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You are here:   OldClasses > 2012 > Cymodoce pelsarti | Amelia Armstrong






Fact Sheet



Physical Description


Life History & Behaviour

Anatomy & Physiology

Evolution & Systematics

Biogeographic Distribution

Conservation & Threats

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Life History & Behaviour

Collection Location & Behaviour

Due to favorable tides throughout this study period, C. pelsarti was collected on the nightly high tide from the jetty using fine mesh plankton nets. It was observed that application of a bright white LED torch beam attracted tens of individuals within the space of 30 seconds. Individuals collected were not at the ocean surface prior to the LED torch application as determined using a red torch beam before sampling. This collection method demonstrates a positive phototactic response by C. pelsarti in its natural environment.

This phototactic response was not replicable under laboratory conditions. The factors potentially influencing this non-response include lack of depth, food availability, time of day and time tested during the tidal cycle (Ruppert et al. 2004). Previous studies examining phototactic responses of isopods found that a strong tidal and circadian rhythm affected results.

Gas Exchange

Members of Isopoda respire using five pairs of biramous pleopods located abdominally. This gill location also occurs in Stomatopods and is unusual within Crustacea. Although gills are almost always associated with appendages, they are usually located within a branchial chamber or within the body wall for most taxa (Ruppert et al. 2004). As with all isopods, the pleopods of C. pelsarti are flattened and consist of a basal section with two branches (rami). These rami have a very thin and permeable cuticle and function as gills, providing large and well-ventilated areas for gas exchange. The concave abdominal space that the pleopods sit is called the branchial chamber upon which the pleopods typically lie flat over (Ruppert et al. 2004). C. pelsarti and other Sphaeromatid isopods are restricted to respiration in the more posterior pleopods only. 

Additional to the locomotory and respiratory ability of pleopods, is the observed adaption to a benthic lifestyle in an environment with high rates of sedimentation. Laboratory observations revealed that when buried beneath sediment, pumping action of the pleopods created an incurrent ventilation hole, allowing C. pelsarti to remain buried in sediment whilst maintaining respiratory function.
Right: Pleopod sketch illustrating paired orientation and biramous rami (Endopod & Exopod)
Below: Video of pleopod motion from C. pelsarti


Members of the family Sphaeromatidae are benthic in life style, feeding on algae. The main method of movement by C. pelsarti is through crawling using its seven paired pereopods (as seen in video below). The dorsoventral flattening of this crustacean has not affected its ability to swim up into the water column. The first three pairs of pleopods are specially adapted for swimming and have limited function for respiration. Isopods observed swimming up into the water column towards torchlight appeared to spiral in movement and did not attain positive buoyancy. Once on the surface, swimming had to be maintained else the animal descended towards the bottom. 



Very few studies have been conducted on the feeding modes of isopods. Based on collection data and anecdotal evidence, the Sphaeromatidae are thought not to be carnivorous, parasitic or filter feeders.Due to the morphology of their mandibles and molar process, they are thought tobe browsers or feed on detritus (Poore & Bruce, 2012). C. pelsarti  exhibited a weak red fluorescence in the gut region, indicating the potential presence ofplant material.


Isopods have a foregut and hindgut only with the midgut represented as digestive ceca of the stomach. They have a muscularised ectodermal eosphagus which moves food into the gut. The dorsal area of the stomach separates indigestible material from the food, processed in the vental area. Intracellular digestion and absorption occurs in digestive ceca of the stomach (Ruppert et al. 2004).


Fertilization is internal in Isopods withmany being gonochoric. Although they have internal fertilization, transfer ofsperm occurs via modified pleopods from the male to the female. Male reproductive organs are located on the 8th thoracic segment with sperm being transferred from the penes to the pleopod and then into the female gonophore. Females have gonophore openings on the 6th thoracic segment and are able to store sperm until fertilization. Fertilization is not possible until after the parturial molt (Ruppert et al. 2004). Sphaeromatidae brood eggs internally and release juveniles that bypass a larval phase (Poore & Bruce, 2012), instead are smaller adult versions missing only the last pair of pleopods.


The cuticle of C.pelsarti is tough and thick, providing protection from their external environment and slowing the rate of desiccation if out of water. The cuticle of marine isopods is composed of Crystalline magnesium calcite, amorphous calcium carbonate, amorphous calcium phosphate, water and organic matrix (Neues et al. 2007). Proportional composition and thickness varies with function and habitat. Characteristic of terrestrial and unrelated Sphaeromatid marine isopods, is the ability to enroll into a ball. C. pelsarti when disturbed would roll itself into a ball, protecting its softer underside inside the smooth and hard dorsal cuticular exterior.