There are ~350 species of mantis shrimp (Stomatopoda). Pictured above is Odontodactylus scyllarus (source: Patek & Caldwell, 2005). They are easily recognisable in the field by their colour and appearance, but more scientifically by their flat carapace that leaves the sides of the thorax exposed and doesn’t even cover the last four thoracic segments. Thoracopods 2-5 are subchelate (claw-like), especially the second thoracopod which is transformed into a proper raptorial appendage reminiscent of a praying mantis’s (hence the common name). They walk on their 6th to 8th thoracopods, and if all else fails, you’ll also recognise them from their huge eyes on moveable stalks.
First, some morphological trivia about them (read: stuff I couldn’t fit in the other sections, but couldn’t leave out):
As in all extant crustaceans, stomatopods have a cephalon with at least five appendage-bearing segments, with the second segment’s appendage reduced. Convergently in the stomatopods and many other crustacean groups (decapods, copepods, remipedes, anaspidaceans, peracarids), a cephalothorax (a.k.a. gnathosoma) develops by the docking of several thoracic segments to the head.
Stomatopods are among a minority of the crustaceans where a nauplius larva is not present, instead being replaced by an egg-nauplius stage (Scholtz, 2000), a trait it shares with the anaspidaceans and many decapods. The larva is free-swimming and is responsible for dispersal.
As in the leptostracans, stomatopodan thoracopods have only one epipod; this is most probably reflective of the malacostracan ancestor (Richter & Scholtz, 2001); the more derived Eumalacostraca have two epipods.
Stomatopods are rather unique in that their circulatory system is very extensively developed, relative to its phylogenetic cousins, with the heart extending all the way to the abdomen and its ostia (Siewing, 1957).
As in some crustaceans (e.g. decapods, ostracods or cirripedes), their chitin exoskeleton is reinforced with small amounts of inorganic minerals within the chitin and biopolymer matrix. In stomatopods, the inorganic fraction is made up of calcium and phosphorus, which may be bound together in the form of apatite crystals (but the latter has, to my knowledge, not been confirmed; Currey et al., 1983).
The random bits are done; on with the structured post!
Stomatopoda are the basalmost Malacostraca (Richter & Scholtz, 2001), a position supported by most phylogenetic analyses. In the past, they were placed as members of the Eumalacostraca, and this is how they are presented in any pre-2000 textbook (and some modern ones too). It’s an outdated view (although, as usual in phylogenetics, a still tenable one). Also, keep in mind that the exact branching order of the basal malacostracans is still rather unclear. While I, and most others, support the Stomatopoda at the base of the Malacostraca, other studies have found the Bathynellacea instead (Jenner et al., 2009) or a clade comprising the Stomatopoda + Mysida + Euphausiacea (Meland & Willassen, 2007). However, the most rigorous analyses support the Stomatopoda hypothesis.
Stomatopodan monophyly has never been seriously challenged, even to the point that molecular and morphological data do not contradict each other (Ahyong & Jarman, 2009). Their apomorphies, according to Richter & Scholtz (2001) are:
- five maxillipeds (one pair transformed to the famous raptorial appendages);
- exopod of first thoracopod absent;
- specialised petasman morphology (petasma = gonopod = modifications of first and/or second pleopods in many crustaceans for sperm transfer). Stomatopodan petasma uniquely modified to only move the genital papillae, not for actual sperm transfer;
- related, no spermatophore;
- chromatin in sperm diffuse, no nuclear membrane;
- pleopod exopods functioning as gills;
- lack of antennal gland;
- lack of superomedianum (superomedianum = “infolding at the transition from the cardia to the pyloric chamber”);
- no embryonic “dorsal organ” (dorsal organ = a group of cells found dorsally on the head of many crustacean larvae with unknown function).
Of these, only the five maxillipeds and the pentasma are truly unique to the stomatopods, with the other characters also having convergent counterparts in other malacostracans. One additional character that is unique to stomatopods, but whose phylogenetic value is uncertain, is the triramous antenna, pictured above (Derby et al., 2003). The reason for the uncertainty is several ambiguities in the terminology between “ramus” and “flagellum”; basically, the difference in the stomatopod antennule is that the second flagelum splits into two rami during development, and it is undecided whether they should be called rami or flagella in their own right (Boxshall, 2010). It can also be called the triflagellate antennule.
In any case, it is a trait only found in fossil stomatopods, in the now-extinct phosphatocopids (best known from the Orsten), and which also convergently evolved in a couple of carideans, the basalmost decapods, e.g. Palaemon squilla (Helm, 1928).
As a historical note, Stomatopoda are also referred to by some authors as Hoplocarida, a tradition dating back to Calman (1904), pictured above, the first malacostracan systematics study. Taxonomically, Hoplocarida is the subclass, with Stomatopoda being the only order in it. Systematically, Stomatopoda refers to the crown group, while Hoplocarida is the total-group (i.e. including stem-group fossils).
The stem-group is known from Carboniferous fossils, from a family called the Aeschronectida (Jenner et al., 1998). They are linked to the stomatopods by their triramous antennule and their pleon shape, but show no other stomatopodan characteristics (such as raptorial appendages).
Within the crown group, are the “palaeostomatopods”, “archaeostomatopods” and unipeltates, the former two being paraphyletic wastebasket taxa (Jenner et al., 1998). Unipeltata is, for our intents and purposes, equal to Stomatopoda (they’re at different taxonomic ranks).
On Recent systematics, the stomatopods are split into five superfamilies: the Bathysquilloidea (deep-sea stomatopods), Gonodactyloidea, Lysiosquilloidea, Squilloidea and Erythrosquilloidea (Manning & Camp, 1993). I am avoiding going into the taxonomical quagmire, because it is a bit of a mindfuck; if you care, ask.
Stomatopods first radiated during the Mesozoic Marine Revolution (Vermeij, 1977). The Mesozoic Marine Revolution was a time of great ecological changes in the oceans, in which predation as an ecological force was truly established. With the radiation of the neogastropods came the radiation of a whole range of durophagous (shell-destroying) predators, including the stomatopods. In the Triassic, shell-crushing cephalopods evolved, as did reptilian molluscivores, such as the placodonts. It was in the Jurassic that the stomatopods radiated, alongside other crustaceans such as the nephropids (lobsters), palinurids (spiny lobsters) and brachyurans (crabs), as well as vertebrate predators such as the sharks and batoids (rays). Even durophagous gastropods radiated in response to their evolutionary cousins’ newly-developed fortifications, with the naticids and muricids (drilling gastropods) and buccinids, fasciolariids, melongenids, volutids, olivids and other predaceous gastropods also radiating. As a result of this great increase in predators, crinoids and brachiopods were forever pushed out of shallow waters – most of the marine ecosystem structure we have today came about as the result of the Mesozoic Marine Revolution (well, we don’t have the reptilian predators anymore, but they’re replaced by cetaceans and sharks).
Stomatopodan predation traces are found on Miocene gastropods (Baluk & Radwanski, 1996), but stomatopods are survivors of the Permian-Triassic extinction event; before feeding on gastropods, they used to feed on trilobites. Their fossil record begins in the Late Devonian (Famennian; Schram, 1982), but due to the low amount of inorganics in their cuticle, their preservation potential is rather low, and this should be taken into account.
Their origin (as palaeostomatopods) comes at the same time as a slight burst in crustacean disparity (morphological diversity), with the first eumalacostracans also originating at this time (Wills, 1998).
Their second most renowned trait (the first being the eyes) are their predatory raptorial appendages (pictured above; van Roy, 2006), whose functional morphology convergently evolved several time in the arthropods (unique taxa in the amphipods, decapods [see McLaughlin, 1980 for the crustacean examples], mantids, mantophasmatodeans, ensiferans, heteropterans, mantispids, vittacids, bethylids and dipterans). As said above, it derives from the second thoracopod (maxilliped 2), and it’s used to either smash prey or spear it with the spines. The spearing is notable for being one of the fastest movements in the entire animal kingdom, clocked at 14-23 m/s, with acceleration at a staggering 10⁵m/s² (Patek et al., 2004).
The bashing movement is somewhat unique and was elucidated by Patek et al. (2004), from where the diagram above also comes from. Basically, the limb is held back under high-tension by a spring – in physical terms, a lot of energy is being stored as elastic energy waiting to be released. Once the spring is let go, the limb is ejected at the same high speeds – so high that cavitation is produced. Cavitation is when something moves so fast through a liquid that a bubble of low pressure forms – this bubble then bursts releasing a lot of energy (including a loud “pop”, as known from pistol shrimp). In total, this results in two impacts that break the opponent: the first is from the limb strike, and 390-480µs later comes the collapse of the cavitation bubble, causing a second impact; the former can deliver up to 1500N of force and the latter 500N (Patek & Caldwell, 2005).
Some enigmatic fossils ranging from the Silurian to the Carboniferous called angustidontids have, by some authors, been placed as stomatopod fossils. Angustidontids are simply fragments with many spines of varying length, and the fossil is interpreted by them to be of the raptorial appendage (e.g. Copeland & Bolton, 1960). While their identity is still a topic of debate and it is off-topic to go into it, a stomatopod affinity can likely be ruled out, as the spines of the stomatopod raptorial appendage are not found in such high numbers and become longer distally, not randomly changing in size as in the angustidontids (Braddy & Dunlop, 2000); but see Rolfe & Dzik (2006) for a reconstruction (above picture) as a predatory crustacean with “possible distant affinities also to the stomatopods”, based on findings of associated body fossils.
Another fossil sometimes attributed to stomatopods is the trace fossil Cruziana, pictured above (Zonneveld et al., 2002). It is rather unlikely though, as Recent stomatopod burrows do not exhibit these types of traces.
As with most arthropods, stomatopods can regenerate lost appendages (Berzins & Caldwell, 1983), including antennae, but they are one of the least-studied taxa in this respect. For the limbs, there seems to be a preferred breaking point at the ischium-merus joint (Wood & Wood, 1932), but this is not certain.
Neurologically, stomatopods were some of the earliest crustaceans to have been studied comparatively with regards to homologisation of brain regions across the animal kingdom, by Bellonci (1883) who looked at olfactory glomeruli in eels, stomatopods and crickets. As for specific studies on stomatopod brains only, Bellonci (1878) was the earliest. It is a shame that relatively little work has been carried out on their brains though: their behaviour, as we will see, is fascinating, and it is all enabled by highly-developed visual and chemosensory systems which would be really cool to know more about.
Their olfactory lobes were first studied by Radl (1902), who found that the lobes are arranged in spherical glomeruli, as in the Leptostraca. These are generally of the same volume as decapods, but there are less of them in stomatopods (60-80) than in decapods (min. 100, up to 1200). With respect to their unique antennule, only one of the extra rami has aesthetascs (the sensillae responsible for smell, occurring only on the lateral flagellum, i.e. the one that is split in stomatopods), and there they are inferior in number (~40) and power (12-20 sensory neurons) than in decapods (Mead & Weatherby, 2002).
Of course, the most famous feature of the stomatopods are their eyes, despite their being, at phylogenetic face value, completely normal malacostracan compound eyes. The picture above (Porter et al., 2010) shows four different eye types. A is the squilloid eye of Squilla, B is the Parasquilloid eye of Pseudosquilliopsis, C is the lysiosquilloid eye of Lysiosquillina and D is the gonodactyloid eye of Neogonodactylus. Keep in mind that in each of these pictures, the orientation is the same. In total, there are 7 types of stomatopodan eyes that can be distinguished (Harling, 2000), but we won’t go into them.
Before discussing the eyes, a small note on some significant historical milestones in the history of stomatopod eye research. They were first studied in 1891 by Exner (1891), who concluded that stomatopods are capable of stereoscopic vision. Parker (1891) reviewed crustacean eyes systematically, including the eye of the stomatopod Gonodactylus bredini (back then called G. chiragra), and was the first to identify the importance of the midband (see later). After a research lull in the first half of the 20th century came the discovery by Milne & Milne (1961) that stomatopods control each eye independently.
The eyes of stomatopods are found on stalks projecting from beneath the rostrum, the same set-up as in the leptostracans, euphausiaceans and mysidaceans. The stalks are exceptionally mobile, and noticeably move as the stomatopod tracks prey or any object of attention. They can see and focus on targets up to 1.5 m away (Caldwell & Dingle, 1975), and they are able to conduct both sweeping observances of the environment and specific targetting of individual objects (Land et al., 1990).
Stomatopod eyes are the most complex eyes in the animal kingdom, with 16 different photoreceptors, capable of seeing both UV and circular polarised light – they have long been thought the only animals that can perceive the latter (Chiou et al., 2008), but new research shows a scarab that can also pull it off (Brady & Cummings, 2010). They have full colour vision, and can distinguish targets by colour alone (Marshall et al., 1996). In the eye picture above, you see the darkened stripes/areas of the eye. Each of these regions is termed the midband and is a specialised area composed of four rows of ommatidia containing eight tiered photoreceptor types (so their vision is ochtochromatic, i.e. they see 8 colours). See the picture above (Marshall et al., 2007), which is a cross-section through a gonodactyloid eye; these are the first four rows.
In every individual ommatidial row, the individual photoreceptor has a very specific spectral band (i.e. range on the colour spectrum) that it responds to; with so many of these unique photoreceptors in the eye, the stomatopod can achieve unparalleled colour discrimination, no matter how the lighting conditions are (Osorio et al., 1997).
In addition, series of filters are found among the rhabdoms of rows 2 and 3 (termed intrarhabdomal filters; the black plugs in the picture above), further improving image quality (Marshall et al., 1991). While such filters are present in many arthropod eyes, stomatopodan ones are unique in that they modify the light itself by refraction as it goes through the photoreceptor (Hardie, 1988), instead of merely reflecting light in weird ways as in other arthropods.
These abilities are not constant for all stomatopods or even in a single species, but are physiologically-mediated and optimised depending on what depth the individual is living in (Cronin et al., 1994): the deeper the environment, the more the stomatopod’s photoreceptors will be adapted to shorter wavelengths (see picture above, Cronin & Caldwell, 2002), as they penetrate deeper. This ability, if not unique to the stomatopods, is very highly-developed in them.
The midband effectively splits the compound eye into two halves, meaning that each eye is capable of binocular vision.
In the bulk of the compound eye, referred to as the peripheral retina, every 8th cell contains UV-sensitive photoreceptors (peaking at 315, 330, 345 and 380nm; Marshall & Oberwinkler, 1999), and the other 7 contain similarly unique photoreceptor pigments specialised for mid-wavelength light (each of the seven has a different absorption spectrum).
As a general note: there are two cell types in arthropodan complex eyes. The ones mentioned in the above paragraph are the retinular cells (R-cells), which are the photoreceptive ones. In stomatopods (and most other arthropods), there are 8 R-cells/facet (Schonenberger, 1977). The other cell type is the crystalline cone (a.k.a. Semper cell), found distally on the ommatidia. In stomatopods, again like in most arthropods, there are four/facet (Schonenberger, 1977). That this 8+4 pattern is found so often is one of the main points supporting the complex eye as an ancestral trait of the arthropods (Melzer et al., 1997).
The remarkable abilities of the stomatopod eye can be traced down to the molecular level. In arthropod eyes, the same visual pigments are used for detecting polarised light and for colour vision. In some stomatopods though, there is divergence, with photoreceptors specialised for detecting polarised light only, and others only for detecting colour – we know this from opsin gene expression (Cronin et al., 2010).
Of course, in addition to the compound eyes, most crustaceans also have their frontal eyes (also called naupliar eyes). In stomatopods, there’s three of them: two dorsal and one ventral, and they are relatively reduced (no doubt due to the highly-advanced compound eyes).
Note that this whole time, I was only talking about the adult eye. The larval eye of stomatopods, while still relatively complex, is different from the adult eye, as can be seen in the picture above (Cronin & Jinks, 2001); larval eye on the left, adult on the right. The difference comes simply because the ways of life of a larva and an adult are very different.
Turning to ecology, stomatopods construct simple burrows (Myers, 1979) comparable to marine astacidean ones, and can memorise the layout, at least in the case of articifical burrows (Reaka, 1980). This is where they are easiest to collect as adults, by the way; larval and juvenile stages (the latter having a special name, the synzoea) are best collected along with plankton (e.g. Richardson et al., 2006). Once the larva settles and turns into the adult, it will not migrate – only larvae disperse, especially in the synzoea stage. Species depth tolerance ranges are unpredictable: some live in shallow water only, others in deeper water; some span the shallowest waters to the abyssal depths. But most species do not go deeper than 50 m, and species diversity is, as expected, highest in coral reefs. There are even some present near hydrothermal vents (e.g. Kelley et al., 2005).
Burrows of different individuals are often nearby (there can be 20 burrows/m²), leading to a lot of aggressiveness and competition; stomatopods can identify each other using chemicals (Caldwell, 1979). In the case of the laztter, they are only one of a handful of crustaceans known to be able to do so (an artefact of a lack of study), joining the hermit crabs (Hazlett, 1969) and shrimp (Johnson, 1977). Stomatopods take it one step further, by being able to remember each other, and especially the results of previous fights (Atema, 1995), and so a sort of dominance hierarchy can be thought to exist (although that hasn’t been shown).
Due to their excellent vision, stomatopods use colour signals to communicate (Chiao et al., 2000), as butterflies do. The threat displays described below are dependent on colour perception, as the colour is an honest signal of level of aggression. Of course, it is also the way males and females recognise each other.
Behaviourally, stomatopods are very antagonistic, both between themselves and towards other species. This is a rather tricky situation though – it’s fine when they’re grown animals, but considering that they often live in very close proximity to each other, the moulting phase can easily become a bloodbath. However, this possibility is eradicated by the fact that moulting is correlated (induced by?) lunar and/or tidal cycles and happens ~ every 2 months – you will not find a population where one freshly-moulted animal is alone, they all do it at the same time.
Nevertheless, at moulting time, stomatopods will bluff with threatening displays intended to put off the others from attacking it. Better safe than sorry. Whether these empty threats are “deceptive” (Adams & Caldwell, 1990) or “manipulative” (Krebs & Dawkins, 1984) is a matter of semantics.
The aggressiveness is solely to defend their burrows. The threat display is fairly stereotypical and is called the meral spread: while their body is in the burrow, they lean out and spread the raptorial appendages, revealing the meral spot, a small, colourful depression on the underside of the appendage. The other maxillipeds are then extended to the ground – the body is not seen, so the opponent doesn’t know the state of the exoskeleton. In aggressive displays, they will form a circle, its size being a direct correlate of body size – and the larger animal is the one that has the best chances of winning (Adams & Caldwell, 1990). In serious fights, an attack will follow after this display; in the moulting bluffs, it will not go beyond this point (unless the size difference is huge and the bluff is called). However, it must also be said that they are not above fleeing.
In sexual displays, after the meral spread, the maxillipeds will be whirled around; whether this is a dance-like display or merely a way to waft attractant chemicals around isn’t known. An interesting twist to the usual sex stories can be found in at least some stomatopods. In Pseudosquilia ciliata, the female does the courtship rituals, not the male – and the male is the one that picks and chooses which female to mate with, choosing the larger females. It’s thought that this is due to the ejaculate being nutrient-rich (Hatziolos & Caldwell, 1983).
During the fighting ritual itself, one individual assumes the telson curl position (diagrammed above; Taylor & Patek, 2010), in which the telson (the “tail”) is curled up. The aggressor will then strike blows on the curled-up telson, while the stomatopod will hit its aggressor from its rather uncomfortable position. This ritual is fascinating from a functional and mechanical point of view. As I stressed before, the raptorial appendage is immensely powerful, yet the stomatopods can bash each others’ telsons without breaking the telsons. This was the subject of a study by Taylor & Patek (2010), who found out that the telson of stomatopods is a natural equivalent to Kevlar: it has folds with stiff and elastic regions, allowing the force of each strike to be evenly dissipated.
As with all arthropods, with moulting comes an increase in body size; depending on the species, reaching maximal body size can take from 3 to 25 years. Each moult also leads to an increase in the number and size of eggs. That said, the size of the species determines brood size and frequency: small species reproduce infrequently, but produce many eggs; large species reproduce more frequently, but with less eggs each time (Reaka, 1979).
As trivia to end this post, here’s some random information: a stomatopod, Nannosquilla decemspinosa, is the only animal known to roll itself into a self-propelled wheel as an escape mechanism – it sommersaults backwards at a speed of 72 revolutions per minute, and 40% of the time, it is in a wheel position (Full et al., 1993); this is different from rolling into a ball and letting gravity do the work. Its escape is rather slow at 3.5 cm/s, but it has been observed as travelling more than 2 m this way.
As a small note of acknowledgement, I would like to thank one peer reviewer who read the first draft of this post and promptly told me to “dump all that crap about their systematics”, an action that effectively halved the size of the post. I am told this is more reader-friendly.
Click here for an update on sound in stomatopods!
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Richter, ., & Scholtz, . (2001). Phylogenetic analysis of the Malacostraca (Crustacea) Journal of Zoological Systematics and Evolutionary Research, 39 (3), 113-136 DOI: 10.1046/j.1439-0469.2001.00164.x
Patek, S. (2005). Extreme impact and cavitation forces of a biological hammer: strike forces of the peacock mantis shrimp Odontodactylus scyllarus Journal of Experimental Biology, 208 (19), 3655-3664 DOI: 10.1242/jeb.01831
Porter, M., Zhang, Y., Desai, S., Caldwell, R., & Cronin, T. (2010). Evolution of anatomical and physiological specialization in the compound eyes of stomatopod crustaceans Journal of Experimental Biology, 213 (20), 3473-3486 DOI: 10.1242/jeb.046508