FISHERY BULLETIN: VOL. 87, NO. 2, 1989 



in nearshore mangrove/estuarine localities in the 

 Townsville region which is located in the Central 

 Great Barrier Reef Province. The littoral-dwelling 

 /. pygmaeus was easily dipnetted off rocks and along 

 mangrove mud banks in the Ross River and Ross 

 Creek estuaries and in the Townsville marina. Spe- 

 cimens used in morphometric analysis were pre- 

 served in 10% formalin buffered with borax, while 

 specimens subsequently used for statolith analysis 

 were frozen. 



Measurements were taken with an Olympus SZ 

 binocular microscope^ equipped with an ocular 

 micrometer eyepiece. All weights are wet weights 

 and were taken by blotting the specimen dry and 

 compressing it to expel any water from the mantle 

 cavity. All lengths measured are dorsal mantle 

 length (DML). 



Idiosepius pygmaeus specimens were maintained 

 in aquaria during 1988 using a recirculating sea- 

 water system. They were fed ad libitum with the 

 sergestid shrimp, Acetes sibogae australis, which 

 were also maintained in the aquaria. The aquaria 

 were kept outside so the specimens would maintain 

 normal diel periodicity. 



Sexual maturity was determined by the presence 

 of spermatophores in Needham's sac (i.e., spermato- 

 phore sac) in the males and mature oocytes and large 

 nidamental glands in the females. 



Statolith Analysis 



Idiosepius pygmaeus statoliths are paired calca- 

 reous structures situated within the bilobed stato- 

 cysts which are located at the posterior base of the 

 cephalic cartilage. They were removed by severing 

 the head at the head mantle margin and carefully 

 teasing the statocysts from the skull. The statoliths 

 usually fell free from the anterior wall of the stato- 

 cysts when the statocysts were pulled apart. Stato- 

 liths were placed on a glass slide, washed with 

 water, and dehydrated with 100% ethanol. Final 

 preparation involved flooding the statoliths with 

 xylene and then mounting them anterior (concave) 

 side down in the synthetic mountant and clearing 

 agent, dibutyl-phthalate-polystyrene-xylene (D.RX.), 

 under a coverslip. This produced adequate clearing 

 in the lateral region of the statolith near the ros- 

 trum. The use of D.P.X. provided a high degree of 

 increment resolution obviating the need for grind- 

 ing techniques. 



Mounted statoliths were viewed with an Olympus 



BH compound microscope (400 x ). The growth rings 

 observed within the statoliths are distinct bipartite 

 structures consisting of a broad and translucent in- 

 cremental zone along with a narrower opaque zone. 

 The rings were counted using a drawing tube at- 

 tached to the microscope to trace lines onto draw- 

 ing paper. Subsequent counts were then taken from 

 the traced image. To avoid bias, lines were deliber- 

 ately not counted during tracing. Ring number was 

 ascertained when the same value was achieved with 

 replicate counts. However if there was some vari- 

 ation in replicates, a mean value was taken from at 

 least 3 counts. 



The nucleus of the statolith was delineated by a 

 prominent dark ring. This feature has been shown 

 to exist in the statoliths of other species of cepha- 

 lopods (Hurley and Beck 1979; Kristensen 1980; 

 Rosenberg et al. 1981; Lipinski 1986) and probably 

 represents a hatching mark or first-feeding mark. 

 Specimen age was thus determined as being the 

 number of growth rings from the nucleus to the 

 outer edge of the statolith. 



Tetracycline Staining 



Specimens used for tetracycline staining were 

 hand-netted in the estuary, maintained in 20 L 

 plastic buckets during collection, transported back 

 to the laboratory and subjected to staining during 

 the same day. Five-hundred mg of tetracycline was 

 dissolved in 2 L of seawater, and the specimens were 

 placed in the solution for 2 hours. A minimum of 2 

 hours was found to be needed for the tetracycline 

 to be incorporated into the statolith. Tetracycline 

 staining produced a distinct band on the statolith 

 when viewed under ultraviolet irradiation (Leitz 

 Dialux UV compound microscope with kp500 filter 

 and ultra high pressure mercury lamp); the inner 

 edge of the band corresponded to the growth in- 

 crement deposited during the time of staining. The 

 temporal mark on the statolith was used to cali- 

 brate the periodicity of the subsequent rings laid 

 down. 



RESULTS 



Idiosepius pygmaeus apparently is predominant- 

 ly a shallow-water estuarine species. No specimens 

 have been captured from benthic sampling on the 

 nearby continental shelf (P. Arnold^). Three-hundred 



^Reference to trade names does not imply endorsement by the 

 National Marine Fisheries Service, NOAA. 



'Peter Arnold, Queensland Museum (North Queensland Branch). 

 Flinders Street, Townsville, Queensland 4810, Australia, pers. 



266 



