The genus Quadrimaera is a member the highly variable order Crustacea, and falls within the class Amphipoda. It is found all over the world, in tropical to subtropical marine environments, and generally found along the coastlines of countries in these waters. There are two possible species of Quadrimaera that the specimens collected from South East Queensland could belong to: Q. serrata, or Q. quadrimana. Due to the morphologically complex nature of amphipods, with species varying ever so slightly from one another, the specimen collected will be described as being either one of these species. The major defining features of amphipods, include their antennae, the second gnathopod, and their urosome. Small structural differences in these body parts are important indicators, as to which species they belong. However, with the lack of specimens, it was difficult to state their species with confidence.

The second gnathopod for both species, is a sexually dimorphic trait, and is highly variable among individuals and life stages. The differences of this gnathopod between sexes, indicates that sexual selection has occurred over time, with it being advantageous for the male gnathopods to be larger. Possible explanations for this difference include: the representation of strength and good genes, and it being advantageous during mating. Their antennae are an important sense organ, not only to find food, but also assisting in the mating process, which can only occur after the females last molting stage. These two features are important for the life history of these species, with antennae sensing out pheromones that indicate when a female is due to molt, while the gnathopods are used during amplexus.

They are often found within coral rubble and macroalgal communities, grazing on these macroalgal particles to gain nutrients. Due to their close proximity to shorelines, and their sensitivity to pollution, they are under threat of sewage runoff created by large storms which will become more frequent in the future. They also face major threats from sea temperature rise, which will largely affect their reproduction and life history stages.

The two specimens examined averaged 3mm in length, with a laterally compressed, convex, and inflated body shape. The gelatinous and translucent body clearly shows the circulatory, and possibly the digestive system along the pereon, and around the head (Figure 1). 

The Head

The head bares two antennae; the first possessing an accessory flagellum that is half the length of the primary flagellum, and the second antenna being shorter than the first. Setae runs along the length of both antennas (Figure 1 & 2). An ovate compound eye is also present on the head. A rostrum is absent, or greatly reduced in this specimen. The typical amphipod mouthparts including: one pair of mandibles, two pairs of maxillae, and maxillipeds, are located on the underside of the head (Chapman 2007).

The Pereon

The pereon is structurally equivalent to the thorax in other arthropod species, and in this case, bares seven pairs of walking legs, the first two of which, are known as gnathoopods (Figure 3). The propodus of the first gnathopod is significantly reduced in comparison to the second gnathopod. The second gnathopod is significantly enlarged, subchelate shaped (dactyl closes against palm at < 90° angle), and identical on both left, and right sides of the specimen. The palm of the propodus is acute, sculptured, lined with various setae, and has a posteroventral spine (Figure 4). The gnathopod of these species are highly variable between sexes and life stages, often making it an unreliable defining feature unless life stage is known (Berents, 2006). The peropods are heteropodus, with peropod 3-4 directed posteriorly, and 5-7 directed anteriorly. This is evident when the body is curled, showing peropods 5 and 6 folded back over the pleon, and peropod 7 folded over the urosome (Figure 1).

 The Pleon

Another major defining characteristic of these species are their urosomes, which are the only dorsoventrally flattened part of the body. The first uropod is peduncle, without long setae, but with one or two robust setae (Figure 5). The telson of this specimen exhibits a cleft (split into two lobes), and laminar (dorsoventrally flattened) morphology (Chapman 2007).

Table 1: A comparison of the defining characteristics of the two species Q. serrata and Q. quadrimana to show how similar they are.

Habitat

These species are known to inhabit similar areas, livingamongst coral rubble, algae, reef rock, and in biofouling communities (Hughes,2015). They inhabit the littoral zone, living a fully motile life, from depthsof 3-10 meters (Berents 1983, Stoddart 2003). These specific specimens werecollected on plates from a biofouling community near Manly Wharf, Queensland.Their common association with these surfaces, gives them protection frompredators, and indicates important resources, such as nutrients, are also foundin these areas. When collecting the specimens from plates, they would oftenscurry to the underside of the plate when it was flipped over. This behaviourgives the impression they seek shelter on the underside of substrates whendanger is present, therefore, making sense that they inhabit coral rubble, asit is very complex with many crevices to hide in.

Feeding

As there is no information regarding their feeding habit in the literature, assumptions are made based on their morphological characteristics and habitat. Due to their association with coral rubble and algae, it can be expected they are grazers, or detritivores. It is highly unlikely that they are filter feeders, as they do not possess any of the structures common to amphipods of that feeding mode. These include, combe like structures present on thoracic limbs, as seen on Corophium volutator (Figure 6),a filter feeding Gammaridae (Riisgård 2015). Filter feeding, also implies sedentary lifestyles, which, as discussed in Locomotion, these species do not exhibit.

Generally the maxillae, and maxillipeds are used forhandling food, and the mandibles for tearing, cutting, and chewing (Glazier,2009). In these species, the mandible palp and setae are well developed,possibly functioning to prevent uneaten food particles from escaping the molars(Watling, 1993). Mandible groups have been identified by Watling (1993) in areview of literature, created groups based on changes in mandible structure,indicating it reflects feeding behaviour of the families within those groups.According to this work, Quadrimaerafalls into Group 1, which is the most ancestral mandible form, withmodifications to this plan being rare, and if so, minor among species. Beingplaced in this group, indicates that these species exhibit a herbivorouslifestyle, being generalist macrophage feeders of organic matter in the sediment,or grazing on macroalgal epiphytes. Due to the environment these specimens werecollected from, their feeding style could be compared to a similar species fromthe same family, M. nitida, which hasbeen found to feed on both, macro- and microphagous particles (Macneil et.al.1997). It seems highly possible that this specimen will follow the samebehaviours.

Locomotion

Like most amphipods, the majority of their movement created to swim is generated by the pleopods, with some assistance from the more rigid uropods (Ruppert et.al. 2004). Swimming, is generally initiated by a backwards thrusting motion of the abdomen, with the uropods assisting, by pushing against the substratum. Slow walking across the substrate, is generated from the pleopods, however, when faster movement is required (i.e. for evading predators), the pereopods are also used, along with a leaning motion towards one side (Ruppert et.al. 2004).

Reproduction

Understanding of their reproduction is based on a related species from the same family, as the structures are similar. However, generally reproduction does not change dramatically throughout amphipod species.

Mating in all amphipod species, is triggered by the final molting stage in females. When the females are premolt, males evaluate the fitness of a female and sense a range of characteristics, such as the females reproductive quality, and the necessary time of investment needed for successful reproduction (Beermann et al. 2015). A precopulatory behaviour that has been observed in many other amphipod species, is mate guarding, which occurs close to the molting stage, and involves the male carrying the female, until her molting stage. This sexual selection strategy, sees the male investing his time and resources into guarding the female, to ensures his fatherhood, and control over her breeding (Barki, 2008). Their movements towards each other for this precopulatory behaviour, is triggered by a release of water borne attractants, that is secreted by both sexes, and picked up using the chemosensory structures around their antennae (See Sensory Systems). External fertilisation, is exhibited immediately after molting is complete, as the cuticle is sufficiently flexible enough, to permit eggs to be released through the genital pore of the brood pouch (Conlan, 1991). This behaviour, is possibly seen in the Quadrimaera species, as it is a common strategy exhibited in many other amphipod species.

Development

Eggs are fertilised, and will hatch within the mothers brood pouch. There are two stages of development that juvenile amphipods undertake with the mothers brood pouch; the embryonic period from ovulation to hatching, and the juvenile period, from hatching to emergence from the brood pouch (Borowski, 1980). As they are direct developers, they do not exhibit a larval stage within their lifecycle. The development of a closely related Melita plumulosa has been observed by Mann & Hyne (2008), and steady embryonic growth is exhibited, taking 8 days to reach free swimming juvenile (Figure 7), and it is believed that Quadrimaera species would follow a similar developmental path. The juveniles emerge from the brood pouch as miniature adults, and are lacking a metamorphosis stage in their life-cycle (Borowski, 1980).

In a large-scale review of Gammaridae reproduction, and life histories by Sainte-Marie (1991), it was found, that a range of environmental factors could influence life history traits. On average, females of species inhabiting warm climates, had smaller brood sizes, with latitude having a large influence on this characteristic. As explained in the Biogeographic Distribution section, these Quadrimaera species inhabit mid to low latitudes, meaning, they live in temperate and tropical areas. This, according to the review, implies that their reproductive potential is high, as they exhibit a large number of broods, with females having a shorter lifespan. However, they show an average smaller body size, heightened maturity, and have fewer embryos per brood cycles (Sainte-Marie 1991). If this review is correct in its analysis, these traits should be exhibited in the Quadrimaera species, yet there are still family dependent variations with these generalised life history traits (Sainte-Marie 1991).

Dispersal is limited in amphipods, due to their lack of larval stage in their life cycle. Therefore, other mechanisms for dispersal have been theorised, such as rafting (Barnard 1970). This theory suggests that amphipods disperse to other areas, by means of detached marine plants, and driftwood. This seems the only plausible method for long term dispersal in these species of Quadrimaera. 

Extra Research - The Second Gnathopod

Both of the possible species exhibit differences in claw shape, and symmetry between male and females, known as a sexually dimorphic trait. This difference, is generally agreed to be an observable result of sexual selection. There is larger variation in the reproductive successes of males, causing the gnathopods of these species to evolve over time (Barki, 2008). Males of both species, exhibit more depressions in their palm than the females (Myers 1985). The reason for this evolution is unknown, and there are a range of functions that could cause this dimorphic trait, such as; attracting females, manipulation during copulation, or it is beneficial during competitive aggressive displays, and post copulatory mate guarding. The second gnathopod of females in both species, is much smaller than that of the males (Myers 1985), indicating size being an important factor in sexual selection, either to attract females, or in competitive interactions with other males. In this section, possible reasons for this gnathopod difference will be discussed, and determined which of these is more plausible. 

A common mating behaviour in many gammaridian amphipod species, is precopulatory carrying, which involves the male carrying the female before she molts (Borowsky, 1984). This is a form of mate guarding, where the male monopolises the female before mating, to ensure his reproductive success (Hyne 2011). It is unlikely, that this is the role of the 2^nd gnathopod, as it has been observed in similar gammaridian amphipod species by Borowsky (1984), that the smaller, first gnathopod, is used for attachment during this precopulatory behaviour, therefore it would not make sense to evolve a possible stronger claw on the second gnathopod.

Another possibility, is the size difference being a observable characteristic of male strength and fitness, with larger claw sizes for stronger males. As the female claw is much smaller, it would seem the males have evolved larger claws as it is an attractive trait to females. It has been found that in a similar Gammaridae species, G. pulex, the second gnathopod functions only in copulation, and is impossible without them. (Hume et.al. 2005). In copulation, they are used for seizing and manipulating females, however, not being used in precopulatory behaviour. Males with their gnathopods cut off in the Hume et.al. experimental study, chose females that were further from their molting stage, and therefore less fit than the females chosen by males with their second gnathopod. This indicates that when the males were lacking their gnathopod, they knew they would not be able to attract the fitter females, so chose the less fit females (further away from molting stage). Therefore, it seems the size of gnathopods, is an important characteristic regarding mate choice in males.

It is unlikely females find this difference in claw size attractive, due to its representation of strength, as across literature, descriptions of males in both species show the claw to be of similar size, and structure. It is hypothesised, that a larger claw size in males of the Q. serrata and Q. quadrimana, gives them the ability to control females during copulation, reducing their chances of escape. In many species of animals, females have the ability to choose their mates, as is the same in amphipods. Therefore, based on readings of other species, this sexually dimorphic trait has arisen in Quadrimaera species, as it gives males an advantage to hold on to their mate, even if she tries to escape copulation.

Skeletal

Unlike other species of Crustaceans, the carapace is absent, instead, overlapping coxal plates are present, covering the coxae of the pereopods, which is functionally equivalent to the carapace in other Crustaceans. This protected space ventral to the thorax, houses the gills, and in females, also the brood pouch, called the marsupium (Ruppert et.al. 2004).

As discussed in the Phylogeny section, these species are part of the Phylum Arthropoda, therefore, undergo molting at several stages throughout their life cycle, for growth (Ruppert et.al. 2004). This involves shedding their exoskeleton, and is an important part of their reproduction (See Reproduction and Development).

Circulatory

The circulatory system of amphipods, is in the form of a haemolymph vascular system, with thoracically located gills. This comprises a haemocoel, a large space extending throughout the body, containing the muscles, bathing them in blood, and also functions in transporting nutrients, and wastes (Ruppert et.al. 2004). A simple Gammaridae body plan, is used to theorise the structure of the Quadrimaera species haemolymph vascular system, as they have similar body plans, and Gammaridae is believed to be the ground pattern of all amphipods (Wirkner & Ritcher, 2007). The system comprises of a tubular heart, running from the border of the cephalothorax, to the end of the seventh thoracic segment (Figure 8). Blood flow is provided to the brain through the aorta, forming a pericerebral ring around the brain. Extensive capillary networks were found in the coxal plates, with a current haemolymph flow through these structures. Taking this into consideration, and the thinness of the cuticle around that region, gives evidence that coxal plates could act as an accessory respiratory structure, along with the thoracically located gills (Wirkner & Ritcher, 2007).

Sensory System

The sensory structures of these species, include the two paris antennae, and the pair of compound eyes located at the head. The compound detects light through light-receiving cells, called ommatidia (Ruppert et.al. 2004). A single ommatidia contains it’s own focusing system, and a light transmitting system. These are very complex systems, enabling amphipods to obtain a mosaic like image. This compound eye, poorly determines distances (20cm or less), but is very effective at detecting motion (Ruppert et.al. 2004).

 Amphipods have chemosensory cells, which attach to specific sensillae (hair like structures called aesthethascs), on the distal segments of the first antennae (Beermann et al. 2015). These structures, are essential for determining the location of food, potential mates, and predators. It is especially important in mating, as they need to sense the pheromones released from the females at premolt. 

Mechanoreceptors are also present in the form of hollow setae, with sensory neurons inside, detecting vibrations in the water. To assist with balance, a statocyst may also be present in this species, however very little information is available in the literature supporting this (Ruppert et. al. 2004).

Records from literature show that they inhabit warm, shallow, tropical waters, around the world. Specifically, they have been found in Australia, Hawaii, Micronesia and Polynesia, South Africa, The Red Sea, and Madagascar (Apaddoo et.al. 2002, Krapp-Schickel, 2009). The map below shows their distribution around Australia, and surrounding areas. Their rafting mode of dispersal, using currents, has allowed them to inhabit other areas available to them, with temperature seeming to be the main characteristic they seek in an environment. As is shown in Figure 9, their distributions have a tendency to overlap each other, indicating that they inhabit similar areas. Due to this environmental overlap, they have not needed to evolve different structures, and have similar environmental pressures. This could be the reason why they are morphologically similar, and therefore, difficult to distinguish from each other. They have also been found in a range of habitats, including lagoons, around boat jetties, and on coral reefs (Hughes, 2015).

In the past, there has been some confusion regarding the classification of Quadrimaera to the family, and genus level. Originally, species were classified as being part of the genus Maera (Berents 1983). Krapp-Schickel & Ruffo in 2000, established the genus Quadrimaera, and some of the Maera species, including what is now called Q. quadrimana, were placed into this genus (Lowry & Springthorpe, 2005). Before 2002, Quadrimaera species were placed in the family Melitidae (Apadoo et.al, 2002). This was then abandoned, and Krapp-Schickel (2008), formalised the family Maeridae, placing 26 genus from the family Melitidae, including Quadrimaera into the new family (Hughes, 2015). The struggle of placing this genus into a family, is possibly due to the high variability of the species at different life stages, and the genus Maera being negatively classified (Krapp-Schickel et.al, 2009).

Many recent papers still find it difficult to classify them into the correct order. In differing papers, they are part of Gammaridae, and in others, they are part of the Senticaudata. For the purpose of this species description, I have used the most recent classification of them, placing them into the Senticaudata order, with their systematic as follows:

Kingdom: Animalia

Phylum: Arthropoda

Class: Crustacean

Order: Amphipoda

Suborder: Senticordata

Infraorder: Hadziida

Parvorder: Hadziidira

Superfamily: Hadzioidae

Family: Maeridae

Genus: Quadrimaera

Species: serrata

Kingdom: Animalia

Phylum: Arthropoda

Class: Crustacean

Order: Amphipoda

Suborder: Senticordata

Infraorder: Hadziida

Parvorder: Hadziidira

Superfamily: Hadzioidae

Family: Maeridae

Genus: Quadrimaera

Species: quadrmana

There are a range of current and future threats to these amphipod species. Due to their sensitivity to contaminants, locality, and closeness to coastlines, runoff by storms, poses a major threat to their reproductive system (Hyne, 2011). Runoff of sewage pollution reduces the abundance, and species richness of amphipods closest to the source, however, burrowing species may be more tolerant to runoff contamination (De-la-Ossa-Carretero et.al. 2012). These Quadrimaera species are at high risk of population decline resulting from pollution, due to their motile life, and closeness to land. As discussed in Reproduction and Development, temperature can have a dramatic effect on life history characteristics (Sainte-Marie 1991). Future ocean temperatures are predicted to rise, which will speed up physiological development, and reproductive processes. This will increase the rate of sexual maturity, causing these species to have a smaller body size at sexual maturity, and less time between molting stages (Glazier, 2009). This will have a negative effect on their dispersal, as there will be less time reaching sexual maturity, and therefore less distance will be covered, stopping them from inhabiting new areas.

Microplastics are another threat to these species of amphipods. It has been found, that the assimilation rate of food particles for amphipods exposed to polyamide fibres, was significantly reduced. However, this not the case for all amphipods, and is highly dependent on the feeding mode, and physiology of the digestive tracts. It is not only the chemical makeup of the PA fibres that affect the amphipods, but also their shape can mechanically harm, and disrupt the digestive tract, resulting in decreased assimilation efficiency, and in the long term, reduced size and fecundity (Blarer & Burkhardt-Holm, 2016).

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