Isopoda is a large and diverse monophyletic order of the phylum Crustacea. The sphaeromatid isopod, Paracerceis sculpta (Holmes, 1904) is native to the North East Pacific and has been introduced globally. The species has a wide range of water tempreture tolerance and are found over a large range across temperate, tropical and subtropical habitats all over the world. P. sculpta in its native habitat is associate with coralline algae as feeding ground and living in cavities of calcareous sponges whereas they are commonly found within bio-fouling communities in shallow water in the introduced locations, being scavengers and browsers (Hewitt & Campbell 2001).

The P. sculpta specimen examined here is a male individual collected from Heron Island (Figure 9). The isopod is 12mm in length and 3.5mm in width, bilateral symmetrical. Its dark brown colour and pattern resemble the inhabiting bio-fouling environment, made it hard to be detected. Like all other sphaeromatids isopods, P. sculpta specimen has a compact body, with tough exoskeleton and are able to roll themselves into a ball when threated, protect the soft underside in the exoskeleton ball. Cephalosome surface is smooth, two pairs of antennae at the front of head, the anteriorly second pair of antennae is slightly longer than the first pair. Two sessile compound eyes present laterally (Figure 4). Mouthparts consist of one pair of maxillipeds, two pairs of maxillae, and a mandible. 

Its carapace reduced into pereon, comprised of seven segmented pereonites, under each pereonite, and each coxal plate bears a pair of pereopods (walking appendages). The male has five pairs of pleopods under pleon, consisting of six somites or pleonite. The last pleonite fused to telson and becomes a pleotelson. P. sculpta possess ornamented sculpture of pleotelson to which a pair of elongated uropods is attached (Figure 5-8). 


Sexual Dimorphism

P. sculpta exhibits obvious sexual dimorphism where males distinguish from females by possessing larger body size, elongate uropods, ornate pleotelson, and spines on the pereon (Figure 3).

Male Polymorphism

P.sculpta is famous for its male polymorphism (Figure 2), which means that matured males come in three distinct body form with deferent genotypes (Figure xxx): 81% of the males are ornamented α-males, possessing elongated uropods and enlarged pleotelsons; β-males comprise 4% of the male population, smaller in size, lack pronounced long uropods and resemble sexually matured females in morphology; γ-males are the smallest individuals, lack ornamentation, morphologically similar with the juvenile form (Munguia & Shuster 2013). The stable coexistence of three types of male morphs in natural environment is a result of genetic variation and under control of alleles, which is consistent with the Hardy- Weinberg equilibrium (Shuster & Wade 1991). The three types of discrete male phenotypes occurs at frequencies at which their reproductive success and fitness are equal (Shuster & Wade 1991). 

P. sculpta is native to the Pacific Coast of North and Central America and has been introduced globally by maritime traffic. The species is found inhabiting in spongocoels of a particular calcareous sponge species Leucetta losangelensis and feed on subtidal coralline algae in its native habitat- the Northern Gulf of California (Shuster 1989). However, in introduced locations globally, they are found to be prevalent in hull-fouling communities associated with vessels and harbours with ballast water discharge (Hewitt & Campbell 2001), Human disturbances have altered their niches and behaviours greatly (Vincenzi et al. 2013). P. sculpta is a robust species that establishes their population in diverse habitat with a wide range of water temperatures and chemistry (Hewitt & Campbell 2001). Vincenzi et al. (2013) conducted a study of the invasive species P. sculpta in Northern Adriatic lagoons, the field data suggest that P. sculpta is a pest species. Except for the evidences that it is much more competitive than the two native sphaeromatid species in the lagoon, there is no proof that the species is having a detrimental impact on its invaded ecosystem. 

Reproductive biology

Similar with most of the animal species, the availability of sexually matured females is the limiting factor of male reproduction. Males have to compete for mating with sexually receptive females (Darwin 1883). The fascinating way of P. sculpta males defending and competing with each other to breed with females is probably the most frequent studied aspect of this species.

P. sculpta usually inhabit and breed predominantly within the spongocoels of intertidal calcareous sponges (Shuster & Arnold 2007). These isopods are also found living on tunicates, seaweed, sea squirts, barnacles and rocky substrates, when sponges are not available, however mating behaviours outside of sponges are poorly known (Shuster & Arnold 2007).

After moulting, the adult α-male leave the feeding ground- coralline algae and settle themselves in the sponge cavities. As illustrated in Figure xxx, they position themselves at the osculum with the head pointing inside of the sponge, while the telson and uropods exposed to the outside as courtship display or to scare off other rivals (Shuster 1987). Females show courtship by pulling the α-male’s uropods using their mouthparts, the α-male respond by grasping the female by uropods and let them enter its territory. The previous study has found one α-male can collect up to 19 females into their harem (Shuster & Arnold 2007). After entering the occupied spongocoel, females undergo the final moulting- after which the female become sexually receptive, allows mating with the α-male. All female P. sculpta are semelparous, breed the young in a brood pouch, and die within two weeks due to depletion of somatic resource and incapability of feeding (Shuster 1989).

The other two types of P. sculpta males use different strategies for mating. While α-male is defending their territories, β-male can enter spongocoels effectively by mimicking the morphology and behaviour of a receptive female. γ-male, as mentioned, resembles juveniles, they sneak into spongocoels by stealth. They firstly wander around the osculum, waiting for the opportunity to rush into the spongocoels and fertilise as much female as it can, taking advantage of their fast movements and small size.

The YouTube video above best illustrates the alternative mating strategies of P. sculpta, credited to Louisa Pitney and Casey Dunn (2014). (http://creaturecast.org/)

Development and Life History

The P. sculpta is a sedentary species. Juviniles directly develop in female adult’s ventral brood pouch (Shuster, 1989), do not undergo a planktonic phase as commonly seen in other crustaceans species (Hewitt & Campbell 2001). The embryo complete development within the female for approximately four weeks and leave them as mancae (a post- larval juvenile stage, resembles miniature female adult morphology, only possesses six pereonites). After leaving the spongocoel, the young isopods disperse to feed on intertidal or subtidal coralline algae, undergo biphasic molting. As male adult samples are rarely collected in algal samples, the males appear to leave algae and migrate to sponges during the same period of their adult molting (Shuster 1991).

Locomotion 

P. sculpta are benthic crawlers and swimmers, although they are described as sedentary (Hewitt & Campbell 2001), prefer not to move. They normally use their seven pairs of pereopods to craw in a slow speed, the lateral uropods can generate a fast propulsion when its threated (under laboratory conditions). See video below:

 The footage of P. sculpta locomotion, filmed in University of Queensland.

Anatomy
Anatomy of P. sculpta is discussed in Physical Description section.



Reproductive System 

P. sculpta males possess a pair of penises. Sperm is transferred into the female’s gonophore by the modified second pair of pleopods. Fertilisation occurs after the sexual receptive molt, after which the connection is made between the oviduct and semen receptacle (Fox 2006). The young isopods are brood by the female within the marsuoium (oöstegites) and release as a post-juvenile form known as mancae (Shuster 1987).

Sensory & Nerve System

P. sculpta’s sensory system is poorly studied, however the species is the member of class Malacostraca, the sensory mechanism of the class has been intensively investigated.  Malacostracans has three distinct sensory pathways: Olfactory for chemical communication, antennular chemo and mechanosensory and statocysts sensory pathway (Schmidt-Rhaesa et al. 2015).

P. sculpta originally occurs from 32°N in Southern California to 18°N along the Mexican Pacific coast (Espinosa-érez & Hendrickx 2001). It is now having a wide spread introduced distribution globally as a result of human disturbance and the increasing vessel traffic. P. sculpta specimens have been collected in the Gulf of Mexico (Munguia & Shuster 2013), which is at the northern side of Mexico and a number of other tropical and subtropical habitats, include: Hawaii (Miller, 1968), Brazil (Pires, 1981), Hong Kong (Bruce 1990), the Atlantic coast of Europe (Rodriguez et al., 1992), and Australia (Hewitt & Campbell 2001) (Figure 11).

The P. sculpta specimens collected from Townsville in 1975 are the first record of the species in Australia (Harrison & Holdich 1982). The species is known to exist in 13 localities so far, range from tropical to temperate habitats in Australia (Harrison & Holdich 1982) (Figure 10). It is further hypothetically inferred to continue expanding its distribution in Australia marine environment due to its capacity to survive in wide range of habitat (Harrison & Holdich 1982). 

Isopoda is one of the nine orders in the superorder Peracarida within crustacean subphylum. The Peracaridas are described as “marsupial crustaceans”, which is distinct from the other superorders (Hoplocarida, Syncarida, Eucarida) by females bearing an internal pouch called oöstegites in which their embryos are brood (Brusca et al. 2001). P. sculpta is a member of Sphaeromatidae (or marine pill bugs) family which contains a variety of species in about 100 genera. Sphaeromatid isopods are probably the most commonly observed isopod family in shelf waters and rocky shores. Within the genus Paracerceis, there are 13 described Paracerceis species up to date (Kensley et al. 2004).

The specimen was identified using the key published in "A guide to the coastal isopods of California." (Brusca et al. 2001) (Figure 12). Available at http://tolweb.org/notes/?note_id=4175.

Classification

Kingdom          Animalia
Phylum            Arthropoda
Subphylum      Crustacea
Class                Malacostraca
Subclass          Eumalacostraca
Order               Isopoda
Suborder         Flabellifera
Family             Sphaeromatidae
Genus              Paracerceis
Species            Paracerceis sculpta 

Information is obtained from WoRMS (http://www.marinespecies.org/isopoda/)

Synonyms

P. sculpta has been recorded with 3 other different synonymized names while Paracerceis sculpta is the most commonly used and accepted name of the species (Harrison & Holdich 1982). The previously used names are: 

Dynamene sculpta (Holmes, 1904) 

Cilicaea sculpta (Holmes, 1904)

Sergiella angra  Pires, 1980

The P. sculpta is not listed as an endangered species, instead, it is expanding their distribution globally. There are currently no efforts are contributed to the conservation of this species. However, the threats could be the result of human disturbance, since the increasing of shipping can transport the species to numerous locations around the world, causing a dramatic shift in their habitat and thus influence the population genetics.

Brusca, R.C., Coelho, V. & Taiti, S., 2001. Aguide to the coastal isopods of California. Internet address: http://tolweb.org/notes.

Darwin, C., 1883. The Descent of Man and Seletion in Relation to Sex,Рипол Классик.

Espinosa-érez, M.D.C. & Hendrickx, M.E., 2001. The Genus ParacerceisHansen, 1905 (Isopoda, Sphaeromatidae) in the Eastern Tropical Pacific, Withthe Description of a New Species. Crustaceana, 74(11), pp.1169–1187.

Fox, R., 2006. Invertebrate Anatomy Online. Lander University,pp.1–7. Available at:http://webs.lander.edu/pf.htm?&pf=/rsfox/invertebrates/thelyphonus.html.

Harrison, K. & Holdich, D.M., 1982. New eubranchiate shaeromatidisopods from Queensland Waters. Mem. Qd Mus., 20(3), pp.421–446.

Hewitt, C.L. & Campbell, M.L., 2001. The Australian Distribution ofthe Introduced Sphaeromatid Isopod, Paracerceis sculpta. Crustaceana,74(9), pp.925–936. Available at: http://www.jstor.org/stable/20105328.

Kensley, B., Schotte, M. & Schilling, S., 2004. World list of marine,freshwater and terrestrial isopod crustaceans. Smithsonian Institution,Washington, DC Disponible en: http://www. nmnh. si. edu/iz/isopod.

Munguia, P. & Shuster, S.M., 2013. Established populations ofParacerceis sculpta (Isopoda) in the Northern Gulf of Mexico. Journal ofCrustacean Biology, 33(1), pp.137–139.

Schmidt-Rhaesa, A., Harzsch, S. & Purschke, G., 2015. Structure andevolution of invertebrate nervous systems, Oxford University Press.

Shuster, S. & Arnold, E., 2007. The effect of females on male-malecompetition in the isopod, Paracerceis sculpta: a reaction norm approach tobehavioral plasticity. Journal of Crustacean Biology, 27(3), pp.417–424.Available at: http://www.bioone.org/doi/abs/10.1651/S-2784.1.

Shuster, S.M., 1987. Alternative reproductive behaviors: three discretemale morphs in Paracerceis sculpta, an intertidal isopod from the northern Gulfof California. Journal of Crustacean Biology, 7(2), pp.318–327.Available at: papers2://publication/uuid/0F0F0B6F-3DAB-4474-BA0F-B5C648F35303.

Shuster, S.M., 1991. Changes in female anatomy associated with thereproductive moult in Paracerceis sculpta, a semelparous isopod crustacean. Journalof Zoology, 225(3), pp.365–379.

Shuster, S.M., 1989. Male Alternative Reproductive Strategies in a MarineIsopod Crustacean (Paracerceis sculpta): The Use of Genetic Markers to MeasureDifferences in Fertilization Success Among α-, β-, andγ-Males. Evolution, 43(8), pp.1683–1698. Available at:http://www.jstor.org/stable/2409384.

Shuster, S.M., 1992. The Reproductive Behaviour of α-, β-, and γ-MaleMorphs in Paracerceis Sculpta, a Marine Isopod Crustacean. Behaviour,121(3), pp.231–257.

Shuster, S.M. & Wade, M.J., 1991. Equal mating success among male reproductivestrategies in a marine isopod. Nature, 350(6319), pp.608–610.

Vincenzi, C. et al., 2013. Alien species in the northern Adriatic lagoons:Paracerceis sculpta (Isopoda: Sphaeromatidae). In Rapport du 40e Congrès dela Commission Internationale pour l’Exploration Scientifique de la merMéditerranée. p. 588.