Kilada et al.i An age-determination method for 3 commercially important crustaceans 
45 
A 
Figure 2 
Histological sections (6-pm thickness), stained with Masson’s trichrome, of the 
(A) dorsal ossicle of a red king crab (Paralithodes camtschaticus) that was col- 
lected in southeast Alaska in 2014 and used to represent both the red king crab 
and the southern Tanner crab (Chionoecetes bairdi). Included are the mesocar- 
diac (m), the urocardiac (u), and the medial tooth (mt). Higher magnification of 
the ossicle region denoted by the box in panel A revealed the cuticular layers 
of the arch of the mesocardiac, as evident in the high-magnification images of 2 
other specimens collected in 2014: (B) the southern Tanner crab and (C) the red 
king crab. The epicuticle (Ep) and membranous layer (MI) stained red and the 
exocuticle (Ex) stained a darker shade of blue than the endocuticle (En). 
Presence of growth bands 
Thin sections were successfully obtained from the me- 
socardiac ossicles from 19 red king crabs and 32 south- 
ern Tanner crabs and single eyestalks from 18 spot 
shrimp. For mesocardiac sections of both crab species, 
bipartite band patterns (exclusive of the lamellae) were 
clearest in the location of maximum endocuticle thick- 
ness in relation to the exo- and epicuticle that was at 
the proximal end of the structure (Figs. 2A, 4, A-B). 
For spot shrimp, bands were clearest at the proximal 
end of the eyestalk with respect to the anterior tip of 
the cephalothorax (Figs. 3A, 4C). 
Ossicular retention throughout ecdysis 
Comparison of gastric mill structures in exuviae with 
those of postmolt stomachs of crabs indicated differ- 
ences in presence of regions of the gastric mill. The 
cusp of the medial tooth of the urocardiac and zygocar- 
diac ossicles of the gastric mill, along with the portion 
of the ossicle adjacent to each tooth, were the primary 
structures present in the exuviae of both southern Tan- 
ner and red king crabs (Fig. 5A). For both crab species, 
the anterior portion of mesocardiac, the pterocardiac, 
and the anterior portions of the zygocardiac ossicles 
were not visible in the exuviae, but they were present 
and robust in a postmolt crab, indicating that these 
portions were potentially retained or resorbed and sub- 
sequently replaced (Fig. 5, B and D). 
Discussion 
The presence of bands in the endocuticle layer of the 
mesocardiacs of southern Tanner and red king crabs 
and in the eyestalks of spot shrimp indicated that de- 
termining age on the basis of band counts may be fea- 
sible, as it is for other crustacean species (e.g., Leland 
et al., 2011; Kilada et al., 2012; Leland et al., 2015; 
Kilada et al., 2015; Sheridan et al., 2015; Kilada and 
Ibrahim, 2016; ICrafft et al., 2016). Histological charac- 
