Occurrence and quantification of the age-pigment lipofuscin in two important commercial shrimp species of southern Brazil: Pleoticus muelleri (Decapoda: Solenoceridae) and Xyphopenaeus kroyeri (Decapoda: Penaeid)
Pleoticus muelleri (Bate, 1888) is an endemic shrimp species of the South American coast, occurring from Rio de Janeiro (20º S), Brazil, to Santa Cruz (50º S), Patagonia, Argentina. The shrimp Xiphopenaeus kroyeri (Decapoda: Penaeidae), is widely distributed in the western Atlantic Ocean, between Virginia, United States, and southern Brazil. The quantification of lipofuscin age pigment in nervous tissues by microscope-based approach has been succesfully established as a reliable and more precise methodology to resolve age groups in natural populations of crustaceans than size-based approaches, and is beggining to inform stock assessment for commercialy importante species, like P.muelleri and X.kroyeri. To date, there is no information about the accumulation of neurolipofuscin in both species, and the present study provides the first evidence that the age pigment lipofuscin occurs at quantifiable amounts in the nervous tissues of P.muelleri and X.kroyeri. Specimens of the red and seabob shrimp were sampled in the spring of 2012. Each individual was measured (Carapace length – mm), and the lipofuscin content was verified and quantified in 6 samples of P. muelleri and 8 samples of X. kroyeri, of different age/size classes, collected in shallow coastal waters (7-20 m) of the Balneário Camboriú city, Santa Catarina, Southern Brazil. The brains were carefully dissected. The samples were embedded in paraffin wax on the vertical plane, with the supra-esophageal ganglion facing the cutting edge of the wax block. Serial sections (6µm) of the brains were dewaxed through three xylene changes (2 minutes in each).
Neurolipofuscin was identified in histological sections for both species, in the olfactory lobe cell mass (OLCM). Most of the lipofuscin granules observed had diameter varying between 0,15 and 1,92 µm. The fraction area of accumulation had a size between 1,2 and 22 µm. The quantification showed a size dependent accumulation. Then, the lipofuscin was indeed found in both species, with the granules showing yellow autofluorescent emission when excited by blue light; the deposits of lipofuscin seems to be intracellular, which is consistent with the observations of neurolipofuscin in crustaceans and there is a indicative of a size/age dependency in both species. The results reported showed that the quantification of lipofuscin is possible for both species, and its use is a reliable tool for age determination of commercial shrimp species.