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Summary |
An overview | |
Carukia barnesi is a small transparent box jellyfish that's recognised by the cuboidal shape of its bell. As a member of Order Carybdeia, C. barnesi possesses only four tentacles in total, with a single tentacle per pedalium. On its tentacles are bands of widely spaced nematocysts ending in a little tuft to the side, that identifies C. barnesi from the rest of the Carukiidae (Gershwin, 2005). It is usually found in slightly deeper waters around near shore reefs as an adult, and feeds on larval fish (Courtney et al., 2015). As a Cubozoan, it is a strong active swimmer, unlike other medusoid Cnidarians, and has highly developed eyes that allow it to navigate the reefs (Colin et al., 2013). The venom of the C. barnesi, while not considered lethal to humans, causes excruciating muscle cramps and nausea, and has been the subject of medical study for years (Barnes, 1964).
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Introduction | |
As early as 1922, beachgoers in Queensland occasionally suffered from painful symptoms, such as nausea and muscle contractions following a sting from an unknown organism. Efforts made to capture and identify the organism responsible were largely unsuccessful, until Dr John Barnes was appointed to the case. With his aid and expertise, and a contraption made with a rat trap and a flour sifter, amongst other things, the organism responsible was captured and identified in 1964 (Barnes, 1964).
This cubozoan would be later named Carukia barnesi, by R.V. Southcott, in honour of the doctor who identified it. Since then, numerous other studies regarding this organism have been published, and in this web-page, I will attempt to consolidate the information available in order to provide a detailed summary of the cubozoan.
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Physical Description | |
Carukia barnesi (C. barnesi) has a translucent cube shaped bell characteristic of the class Cubozoa, spanning at most 2cm in width and 2.5cm in length (Fig. 1.) (Southcott, 1967). Though translucent, the shape of the digestive system can be seen below the apex, as a slightly more opaque ‘T’ shaped structure. The stomach is located just under the apex, with an esophageal-like extension downwards called the manubrium, at the end of which is where the mouth is located. The bell is slightly narrowed at the apex end, though maintaining a more rounded dome that distinguishes it from other carybdeids, and has a broad, unbranched, pedalium present at each corner that sets carybdeids apart from the other Cubozoan order, Chirodropida (Gershwin, 2005). A single tentacle extends from every pedalia, and has been observed up to lengths of 50nm (Underwood and Seymour, 2007). Upon the tentacles are raised rings of Type 1 Nematocysts, also known as homotrichous basic euryteles, that give the tentacles a banded appearance. The bands of the C. barnesi are more widely spaced apart compared to the other species, and extend slightly horizontally outwards in a little tuft, often called ‘neckerchiefs’ following an apt comparison made to John Wayne’s western movies (Gershwin, 2005).
When viewed from the side, the bell of C. barnesi is far from smooth. Surface furrows run vertically downwards on the external surface of the bell, starting from a third down of the bell’s length. Additionally, the surface of the bell is dotted with pinkish wart-like structures known as ‘mammillations’. Mammillations are clusters of homotrichous haplonemes (Type 2 nematocysts), and number roughly between 66 to over 220 on a bell (Underwood and Seymour, 2007). The top of the bell and its adjacent areas tend to have higher densities of mamillations, as compared to the rest of the bell where the mammillations are more dispersed. (Southcott, 1967). Members of the genus Carukia are distinguishable from other carybdeids as they consistently possess either none, or only one mammillation in each octant of the velarium (Bentlage and Lewis, 2012).
Alternating with the tentacles, each lateral face of the C. barnesi possesses a rhopalium, that to the naked eye looks like a dark pigmented spot on the bell (Fig. 3).
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| Figure 1 |
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| Figure 2 |
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| Figure 3 |
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Ecology | |
Generally, medusae are found drifting in the open ocean, where a lack of complex environmental structures ensure that the bells of the medusae are undamaged. However, cubozoans tend to be found near shore, in complex habitats. This change in habitat is attributed to the sophisticated visual system of the cubozoans, elaborated on in Anatomy and Physiology, that allow it to detect obstacles, and their ability to swim actively and avoid obstacles (Nilsson et al., 2005).
While C. barnesi typically don’t occur near to beaches, a strong incoming tide and a sustained wind towards the coast might wash them closer to shore, where they come into contact with swimmers (Lyons, N.D.)
In a stark contrast to most medusoid Cnidarians that drift and wait for their prey to come into contact with their tentacles, Cubozoans are known for being active hunters that are able to target their preferred prey (Courtney et al., 2015).
When swimming prey come into contact with their drifting tentacles, the nematocysts in the tentacles will discharge, envenomating the prey and attaching it to the tentacle (Schlesinger et al., 2009). More details regarding the mechanism of the nematocysts will be covered in Anatomy and Physiology. While the exact components of the venom are unknown, a study done by Winkel et al. (2005) concluded that the venom contains a neural sodium transmitter that induces the releases of catecholamines in atrial tissue, leading to the tachycardia often observed in those that have been stung by C. barnesi. Following the death of prey, the tentacle contracts to transfer prey to the mouth for digestion (Regula et al., 2009). As the prey is brought up to the underside of the bell, the pedalia push the prey inside the bell, and the manubrium extends to engulf the prey (Matsumoto, 1995).
In general, not enough research has been done to state if C. barnesi plays any ecological roles, as their small and transparent nature and variable, patchy, distribution make it difficult to run studies on C. barnesi populations (Mooney and Kingsford, 2017).
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Life History and Behaviour |
Reproduction and Development | |
As a cnidarian, C. barnesi possesses a biphasic life cycle involving a larval stage, an asexually reproducing polyp stage, and a sexually reproducing medusoid stage (Werner et al., 1971). However, there are some differences that set aside the cubozoans, and by extension, C. barnesi from the rest of the cnidarians.
While the fertilisation methods of C. barnesi have never been recorded, cubozoan eggs are internally fertilised and develop in the gastral cavity of the female (Courtney et al., 2016). A fertilisation membrane is developed after 2 hours of fertilisation, and blastulation occurs within 30 to 48 hours of fertilisation. After development in the egg capsule, the planula larvae of the C. barnesi enter a dormant phase for a period that can last between 6 days to 6 months (Courtney et al., 2016). It is currently unknown as to why the larvae enter a dormant phase.
Post hatching, the planula larvae sink to the benthos and move along the substrate to seek out a suitable site for settlement. Other cubozoan larvae possess nematocysts for the purpose of attaching to the substrate (Hartwick, 1991), but C. barnesi are unique in that regard, as they do not use their nematocysts for attachment, and the function of the larval nematocysts at the end of their tentacles remain unknown (Courtney et al, 2016 [1]).
Once the larva has settled, it undergoes metamorphosis to form the primary polyp. Metamorphosis to a primary polyp begins with the formation of a stalk and one tentacle, followed by subsequent tentacles (Courtney et al., 2016 [1]). The primary polyp undergoes asexual reproduction to form secondary polyps after about 28 days, after at least 4 tentacles have been formed. At this point, the primary polyp consists of a base that’s attached to the substrate, a stalk that connects the base to the calyx, which is a cup-shaped structure with a mouth encircled by tentacles. A ciliated swimming polyp forms on the side of the calyx, with two tentacles and one stalk, detaches within 4 days and disperses to settle elsewhere (Courtney et al., 2016 [1]).
While the medusoid form of the C. barnesi is typically found in deeper waters around reefs, the polyp stage may occur instead in estuarine areas, as their proliferation rates are higher in lower salinities (Courtney et al., 2016 [2]). Unlike other cnidarian polyps that undergo transverse fission to give rise to multiple medusae (Fautin, 2002), each cubozoan polyp will develop into only one medusoid individual (Werner et al., 1971). During development, the stalk shortens, and the calyx extends and widens to form the beginnings of a bell. The tentacles will then shift towards four points and merge into one tentacle at each corner, giving rise to the characteristic form of a cubozoan. Following the fusing of the tentacles, the rhopalia, manubrium, mammillations and pedalia form. During the formation of the rhopalia, small crystal structures enveloped in a statocyst membrane below the eyes fuse to form a single statolith (Tiemann et al., 2005). The currently upside-down C. barnesi begins to pulse, and detach when they are fully formed (Courtney et al., 2016 [1]).
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| Figure 4 |
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Behaviour | |
Primary polyps creep around the benthos, and may slowly swim around, but generally remain stationary. Polyps do not feed until the third tentacle is formed, where they will start to capture and consume rotifers and other planktonic invertebrates. The secondary polyps, in contrast, are slightly better swimmers than the primary polyps. (Courtney et al., 2016 [1]).
Juvenile medusae of the C. barnesi feed preferentially on marine invertebrates (Courtney et al., 2015). As they mature and increase in size, their preference shifts to larval fish, as they provide more proteins and nutrients.
Cubozoans have been observed to have two markedly different states of diurnal activity, possibly as a result of the nature of larval fish, their preferred prey (Seymour et al., 2004). Larval fish are visually attracted to the clusters of Type I nematocysts on the tentacles, and are lured by C. barnesi twitching the tentacles in a mimicry of prey organisms. (Courtney et al., 2015). As this lure is ineffective at low light levels, cubozoans mostly hunt in the day, near the water surface where the larval fish concentration is the highest. At night, cubozoans have been observed to ‘lie’ dormant near the benthos, possibly as an energy conserving measure (Seymour et al., 2004).
Additionally, the frequency of their luring twitches have been shown to increase with the size of their bells, to reflect the ontogenetic shift in prey from invertebrate prey to larval fishes as the medusae mature (Courtney et al., 2015). As the venom profiles of C. barnesi are loosely specific to the type of prey that they target, the maturation from juvenile to adult is accompanied by a change in the venom profiles (Underwood and Seymour, 2007).
Here is a video demonstrating how luring and feeding occur in C. barnesi.
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Anatomy and Physiology |
Nematocysts | |
A nematocyst is divided into three main components: a capsule, a tubule, and a shaft. The capsule contains the shaft and a tightly coiled tubule. The nematocyst generally requires a mechanical and a chemical signal to trigger, and when both are present, the tubule extends outwards rapidly, allowing the shaft to penetrate the outer epithelium of the prey. While other cubozoans tend to have different nematocyst types during their polyp and medusa forms, C. barnesi exhibit no change, and are unique in that regard (Courtney et al., 2016 [1]).
Type 1 nematocysts, or homotrichous basic euryteles, are found on the tentacles as described in Physical Appearance. The shaft structure of a C. barnesi nematocyst is a eurytele; the tip and the base are roughly the same diameter, and the central portion is expanded (Southcott, 1967). However, the shaft structure is still debated, with some choosing to use ‘tumitele’ as an alternative classification C. barnesi nematocysts. As it is homotrichous (also known as holotrichous), spines of the same type cover the entire length of the tubule (Gershwin, 2006).
Type 2 nematocysts, or homotrichous haplonemes, form the mammillations present on the bell of C. barnesi. Haplonemes are a group of nematocysts that lack a well-defined shaft (Gershwin, 2006).
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Digestive System | |
The mouth of C. barnesi is shaped like a four-petaled flower (Fig. 5), and is capable of expanding to engulf its prey when the prey is brought closer by contracting tentacles (Southcott, 1967). The mouth is connected to the stomach by the manubrium, a tube that is capable of moving around to bring the mouth closer to the prey. The stomach possesses short furrows, internal rugae, that are thought to serve a digestive function. However, unlike other carybdeids, the stomach of the C. barnesi lacks gastric phacellae and cirri, structures that in other cnidarians kill and paralyse prey (Dawson, 2003).
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| Figure 5 |
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Reproductive System | |
The gonads of the C. barnesi are located on the inner walls of the bell, encompassing most of the length, and extending into the gastrovascular cavity of the bell (Southcott, 1967). Gametes are released into the gastrovascular cavity, where fertilisation occurs in females. Currently, it is not known what environmental cues trigger reproduction in C. barnesi (Courtney et al., 2016 [1]).
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Sensory Structures and the Nervous System | |
The following segments cover the sensory structures and the nervous system of cubozoans in general, as specific studies regarding these systems in C. barnesi have not been conducted. However, the functioning of these systems are highly unlikely to differ between the different cubozoan species.
Rhopalia
A rhopalium is a club shaped cavity that hangs from a stalk and opens on the external wall of the bell, arranged in an alternating fashion with the tentacles and totalling 4 in number. The rhopalia contain the eyes and the statocysts, and are vital to coordinating swimming responses and directional changes in response to visual input (Parkefelt et al., 2005).
The cubozoans are known for their well developed eyes, contained in the rhopalium. A cubozoan has four types of eyes, and a total of 24 eyes (Nilsson et al., 2005). Each rhopalium has a large complex eye, a small complex eye, a pair of pit eyes, and a pair of slit eyes (Fig. 6) (Coates, 2003). The small complex eye and the pit eyes are outwards and upwards facing, while the large complex eye and the slit eyes are directed towards the center of the bell, and slightly downwards (Parkefelt et al., 2005). Ciliated photoreceptors are present in all of the eye types, and are structured as such: A cluster of microtubules, where the outermost segment receives external information, that is then screened through pigment, and then transmitted through the nuclear layer to the nervous system (Yamasu and Yoshida, 1976).
The complex eyes found in cubozoans have been compared to vertebrate eyes, as they share a similar structure, with a cornea, cellular lens, a possible vitreous space, a retina, and pigmented cells. These eye types are also called camera-type eyes, and are thought to function in a similar fashion to vertebrate eyes (Conant, 1898, as cited in Parkefelt et al., 2005). The two complex eyes are mostly differentiated by size, and contain roughly the same types of cells with variations in cell numbers (Nilsson et al., 2005). The cellular lens contains a crystallin protein that is unique to the cubozoa, and is thought to have evolved independently (Nilsson et al., 2005). These complex eyes are theoretically capable of forming blurred images, but cubozoans lack the brain to process images, so it is possible that these eyes are used to detect the ‘shadows’ formed by organisms or structures in their visual environment (Nilsson et al., 2005).
The pit eyes and the slit eyes are the two forms of the paired ocelli that are formed by cavities in the epithelium that are lined with pigments (Ng, 1974, as cited in Coates, 2003), as well as ciliary photoreceptors. The cavities are coated with a refracting secretion that is hypothesised to be a form of protection for the photoreceptors (Yamasu and Yoshida, 1976).
Statoliths are hard structures found in the rhopalia of cubozoans that help them to orientate themselves (Mooney and Kingsford, 2017). A statolith is a small round structure made out of calcium sulfate that is laid down by the organism on a daily basis, forming growth rings that can be used to approximate the organism’s age (Gordon et al., 2004). The statolith is contained in a statocyst membrane, and the movement of the statolith is picked up by the ciliated sensory cells located at the ends of the statocyst. (Sotje et al., 2011). The statolith is also said to function as weight so that the eyes are always facing the same direction regardless of orientation (Coates, 2005).
Nervous System
All of the information collected by the eyes and the statoliths are relayed by the nervous system and used to coordinate directional swimming. The nervous system comprises of a nerve ring near the bell margin, and a nerve net (Satterlie, 2002). Neurons extending from the ring into the stalk connect the rhopalium to the rest of the system and the other rhopalia, though neural processing occurs in the individual rhopalium (Laska and Hundgen, 1984, as cited in Parkefelt et al., 2005). Within the rhopalia, neurons are arranged in bilateral groups associated with the eyes (Fig. 7) (Parkerfelt et al., 2005).
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| Figure 6 |
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| Figure 7 |
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Musculatory System | |
Cubozoans possess two main types of muscles: longitudinal and circular muscles. Circular striated muscles run alongside the entirety of the subumbrella surface (Satterlie et al., 2005). These muscles contract to generate thrust that propels the cubozoan forward, allowing them to swim. Additionally, the force of the thrust is enhanced by the highly flexible nature of their velariums that incorporate water from outside the bell, allowing them to maintain a forward momentum even when relaxing (Colin et al., 2013). Strips of smooth muscles run longitudinally from above the rhopalia to the manubrium, with short extensions that branch out perpendicularly.
The muscles in the pedalia and the tentacles are longitudinal, and located on the oral side of these structures. These muscles aid in the contraction of tentacles to reduce drag when cubozoans are actively swimming, and to bring food to their mouths (Matsumoto, 1995).
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Biogeographic Distribution | |
C. barnesi is native and endemic to Australia, and is found along the Northern coastline, ranging from Broome to Rockhampton (Tibballs et al., 2012), indicating that the species has a preference for tropical zones. They are usually found between the depths of 10m-20m around mainland, near shore, and mid-shelf reefs. Reefs fringing islands had higher abundances of C. barnesi when compared to reefs that were fully submerged (Kingsford et al., 2012). While the study done by Kingsford et al. (2012) had no explanation for why C. barnesi were not found in offshore reefs, the absence could possibly be a reflection of the lower salinity preferences of C. barnesi polyps, as mentioned in Life History and Behaviour. The lack of estuarine habitats, combined with the nature of Carukia populations to be highly localised (Pitt and Kingsford, 2000), could explain the absence of C. barnesi from fully submerged reefs and offshore reefs.
Embedded below is an interactive map from eAtlas that shows the locations of sting events in Queensland, built from a public database made by compiling sting events and studies. Please note that sting events in this database include stings from species other than C. barnesi.
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| Figure 8 |
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Evolution and Systematics | |
Classification table of C. barnesi (Southcott 1967)
Phylum: Cnidaria
Class: Cubozoa
Order: Carybdeida
Family: Carukiidae
Genus: Carukia
Cubozoans were, up until the 1970s, regarded as a part of the Scyphozoa (Gershwin, 2005), until a revision by Werner (1973) set them aside as a class based on unique features present in their polyps (See Life History and Behaviour). This separation was later further supported by protein analyses of cubozoan venom, and no protein homologies could be found outside of Cubozoa, suggesting that the Cubozoans had evolved a novel family of proteins (Brinkman and Burnell, 2009). However, there has been some debate regarding the phylogenetic position of Cubozoa in relation to Scyphozoa and Hydrozoa (Gershwin, 2005). The more widely accepted hypothesis is that the scyphozoans and the cubozoans form one clade, the Acraspeda (Collins et al., 2006), with the cubozoans having been derived from a scyphozoan ancestor.
When Sotje et al. (2011) compared the rhopalia of a cubozoan and a scyphozoan, they found several homologous characteristics that strongly supported the hypothesis of a scyphozoan ancestor. The statoliths of both classes both shared similar origin areas, and were formed from the same compound: bassanite. In contrast, hydrozoan statoliths were mostly made of calcium manganese phosphate (Sotje et al., 2011).
Within the Cubozoa, the Carybdeids have been grouped according to the solitary tentacle arising from a pedalium, and the Carukiidae created to resolve issues of paraphyly within the order (Bentlage et al., 2009). It has also been suggested that the last common ancestor of the carybdeids possessed an underlying mechanism of the Irukandji syndrome, which led to the similar effects, yet differing levels of toxicity of carybdeids. According to Bentlage and Lewis (2012), the Carukiidae are the second family to diverge from the last common carybdeid ancestor, set apart by rhopaliar niche ostium with rhopaliar horns, and their lack of gastric cirri (Fig. 9).
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| Figure 9 |
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Conservation and Threats | |
Little is known of the global C. barnesi population, due to its demure size and the lack of information regarding the environmental cues for reproduction (Courtney et al., 2016 [1]). As such, C. barnesi currently has not been evaluated and assigned any conservation status by the IUCN Red List. However, a study done by Boco et al. (2019) has shown that C. barnesi polyps are not strongly affected by projected climate scenarios, and will survive the most extreme scenarios, though with negatively impacted mobility. It is also possible that with warming temperatures, the range of C. barnesi may increase southwards.
The decline in numbers of one of their only natural predators, sea turtles, may also lead to increased population sizes of the C. barnesi, though no formal study has been made regarding that hypothesis.
In brief summary, the numbers of C. barnesi are highly unlikely to be in danger of decline or extinction in the near future.
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References | |
Avila-Soria, G. (2009). Molecular characterisation of Carukia barnesi and Malo kingi, Cnidaria; Cubozoa; Carybdeidae. PhD Thesis, James Cook University.
Barnes, J.H. (1964). Cause and Effect in Irukandji Stingings. The Medical Journal of Australia. 1(24), 897-204.
Bentlage, B., and Lewis, C. (2012). An illustrated key and synopsis of the families and genera of carybdeid box jellyfishes (Cnidaria: Cubozoa: Carybdeida), with emphasis on the "Irukandji family" (Carukiidae). Journal of Natural History. 46(41-42), 2595-2620.
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Websites included:
eAtlas, available at: "https://eatlas.org.au/home"
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