpheid shrimp have a complete gut which consists of the foregut, midgut and hindgut. Food passes down the oesophagus and directly into the foregut where it is sorted and masticated (Felgenhauer, 1992). It must then pass through the gland filter which separates small and large particles (Felgenhauer, 1992). small particles are moved into the digestive gland (hepatopancreas) where digestive enzymes are released, and nutrients are absorbed (Felgenhauer, 1992). In contrast, larger particles are moved directly into the midgut where remaining nutrients and water are absorbed before the food particles are moved through to the hindgut and out through the anus (Felgenhauer, 1992).
Snapping shrimp also have an open circulatory system. The dorsally located heart pumps blood containing various cell types (various types of amoebocytes, explosive cells associated with blood clotting and oxygen carrying cells) through several vessels and into the open hemocoel (Brusca et al., 2016). Open circulatory systems require efficient removal of waste products as the body tissues essentially bathe in the blood.
Alpheid shrimp are ammonotelic (excrete nitrogenous waste as ammonia) organisms which possess excretory antennal glands located at the base of the antennae (Brusca et al., 2016). These glands function similar to nephridia and remove toxic ammonia from the shrimps’ body (Brusca et al., 2016).
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Physiology | |
Members of the genus Alpheus have the remarkable ability to regenerate their chela. If the large chela is removed by competition of predation, the small chela will undergo physiological changes and grow into the large chela over successive moults (Koukouvinis et al., 2017). Furthermore, the missing large chela will be replaced by a new small chela (Koukouvinis et al., 2017). Guchardi & Govin (1989) studied the vessel sizes in the chela of Alpheus species and found that larger vessels are found in the large chela. The researchers also demonstrated plasticity in the adult vascular tissue by showing how reversal of chela asymmetry resulted in an increase in the size of the vessels found in the small chela, which eventually grew into the large chela (Guchardi & Govin, 1990).
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Biogeographic Distribution | |
As previously mentioned, snapping shrimp are most commonly found in tropical waters across the globe. As these specimens are of an unknown species, I am unable to provide even an approximation of their distribution.
However, the species which these specimens most closely resemble (A. strenuus Dana) is found throughout the Indo-Pacific, from the Red Sea in the west to French Polynesia in the east (Banner & Banner, 1982).
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Evolution and Systematics | |
The crustacean fossil record dates back to the early Cambrian, and possibly even as far back as the Ediacaran (Brusca et al., 2016). Remarkably, detailed Cambrian fossils of both adults and nauplii larva share many characteristics of the modern-day crustaceans (Brusca et al., 2016). It is widely accepted that Hexapods arose from a group of crustaceans (most likely the Cephalocarid or Remipede clade), rendering the crustaceans a paraphyletic group (Brusca et al., 2016).
Many lineages have emerged from the group Crustacea, including the family Alphidae to which snapping shrimp (Alpheus) belongs. Very few phylogenetic studies have been conducted on alpheid shrimp which explains why advances in knowledge often result in the rearrangement of species into different genera or families (Anker et al., 2006).
The most recent alpheid phylogenetic study by Anker et al. (2006), as seen in figure 12, positions Alphidae (a monophyletic group) as the sister group of Ogyrididae. Furthermore, the researchers believe that the ability to produce cavitation bubbles evolved only once and caused a huge radiation, resulting in the diversity seen today (Anker et al., 2006).
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| Figure 12 |
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Conservation and Threats | |
Only one species of snapping shrimp, A. cyanoteles, is identified to be under threat by the IUCN. This shrimp is endemic to a small area in Malaysia (Yeo & Ng, 1996). Little is known regarding the extinction risk of the other members of the genus Alpheus.
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References | |
Anker, A., Ahyong, S. T., Noël, P. Y., Palmer, A., R. (2006). Morphological Phylogeny of Alpheid Shrimps: Parallel Preadaptation and the Origin of a Key Morphological Innovation, the Snapping Claw. Evolution, 60, 2507-2528.
Banner, D. M. & Banner, A. H. (1982). The alpheid shrimp of Australia Part III: The remaining alpheids, principally the genus Alpheus, and the family Ogyrididae. Records of the Australian Museum, 34, 1–357.
Bauer, R. T., and Caskey, J. L. (2006). Flagellar setae of the second antennae in decapod shrimps: sexual dimorphism and possible role in detection of contact sex pheromones. Invertebrate Reproduction & Development, 49, 51–60.
Brusca, R. C., Moore, W., Shuster, S. M. (2016). Invertebrates. Sinauer Associates, Sunderland, Massachusetts U.S.A, 3rd Edition, 728-836
Chen, P., Lin, A. Y., McKittrick, J., Meyers, M. A. (2008). Structure and mechanical properties of crab exoskeletons. Acta Biomaterialia, 4, 587–596.
Correa, C. & Martin, T. (2003). Mating systems in caridean shrimp (Decapoda: Caridea) and their evolutionary consequences for sexual dimorphism and reproductive biology. Revista Chilena de Historia Natural, 76, 187-203.
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Felgenhauer, B. (1992) Internal anatomy of the Decapoda: an overview. In: Harrison F, Humes A (eds) Microscopic anatomy of invertebrates. Decapod Crustacea, vol 10. Wiley-Liss, New York, pp 44–75
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Nauen J. C., Shadwick R. E. (2001). The dynamics and scaling of force production during the tail-flip escape response of the California spiny lobster Panulirus interruptus. J. Exp. Biol. 204, 1817–1830.
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Spence, H. R. & Knowlton, R. E. (2008). Morphological and developmental differences in three species of snapping shrimp genus Alpheus (Crustacea, Decapoda). Southeast Naturalist, 7, 207–218.
Tracey, E., Pereira, A., Hughes, M., Ko, C. A. 2002. THE EMBRYONIC DEVELOPMENT OF THE SNAPPING SHRIMP, ALPHEUS ANGULOSUS MCCLURE, 2002 (DECAPODA, CARIDEA). Crustaceana 86 (11) 1367-1381
Versluis, M., Schmitz, B., von der Heydt, A., Lohse, D., (2000). How snapping shrimp snap: Through cavitating bubbles. Science, 289, 2214-2117.
Vickery, R., Hollowell, K., Hughes, M. (2012). Why have long antennae? Exploring the function of antennal contact in snapping shrimp. Marine and Freshwater Behaviour and Physiology, 45, 161–176.
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