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March 4, 2016

United States

Minnesota—Use a Microscope to Determine the Age of Shrimp

 

The age of most aquatic animals can be found by counting annual growth bands in hard structures, such as the otoliths (stone-like structures in the ear that are important for balance and orientation) in fish and the shells of bivalves.  Because crustaceans grow by molting and are assumed to lose and replace all calcified structures, including the cuticle (exoskeleton), their ages could not be determined.  Now, however, a technique to directly and accurately age individual crustaceans has been found.

 

   

 

In a study, researchers examined the eyestalk and gastric mill (the first of a two-part stomach lined with chitinous teeth that acts as a gizzard) of four decapod crustaceans: American lobster (Homarus americanus), snow crab (Chionoecetes opilio), sculptured shrimp (Sclerocrangon boreas), and northern shrimp (Pandalus borealis).  Their eyestalks and gastric mills were embedded and then sectioned transversely or longitudinally.  Growth bands were recognized as paired light and dark bands in the endocuticle (from the outside in, the crustacean cuticle has 3–4 main layers: the epicuticle, exocuticle, and endocuticle).  The emesocardiac ossicle (one of several small ossicles in the gastric mill) also demonstrated growth bands in the American lobster.  In the crab, growth bands were most easily interpreted in transverse sections of the eyestalk, while in the two shrimp species, longitudinal sections of eyestalk were best.  In lobster, growth bands were most clearly seen in longitudinal sections of the mesocardiac ossicle.  Up to six growth bands were seen in the shrimps and up to 10 or 15 in the crab and lobster, respectively, agreeing with longevity expectations.  Additional data confirmed that the growth bands provided an accurate correlation with known ages.  The match between growth bands and estimated age, however, was not good for a certain population of larger lobsters.  This discrepancy could not be explained, however, similar challenges have been encountered in the application of well-established, otolith-based ageing techniques in some fish species.  

 

Further detailed statistical analyses supported the concept of growth bands being a reliable indicator of age in the four crustaceans tested.  To determine whether any mineralized features of the cuticle were conserved through molting, some juvenile lobsters were immersed in seawater containing the fluorescent marker calcein (a calcium-binding compound).  The results provided the first evidence that a mineral in the cuticle survived molting, which provides a possible mechanism for growth bands to accumulate and record age in the endocuticle throughout the life of the crustacean.

 

Information: Stephen W. Carmichael Mayo Clinic, Rochester, Minnesota 55905, USA (Email carmichael.stephen@mayo.edu).

 

Source: Microscopy Today.  Carmichael’s Concise Review.  Look in a Microscope to Determine the Age of a Shrimp, Crab, or Lobster!  Volume 21, Number 3, Page 8,  May 2013.

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