NOTES 



CONJOINED TWIN ADULT SHRIMP 

 (DECAPODA: PENAEIDAE) 



A two-headed roughback shrimp, Trachypenaeiis 

 similis (Smith), caught at the entrance to Galveston 

 Bay, TX 10 May 1987 by Harold Fraley, together 

 with a color photograph of the specimen taken sub- 

 sequent to capture, was sent to me for identifica- 

 tion, morphological examination, and deposit in the 

 crustacean collection of the National Museum of 

 Natural History (USNM 234419), Smithsonian 

 Institution. 



The specimen (Fig. 1) is composed of two cephalo- 

 thoraxes (heads) perfectly aligned with the median 

 sagittal plane and conjoined posteriorly to an un- 

 paired, normally segmented abdomen. The cephalo- 

 thoraxes and abdomen were disarticulated when the 

 preserved specimen reached me in October 1987, but 

 were restored to normal position easily with aid of 

 the photograph as a guide. The lower cephalothorax 

 is that of an adult female, carapace length including 

 rostrum 32 mm, short carapace length (orbital 

 margin to posteromedian edge of carapace) 21 mm; 

 respective measurements for the upper carapace are 

 34.4 mm and 21.7 mm. The abdomen is flexed and 



twisted to the left, and the fourth and fifth segments 

 are damaged, hence its length cannot be measured 

 accurately. Comparison of the specimen with sper- 

 matophore bearing females of the species in the 

 USNM crustacean collection indicates that it is adult 

 in size, about 85 mm total length. 



Shrimps are sometimes caught and preserved 

 while in the act of molting. In that event the cara- 

 pace being molted tends to be loosened at the 

 thoraco-abdominal juncture so that its posterior end 

 can be flipped dorsally and away, freeing the husk- 

 like old carapace from the underlying soft new cara- 

 pace. A first impression that this specimen was 

 caught and preserved while in the act of molting was 

 not borne out by the structures observed. 



The integument of each carapace is firm, as is that 

 of the other exoskeletal parts. Both carapaces are 

 similar in shape and structure, including the part 

 of the lower carapace that is hidden by the upper 

 one. It is noteworthy that the upper carapace is 

 larger than the lower, just the opposite of what 

 would be expected if the upper one represented an 

 ecdysial discard. Eyes in both heads have normal 

 dark corneal pigment, though the corneal surfaces 

 are shriveled by preservation. Antennules, anten- 



FlGURE 1.— Conjoined twin Trachypenaeus similis in diagrammatic lateral view, distal parts of appendages inten- 

 tionally deleted, except for those of uropods. Scale = 1 cm. 



FISHERY BULLETIN: VOL. 86, NO. 3, 1988. 



595 



