Dulichiella pacifica is a marine amphipod species found along the northeast Australian coastline and southeast Asian oceans (Lowry & Springthorpe, 2007). Amphipods are thought to have evolved from Mesozoic times in tropical waters and have spread throughout the world including into arctic waters (Węsławski & Legeżyńska, 2002).

Lifestyle

Dulichiella sp. is usually gregarious species, males and females often arriving and dispersing into new habitats in mating pairs (Manguia & Heldt, 2016). This correlated with this reports research, as many individuals were found in small groups. Interestingly, right-clawed males tend to be more gregarious than left-clawed (Munguia & Heldt, 2016).

Densities of D. appendiculata has a positive correlation with macroalgae abundance and because of the similarities between D. appendiculata and D. pacifica it is likely that they play a similar role in the ecosystem (Morales-Nùñez & Chigbu, 2016). Macroalgae is typically a sign of low reef or water system health and blooms in favourable conditions (Diaz-Pulido & McCook, 2008). This indicates that D. pacifica populations could be a density-independent or a r-selected type and may explain the previous absence of this species from the previous years reports. High densities of D. pacifica or related species may also be used in assessing reef health, however scientific research into this correlation would be required.

Feeding

As mentioned, the density of Dulichiella sp. correlates with macroalgae cover as it is their main food source (Morales-Nùñez & Chigbu, 2016). It is likely that this species is herbivorous like the closely related D. appendiculata which also only eats macroalgaes such as Sargassum (Lowry & Springthorpe, 2007). Specific macroalgaes of the D. pacifica diet hasn’t been recorded however a study by Duffy and Hay (2000) was conducted to determine the overall effect of amphipods on macroalgae dominance using many amphipod species such as D. appendiculata. They found that herbivorous amphipods effect community structure of more or equal magnitude as fish (Duffy & Hay, 2000). Amphipods play a crucial role in a trophic cascade system that appears to be almost textbook. Major predators of amphipods are juvenile fish and so fish populations control amphipod populations with a negative relationship. This in turn effects brown seaweed cover, following an opposite relationship (Brawley & Adey, 1980; Duffy & Hay, 2000). Brawley and Adey (1980) recognised the complexity of this relationship and said that this tropic cascade is highly specialised the outcome of changes will be dependent on the amphipod species. Because of this insight, without research into the effect of D. pacificia no reliable conclusions can be made on the effect of D. pacifica can be made however it is likely it plays an important role in the health of marine ecosystems.

Actual eating process by D. pacifica hasn’t been documented. According to Duffy and Hay (2000), D. appendiculata feeds on organic detritus and this may be similar for D. pacifica. The mouth consists of four appendages; mandible, maxillule, maxilla and the largest appendage, the maxilliped.

Courtship

Many papers have investigated the role of the enlarged gnathopod 2 in males and sexual selection. It is agreed that larger claws are selected for by females, giving those males a distinct advantage in reproductive success. However, this mating system is highly complex as males with an enlarged right claw are preferred over those with an enlarged left claw (Munguia & Heldt, 2016). This trait and the associated behaviours are further discussed in the anatomy section. 

Life history

Morales-Nùñez and Chigbu (2016) have extensively investigated the reproduction and life history of D. appendiculata and will be reviewed here. This study was conducted in Marylands Coastal Bay of the United States of America. It is likely that D. pacifica follows a similar pattern and however will be shifted by 6 months due to the difference seasons seen between the hemispheres. This is summarised in figure 3.

D. appendiculata, and likely D. pacifica, is classified as an iteroparous semi-annual breeder. Iteroparous individuals have multiple reproduction events throughout their lifespan but generally produce fewer offspring with greater investments for each (Reece et al. 2012). Although reproduction is continuous throughout the six month breeding season, two major breeding events are seen (Morales-Nùñez & Chigbu, 2016). This is caused by the reproductive diapause which occurs over winter.

Overwintering females will sexual mature and begin reproducing in April which will be referred to as the spring breeding event in this report. This high density of ovigerous females results in a high increase in the number of juveniles. These new juveniles will form the ovigerous females reproducing in the second breeding event which will be referred to as the autumn breeding event in June-July. A key difference between the females of each breeding event is their body size. The overwintering females mature quickly after winter and will then reproduce multiple times. They correlate more closely to other amphipod species from lower latitude which also have relatively short maturity times and high reproductive potentials as they are able to reproduce multiple times throughout the breeding period (Sainte-Marie, 1991). The ovigerous females in the autumn breeding event have a smaller body size by comparison as they only spend approximately one month maturing before beginning to reproduce. This may mean that their reproductive potential is lower due to lower reproductive fitness as well as the breeding period shorter.

The breeding periods are controlled by environmental factors. These factors can control the body size of the ovigerous females, brood size and embryo development rates (Morales-Nùñez & Chigbu, 2016). Steele and Steele (1986) found that the reproductive diapause occurs when the length of day is reduced during winter. Furthermore, temperature controls the timing and maturation rate of females with 94% of D. appendiculata females being reproductively mature in waters with an average temperature of 14.7°C while 0% were mature in water temperatures of 9.6°C (Morales-Nùñez & Chigbu, 2016). So it is clear that a range of physical factors control the reproduction of Dulichiella sp.

Locomotion

While D. pacifica individuals were on the settlement plates, the males were able to move quickly and efficiently between worms, algas, sponges etc. and avoid being in the open area. When moved into a petrie dish, the males struggled to move due to the presence of their enlarged gnathopod. Instead, they would ‘spin’ around their gnathopod. Females, who lacked the enlarged gnathopod did not seem to change in their locomotion. From this, it can be inferred that the species, or more specifically the males, rely on the uneven benthos for locomotion. This trade-off in locomotion for claw size would come at a great expense of the individual, and thus sexual selection must be extremely high.

Due to the sexual dimorphism seen in this species I will take a closer look at the anatomy of D. pacifica. The specimen I collected consisted of six males and 2 females. Of the six males, three had an enlarged right claw and the others an enlarged left claw. This would indicated that there would be no benefit to being left- or right-clawed and thus there is a 0.5 probability of being left- or right-clawed. However, Munguia and Heldt (2016) found right-clawed males are more attractive to females. The sexual selection for this specific trait ensures the continuation of the ‘right-clawedness’ trait and would mean the eventual decline of left-clawed males. This was not seen in the individuals collected for this study. Therefore there must be both benefits and disadvantages to being left- and right-clawed.

After observing the males in a lab environment it is clear the disadvantage of the large claw. When agitated using the end of a pipette tip, males ‘snap’ their claws and try to move away however tend to spin in circles around their large claw. The enlarged claw inhibits movement and locomotion. Manguia and Heldt (2016) found that enlarged gnathopods are almost always larger in right-clawed morphs than left-clawed morphs. The left-clawed morphs would therefore be able to move more effectively than right-clawed morphs and thus have a distinctive advantage. This is supported by research showing that left-clawed individuals disperse further (Manguia & Heldt, 2016). These left-clawed males are found in 85% of habitats in comparison to right-clawed morphs which are only present in 65% of habitats. However, in general, both morphs are found in all habitats coexisting. Thus, although both morphs have distinct advantages and disadvantages, neither trait is more selected for than the other and therefore both persist.

In contrast, some other hypotheses have been put forward for the function of the enlarged gnathopod 2. Walker (1904) hypothesized that the gnathopod 2 was enlarged for protection and this was further developed by Conlan (1991). She separated amphipod families into two functional groups; mate-guarding species and non-mate guarding species. Mate guarding species were then further classified into carriers and defenders. Carriers were found in the superfamilies;  gammaroidea, talitroidea and hadzioidea, who carry or assist with locomotion using their enlarged gnathopod 2. Defenders include the superfamilies,  corophioidea and caprellidea who guard their mate by maintaining a close presence. This hypothesis is supported by the findings of this report as individuals were often found in pairs or small groups. In addition, the findings by Manguia and Heldt (2016) further support this, as males with an enlarged right-claw, generally have a larger claw and are preferred by females. A larger claw may indicate greater protection during this courtship period and easier carrying.

Some hypothesized secondary functions of the gnathopod 2 are described by Conlan (1991) and include, increased sensory abilities and increased aggressive behaviours which were also seen in the individuals used for this report when provoked. In marine environments this increased aggressive behaviour is theorised to be more intense, to the extent that the non-enlarged gnathopod 2 is used to carry the female, and the enlarged gnathopod 2 is used to defend against other males and also assess the reproductive stage of the female (Conlan, 1991).

A final hypothesis for the function (or secondary function) of gnathopod 2 was presented by Jarrett and Bousfield in 1996. They hypothesized that it is used for sound production which may be a secondary function of gnathopod 2 to aid in defence. This is thought to be particularly enhanced in Dulichiella species due to the large socket in the propod which the dactyl fits into. Snapping of this appendage would produce a sound. The snapping motion described was recorded by Lowry and Springthorpe (2007) and also observed in the specimen captured for this report when provoked.

Skeletal Structure

There are a few defining characteristics of the Dulichiella genus and the specific species therein. The spines found in males on the distolateral crown of the enlarged gnathopod 2 are characteristic of Dulichiella species. Variations in the number of spines can be used to identify  the species and the different combinations are outlined by Lowry and Springthorpe (2007) in figure 4. Figure 5 displays the distolateral crown of gnathopod 2 from species collected from the settlement plates.

A second anatomical feature that is used to identify this species is the dorsal spine formula found on the pleonite and urosomite. Each species formulae is presented by Lowry and Sprintghorpe (2007). The pleonite/urosomite dorsal spine formula for Dulichiella pacifica is 7-7-7-5-4/6-2 (Lowry & Springthorpe, 2007; Lowry & Springthorpe, 2009). Urosomite 1 had some variation between individuals, with either 4 or 6 spines. Due to this species having a laterally compressed body shape, it was difficult to use this characteristic to identify the species. However, figure 6 shows some of the spines on one side.

Thirdly, males perepods, 6 and 7, have long slender setae on the propodus and carpus (Lowry & Springthorpe, 2009). This is seen in figure 7. On perepods 5, 6 and 7 there is also an accessory spine found on the dactylus.

Circulatory System

A study into the hemolymph vascular system by Wirkner and Richter (2007) compared the circulatory system of gammaridea and caprellida, two families within senticuadata. There were some key differences between the two groups and some inferences can be made about the family melitidae, containing D. pacifica due to their close evolutionary history.

The heart found in amphipods is a tubular heart, relatively primitive and extends from the cephlothorax through most of the peron (Stachowitsch, 1992; Wirkner & Richter, 2007). The cephlathorax is made up of fusion of the cephalon (or head) and either the first or first and second thoracic somites. Gammaridean cephlothorax has only the first thoracic somite fused whereas the carellida cephlathorax is the head first two thoracic segments. Morphologically, melitidae is similar to gammaridea, so much so, that the family was formally found within gammaridea (Englisch, Coleman & Wägele, 2003). Examination of the specimen also indicates that the cephlorthorax is made up of the head and the first thoracic segment only (Figure 8). Due to this morphological similarity is it likely that the head of D. pacifica includes only the first thoracic somite. From this, it is likely that the heart will extend from the cephalothorax (head and first thoracic somite) to the end of peronite 7, similar to gammaridea.

At both the anterior and posterior ends of the heart, aortas allow for blood flow to continue to the rest of the body (Wirkner & Richter, 2007). Amphipods also have three ostia, or muscular openings in the heart (Stachowitsch, 1991). These link to the gills found at the base of each thoracic limb (Steele & Steele, 2007).  Interestingly, Wirkner and Richter (2007) found that both gammaridea and caprellida, and therefore most likely D. pacifica has a reduced arterial system compared to other malacostracan species. No hypothesis for this was discussed.

Reproductive System

At the superorder level, pericardia, males actively seek females using antennae in response to pheromones released (Sea Life Base, n.d.). The male releases sperm into the subthoracic marsupium, followed by the female’s egg release. Fertilisation occurs in the marsupium and eggs are brooded and in amphipods, the offspring fully develop and are released as fully formed adults (Lowry, 2003).

Research on D. appendiculata from the southern hemisphere found that a brood consists of 23 eggs on average (Munguia & Heldt, 2016). In comparison, the northern hemisphere broods can have an average brood size of anywhere between 5.5 and 18.3 eggs. This difference in brood size could be affected by environmental factors or whether the females studied were part of the spring or autumn breeding periods. As mentioned, environmental factors greatly effect brood size and juvenile development (Morales-Nùñez & Chigbu, 2016).

D. pacifica is found in a marine epibenthic environment like many amphipod species (Lowry & Springthorpe, 2009). They are often found in coral rubble, sand and areas with low algal turn over and usually to depths down to 20 meters (Lowry & Springthorpe, 2009). Figure 9 shows that this species is restricted to Australia, however species from the genus Dulichiella can be found worldwide. Marchini & Cardeccia (2017) attribute this widespread distribution to boating movement.

The individuals used for this study were collected on settlement plates left in Manly Harbour, Brisbane. This is the furthest south they have been recorded along the Australian coastline. As this is the first year they have been spotted in the area, this could potentially be linked to warming temperatures or an increase in macroalgae. This could become a potential problem to the harbour as many of the species found on the settlement plates are considered pest species due to the low water quality.

The complete taxonomy of D. pacifica is below, according to Lowry and Meyers (2017)

Kingdom: Animalia                                                  

            Phylum: Arthropoda

                        Subphylum: Crustacea

                                    Super Class: Multicrustacea

                                                Class: Malacostraca

                                                            Subclass: Eumalacostraca

                                                                        Super Order: Peracaria

                                                                                    Order: Amphipoda

            Suborder: Senticaudata

                        Infraorder: Hadziida

                                    Parvorder: Hadziidira

                                                Super Family: Hadzioidea

                                                            Family: Melitidae

                                                                        Genus: Dulichiella

                                                                                    Species: Dulichiella pacifica

Figure 10 below shows the phylogenetic tree of the suborder senticaudata. This excerpt was taken from a phylogenetic tree created by Lowry and Springthorpe in 2017 for the entire monophyletic order amphipoda and is currently the most accepted. However this is the first ever phylogenetic tree to be mapped for the entire order and therefore all the relationships are not fully understood.

Due to the significant effect that amphipods can have on ecosystems as previously discussed, measures may need to be taken to conserve macroalgae growths in some areas particularly those that have limited fish grazing. These areas are more susceptible to high amphipod densities as fish, particularly juveniles, are the main predators of amphipods (Brawley & Adey, 1981). Commercial seaweed facilities are another area where controls of amphipod populations may be necessary to maximise their productivity. Duffy and Hay (2000) investigated one insecticide, Sevin, which was shown to be highly successful in controlling amphipod populations.

Atlas of Living Australia, Dulichiella pacificia Lowry & Springthorpe, 2005. Accessed 23 May 2018 <https://bie.ala.org.au/species/urn:lsid:biodiversity.org.au:afd.taxon:33bf92c9-8089-4b5a-a47e-bf99e11a9a2c#>

Brawley, S. H., Adey, W. H. (1981). The effect of micrograzers on algal community structure in a coral reef microcosm. Marine Biology 61(2-3). Pp. 167-177.

Diaz-Pulido, G., McCook, L. J. (2008). Macroalgae (Seaweeds). In The State of the Great Barrier Reef Online, Eds. Chin, A. Great Barrier Reef Marine Park Authority, Townsville. Accessed 30 May 2018 <http://www.gbrmpa.gov.au/corp_site/info_services/publications/sotr/downloads/SORR_Macroalgae.pdf>

Conlan, K. E. (1991) Precopulatory mating behaviour and sexual dimorphism in the amphipod Crustacea. Hydrobiolgia 223. Pp. 255-282.

Duffy, J. E., Hay, M. E. (2000). Strong impacts of grazing amphipods on the organization of benthic community. Ecological Monographs 70(2). Pp. 237-263.

Englisch, U., Colman, C. O., Wägele, J. W. (2003). First observations on the phylogeny of the families Gammaridae, Crangonyctidae, Melitidae, Niphargidae, Megaluropidae and Oedicerotidae (Amphipoda, Crustacea), using small subunit rDNA gene sequences. Journal of Natural History 37(20). Pp. 2461-2486.

Jarrett, N. E., Bousfield, E. L. (1996). The amphipod superfamily Hadzioidea on the Pacific on the Pacific coast of North America: Family Melitidae. Part I. The Melita group: systematics and distributional ecology. Amphipacifica 22(2). Pp. 3-75.

Marchini, A. Cardeccia, A. (2017). Alien amphipods in a sea of troubles: cryptogenic species, unresolved taxonomy and overlooked introductions. Marine Biology 164(4). Pp. 1-14.

Morales-Nùñez, A. G., Chigbu, P.  (2016). Life history of Dulichiella appendiculata (Amphipoda, Senticaudata, Melitidae) in Maryland Coastal Bays, USA. Aquatic biology 25. Pp. 75-82.

Munguia, P., Heldt, K. (2016). Dichotomous male asymmetry in metapopulations of a marine amphipod. Journal of Crustacean Biology 36(4). Pp. 4511-455.

Myers, A. A., Lowry, J. K. (2009). The biogeography of Indo-West Pacific tropical amphipods with particular reference to Australia. Zootaxa 2260. Pp. 109-127.

Lowry, J. K. (2003). Amphipoda. In Zoological Catalogue of Australia. Vol. 19. Csiro Publishing. Pp. 7-18.

Lowry, J. K., Myers, A. A. (2017). A phylogeny and Classification of the Amphipoda with the establishment of the new order Ingolfiellida (Crustacea: Peracarida). Zootaxa 4265(1). Pp. 1-89.

Lowry, J. K., Poore, G., C., B. (2003). Pericarida. In Zoological Catalogue of Australia. Vol. 19. Csiro Publishing. Pp. 5-6.

Lowry, J. K., Springthorpe, R. T. (2005). New and Little-known Melitid Amphipods from Australian Waters (Crustacea: Amphipoda: Melitidae). Records of the Australian Museum 57. Pp. 237-302.

Lowry, J. K., Springthorpe, R. T. (2007). A revision of the tropical/temperate amphipod genus Dulichiella Stout, 1912, and the description of a new Atlantic genus Verdeia gen. nov. (Crustacea: Amphipoda: Melitidae. Zootaxa 1424. Pp. 1-62.

Lowry, J. K., Springthorpe, R. T. (2009). Melitidae, the Melita group. Zootaxa 2260. Pp. 718-735.

Reece, J. B., Meyers, N., Urry, L. A., Cain, M. L., Wasserman, S. A., Minorsky, P. V., Jackson, R. B., Cooke, B. N. (2012). Campbell Biology. 9th edition. Pearson Australia Group Pty. Ltd. Pp. 1212.

Sainte-Marie, B. (1991). A review of the reproductive bionomics of aquatic gammaridean amphipods: variations of life history traits with latitude, depth, salinity and superfamily. Hydrobiologica 223(1). Pp. 189-227.

Sea Life Base (n.d.). Dulichiella anosochir (Krøyer, 1845). Sea Life Base. Accessed 30 May 2018 < http://www.sealifebase.org/summary/Dulichiella-anosochir.html>

Stachowitsch, M. (1992). The invertebrates: an illustrated glossary. Wiley-Liss, Inc. New York, USA. Pp. 438-445.

Steele, D. H., Steele, V. J. (1986). The cost of reproduction in the amphipod Gammarus lawrencianus Bousfield, 1956. Crustaceana 51(2). Pp. 176-182.

Steele, D. H., Steele, V. J. (1991). The structure and organization of the gills of gammaridean Amphipoda. Journal of Natural History 25(5). Pp. 1247-1258.

Walker, A. O. (1904). Report on the Amphipoda collected by professor Herdman, at Ceylon, in 1902. Ceylon Pearl Oyster Fisheries – 1904-supplementary reports. Cited in Lowry & Springthorpe. (2007).

Węsławski, J. M., Legeżyńska, J. (2002). Life cycles of some Arctic amphipods. Polar research 23(3-4). Pp. 253-264.

Wirkner, C. S., Richter, S. (2007). Comparative analysis of the circulatory system in Amphipoda (Malacostraca, Crustacea). Acta Zoologica 88(2). Pp. 159-171.