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Summary | |
Gammarus species are amphipods with a yellow, transparent bauplan distinctively possessing two pairs of geniculate and multiarticulated antennae, and mandibular muscles(although not defined by these characteristics). Acting as an ecosystem provider, gammarus shrimps are able to regulate macroinvertebrate populations and leaf litter percentages on seabeds by espousing various feeding mechanisms.
Male adults employ mating dances before interlocking with females for sexual reproduction. An inverse relationship between mean brooding time and incubation temperature is observable, although gammarus young still undertake a juvenile form as adults do not produce larvae. The movement patterns of the Gammarus genus is unorthodox as it intersperses “side-swimming” whilst it lunges using its pleopods and uropods. After every molting event, a replacement cuticle of organic matrices acts as a placeholder until a new hardened exoskeleton is completed using glycoaminoglycans that produce chitin. Characteristic of an open circulatory system, the internal structure of Gammarus specimens constitutes a heart that runs from the posterior position of the cephalothorax to the pereon segment 7. The male and female reproductive systems differentiate in terms of organs incorporated but adopt similar plasticity respectively in terms of the vas deferens and oviduct.
The geographical distribution of gammarus shrimps circulates predominantly around European and North American bodies of water, with recounts in Australia regarded as rare. Phylogenetic modifications are renowned in relation to constructing the evolutionary relationships of amphipods, with recent changes introducing an entire new suborder taxonomised as Senticaudata. As previously mentioned, although Gammarus do have morphological traits that assist in identifying its classification, there are no true synapomorphies that define the genus. Although most categorised species are within a safe status, there are five out of approximately 200 species that have been listed on the IUCN Red List, thus promoting an urgent need to start conceptualising and implementing conservation strategies for gammarus shrimps before endangerment becomes too common.
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Physical Description | |
The two observed gammarus specimens were considerably tiny in relation to body size, being estimated as respectively 10mm and 7mm in total length. They shared a yellow, transparent bauplan that was visibly segmented approximately thirteen times. Metamerisation was also observable within gammarus shrimps across three metameres: cephalothorax, pereon and pleon. It should be noted that the internal digestive tract of the Gammarus shrimps was externally observable due to the transparency of its body plan.
The cephalothorax consisted of a broad, laterally compressed cephalic lobe with black compound eyes and a slitted anteroventral margin (Lowry & Myers, 2009). The geniculate antennules (they constitute a peduncular article 1 that is slightly larger than article 2 and a long, multiarticulated flagellum) are double the size of the secondary antennae potentially to enhance the sensory capabilities of the organism (Lowry & Myers, 2009; Boxshall & Jaume, 2013). The maxilla of the analysed specimens consisted of maxillipeds of varying sizes positioned on the ventral side of the cephalothorax directly in front of where the gnathopods should be (see “Phylogeny of Amphipoda”) (Lowry & Myers, 2009).
Gnathopods were either absent or indistinctive amongst the numerous peropods observed within the pereon (the thorax component) despite being a common feature amongst Gammarus shrimps (Lowry & Myers, 2009). The following adaptation indicates sexual dimorphism as female gnathopods are similar to other appendages (Gross et al., 2001). The coxae are enlarged within these individuals, specifically around coxae 2 and 3 (Lowry & Myers, 2009).
Curved around the epimera, the abdomen (or pleon) consists of multiple pleopods and three flexed uropods that extend during movement (see“Locomotion”) (Lowry & Myers, 2009). None of the uropods exhibited apical robust setae on neither their inner nor outer ramus (see “Phylogeny of Amphipoda”) (Lowry & Myers, 2009). The telson upholds a subtrapezoidal form with its apex truncated (Lowry &Myers, 2009). The overall pleon structure is used as an indicator to distinguish amphipods from “true shrimps” as they do not resemble the tail fan of the latter (Hibberd, 2009).
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Ecology | |
In terms of freshwater gammarus shrimps, they provide a significant service to their respective ecosystems by controlling both amphipod and macroinvertebrate populations whilst reducing the amount of leaf litter on seabeds. The versatility of feeding mechanisms (from herbivory to predation) adopted by gammarus shrimps allows these organisms to exploit a larger trophic food web; attributive of the strengthening of fitness levels associated with the Gammarus genus. Gammarus shrimps (mostly freshwater species) are capable of assuming various herbivorous diets that allow them to adjust to numerous ecological niches. A primary proportion of herbivorous gammarus shrimps are shredders, consuming detritus (in the form of leaf litter) and particulate organic matter (POM) (MacNeil, Dick & Elwood, 1997). The shredder mechanism also provides nutrition from microbial communities residing on said detritus (Bloor, 2011). It should be noted that there is a dietary preference for elm leaves within herbivory mode (Bloor, 2011). The multifunctional digestive system of Gammarus specimens allows for plant materials to be processed in the foregut and fungi sources in the hindgut (MacNeil, Dick & Elwood, 1997). Denoted as well is the fact that plants containing augmented levels of tannin and lignin prove to be more difficult for gammarus shrimps to digest (MacNeil, Dick & Elwood, 1997).
The predatory side of Gammarus shrimps is diverse as well,with accounts of methods such as cannibalism being recorded. The main approach of predation within this genre of amphipods however is intraguild predation (Dick & Platvoet, 1996). Members of the same guild as Gammarus shrimps are especially vulnerable straight after molting has occurred (targeting crustaceans) due to the replacement cuticle still being relatively soft (Dick & Platvoet, 1996). In Gammarus species such as Gammarus minus pinicollis, cannibalism is an uncommon but accepted form of feeding (MacNeil, Dick & Elwood, 1997). Cannibalism is speculative as being an agent of change pertaining to the natural selection of life histories; this is exhibited by the decline in susceptibility across juveniles of three estuarine species to be cannibalised: Gammarus lawrencianus, Gammarus tigrinus and Gammarus mucronatus (MacNeil, Dick & Elwood, 1997). Sex is an important determinant of cannibalism possibly happening, as females just entering a new instar phase are heavily predated on by male counterparts (Lewis, 2010).
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Life History and Behaviour |
Courtship and Reproduction | |
The courtship rituals of the male Gammarus shrimp are considered swift, persistent and forceful. Once physical contact is made with an attractive female, the male “pursues” her using a unique mating dance (Hendershot & Roles, 2014). If he successfully senses her using his secondary antennae, he then interlocks with her by lodging the dactylus of one gnathopod in the cephalothorax above pereon segment 1 and the dactylus of the other gnathopod in-between the fifth and sixth pereon segments (Sutcliffe, 2010).
Monogamous sexual reproduction is the only mode of reproduction in which Gammarus shrimps undertake. This usually occurs when the female has reached her eighth in star phase and is prepared to molt. Proceeding from the pairing phase, the male begins to thrust his pleon against her repeatedly until placing the female in precopulatory amplexus (Sutcliffe, 2010). The male copulates via two genital papillae protruding from the pereon and the female seizes nearby sperm into her brooding pouch using her pleopods, where they travel through opened oviducts to fertilise the eggs (See “Reproductive System”) (Rodgers-Gray et al., 2004). In some occasions, the female exerts her sexual choice over her mate by cyclically flexing and straightening her pleon at a hurried rate in anticipation of breaking the grasp of the male shrimp (Sutcliffe, 2010).
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Development | |
Brooding time within the brood pouch has been recorded as taking approximately 30 days at 6°C. An inverse relationship between mean brooding time and incubation temperature has been notated, as shown by Martin Sheader, who illustrated a decrease in mean brooding time to just 4.3 days at an incubation temperature of 21°C (1977). When young Gammarus shrimps are hatched,they have already adopted juvenile forms due to having no free-living larval stage (Kolding & Fenchel, 1979). Once the mother reaches her tenth instar stage, she releases her young into the environment, where they grow into the conditions through molting (Kokkotis,1998). The sexes of Gammarus shrimps are not completely observable until juveniles molt into their sixth instar phase (Othman& Pascoe, 2001). The eighth and ninth instar phases indicate there productive phase of the life cycle (see “Courtship and Reproduction”) (Kolding & Fenchel, 1979). The average lifespan of Gammarus shrimps lasts for around one year; during their livelihood (Kolding & Fenchel, 1979).
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Locomotion | |
The genus Gammarus is classified within the family Gammaridae, also known as “sideswimmers” or “scuds” due to the special technique they utilise intermittently throughout their movement. Using their pleopods and uropods, they propel themselves along strata (see “Physical Description”). However, during their quick motions, they simultaneously roll onto their lateral surfaces; an alternative method of swimming they utilise is swimming in circles ( Meadows, & Reid, 1966). The frequency and speed of locomotion is dependent on environmental conditions. For example, through experiments such as those conducted by De Lange, Peeters, and Lürling, it was demonstrated that great concentrations of stressors (e.g. contaminants) can influence an increase in locomotion (2009). This was congruent with the Stepwise Stress Model, which signified locomotion as a proxy for induced stress (De Lange, Peeters, and Lürling, 2009).
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Anatomy and Physiology |
Skeletal Structure | |
Gammarus shrimps develop an exoskeleton during early molting stages that is constantly molted and replaced by a new cuticle proper somatic growth. Following the molting process, the developing cuticle is fundamentally an organic matrix that lacks sufficient glycoaminoglycans (and inherently chitin), hence the cuticle being provisionally flexible (Gagné, Blaise,& Pellerin, 2005). The progress of hardening the exoskeleton with chitin is catalysed by proteins anthropodin and sclerotin (Gagné, Blaise,& Pellerin, 2005). The final stage of this process results in an endocuticle that produces chitin through attached glyaminoglycans and an exocuticle solidified by layers of glycoproteins and the produced chitin (Gagné, Blaise,& Pellerin, 2005).
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Circulatory System | |
The internal morphology of the genus Gammarus consists of an open circulatory system with a heart spanning from the intersection between the cephalothorax and the first thoracic segment to the seventh thoracic segment (Wirkner, Tögel, & Pass, 2013). From the heart, the aorta extends to the brain where it forms a perecebral ring (Wirkner, Tögel, & Pass, 2013). Additionally,capillary networks are observed all over the bauplan, particularly in the coxal plates where haemolymph currents are considered (Wirkner, Tögel, & Pass, 2013).
Although not detected within dissections operated specific to Gammarus specimens, there is an overall census that all gammarideans possess eight cardiac arteries that are paired up (Wirkner, Tögel, & Pass, 2013). It is noted that the first lateral cardiac artery pair is only located within Gammaridea amphipods, delivering blood and nutrients to the mandibular muscles (Wirkner, Tögel, & Pass, 2013).
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Reproductive System | |
Located in pereon segments 6 and 7 are the primary sites of the reproductive system for males (Sutcliffe, 2010). The gonads, signified by carotenoid pigments, sit above the midgut and below the heart (Sutcliffe, 2010). The testes are found within segment 6, where they indirectly form the vas deferens through the expansion of a seminal vesicle (Sutcliffe, 2010). During the period of copulation, stored spermatozoa from the testes are released at the ventral midline of segment 7 via genital papillae (see “Courtship and Reproduction”) (Sutcliffe, 2010).
In females, oviducts protrude at the fifth pereon segment, coiling the midgut and ventral caeca (Sutcliffe, 2010). The joint connecting the fifth pair of pereopods to the pereon is where the oviducts open ventrally to release ovaries (Sutcliffe, 2010). The oviducts optimally function immediately after molting occurs as the oviduct wall becomes temporarily flexible enough for oocytes to successfully pass through; oocytes retain tremendous size compared to the oviduct. (Sutcliffe, 2010). Oocytes are formed in singular strips from oogonia within the ovaries (Sutcliffe, 2010).
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Biogeographic Distribution | |
Despite being a genus encompassing species present globally on a spatial scale, Gammarus populations are mainly distributed amongst European waters, especially within the vicinity of Belgium (World Register of Marine Species, 2016). Gammarus are particularly sparse within the Exclusive Economic Zone of Australia, with minimal confirmed Australian records, thus making these sightings uncommon (World Register of Marine Species, 2016). However, due to the suitability of Gammarus populations to environmental conditions being species-specific, it is understandable that the discovered specimens could withstand the storage conditions of the artificial habitats in which they were located on. The sessile communities formed on the settlement plates that emulated a suitable habitat for these Gammarus individuals were extracted from Moreton Bay at Manly Harbour.
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Evolution and Systematics |
Phylogeny of Amphipoda | |
On a broader perspective, the phylogenetic construction of the order Amphipoda has recently undergone refinements based on new evolutionary findings depicting characteristics that represent relationships between suborders more accurately. The most significant change to the phylogeny of amphipods revolves the formation of the suborder Senticaudata in 2013, which amalgamates some families from the suborder Gammaridea into the outdated suborder Caprellidea (Lowry & Myers, 2013).The preceded theory on the phylogenetic tree of Amphipoda comprised of four suborders: Gammaridea, Caprellidea, Hyperiidea and Ingolfiellidea. Gammaridea encompassed the majority of amphipods as there were no defining features; it utilised symplesiomorphies to distinguish itself from the latter three suborders (Lowry & Myers, 2013). These symplesiomorphies included:
- An advanced abdomen when comparing to organismsunder Caprellidea (Lowry & Myers, 2013); and,
- A complex maxillipeds when comparing to organisms under Hyperiidea (Lowry & Myers, 2013);
However, because of this comparative flexibility in Gammaridea entitling it a significant proportion of amphipods underneath it, this proposed concept on amphipod phylogeny was not a true phylogenetic construction.Therefore further morphological and molecular investigations were undertaken to discover synapomorphies that could appropriately distinguish gammarideans from the other suborders.
Consequently, the current phylogenetic tree for amphipods was formed, implementing modifications instigated on the basis that earlier ancestors of amphipods were separable by the absence or presence of apical robust setae on the rami of uropods 1 and 2 (Lowry & Myers, 2013). From the development of this particular trait arose a drastic incline in amphipod diversity, hence the classing of organisms with this trait into the newly formed suborder Senticaudata (Lowry &Myers, 2013). This allowed Gammaridea to become further exclusive and identifiable. It should be noted though that despite this reconstruction, Gammaridea still encompasses approximately minimal known Amphipoda species, and thus perhaps additional studies into the phylogeny need to be conducted in order to characterise amphipods more thoroughly (Lowry & Myers, 2013).
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Phylogeny of Gammarus | |
Gammarus is a genus exemplary of how both evolutionary scenarios could have been viewed as hypothetically valid. Classified underneath the suborder Gammiridea both prior and after the phylogenetic reconstruction of amphipods, there were noticeable features of the observed Gammarus specimens that acted as potential symplesiomorphies. Under the revised phylogeny though, Gammarus is further evidenced as being within the suborder Gammaridea due to having no sightings of apical robust setae on the rami of uropods 1 and 2 (see "Physical Description").
Overall, the genus can simultaneously be described as a monophylum containing multiple subclades of taxonomically informal aggregates (Hou and Sket, 2015). With this stated, systematising morphologicallydiverse groups such as Gammarus shrimps result in two outcomes:
- polyphetic groups formed by symplesiomorphies (Hou and Sket, 2015); or,
- established aggregates overlapping morphologicallyand biogeographically due to being part of the same monophyletic clade.
Regarding the second outcome, the influence of certain aggregates are excised from the clade due to insignificant differences in cladistic morphology such as the mouth appendages of Fontogammarus compared to Gammarus balcanicus (Hou and Sket, 2015).
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Conservation and Threats | |
When analysing the overall Gammarus genus, there are 5 out of approximately 200 species that have been identified as being at least threatened under the IUCN Red List of Threatened Species (IUCN, 2015). The species most at risk is Noel’s Amphipod (Gammarus desperatus), which is identified currently as being “Critically Endangered”;formerly categorised as an extinct species in a 1996 IUCN assessment (IUCN, 2015). Only one subpopulation remains (at the Bitter Lake National Wildlife Refuge in New Mexico, USA) as a result of other known demes suffering extreme habitat loss from oil and gas industries inducing groundwater contamination (IUCN, 2015). This particular form of disturbance seems to also be the apparent cause of environmental concern for the other four aforementioned species, particularly the “Endangered” Illinois Cave Amphipod (Gammarus acheronytes) (IUCN, 2015).
Conservation initiatives targeting Gammarus populations have not been explicitly researched, let alone implemented. Although there have been studies deriving the implications of biological factors on Gammarus conservation, they only provided contextual significance rather than logistics. For example, a paper conducted by Gervasioet al. as well as Hogg, de Lafontaine, and Eadie investigated the genetic diversity exhibited by Gammarus pecos only to conclude that the extent of heterogeneity existent within the gene pool of the species deemed conservation an urgent matter instead of intrinsically addressing the protocols required to do so (2004). Due to Gammarus collectively including a wide spectrum of habitats as a genus despite their listed species possessing their own unique niche requirements, they are presumably overlooked in conservation plans as highly adaptable organisms.However, their reliance on specific environments theorise Gammarus as being ecologically unfit and thus there is a dire sense of urgency for more research to be conducted regarding the formulation and integration of Gammarus preservation strategies.
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References | |
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