Porter, Marianne C.

Our goal was to assess regional differences in denticle density and skin
tensile properties in four coastal species of shark. We hypothesized that the
denticle density, tensile strength (MPa), stiffness (MPa), and toughness of skin
(MJ·m^-3) would vary regionally along the body of an individual and among
species. An hourglass-shaped punch was used to extract the skin samples at 10
anatomical landmarks and denticle density was quantified. Denticle density
varied significantly among both regions and species, and showed a significant
species by region interaction. Skin samples were tested in tension at a strain
rate of 2 mm-s until failure. We found significant species and region effects for all
tensile and denticle density properties. Also, denticle density increases with skin
stiffness but decreases with toughness. Shark skin toughness is similar to that of mammalian tendons. These data show shark skin functions as an exotendon,
able to conserve energy during swimming.

Sexually active males of the family Blenniidae develop distinctive
glandular swellings of the anal fins. These glands can be divided
into four groups on the basis of their shape and whether they appear
on the anal fin spines or rays or both. Examples of each group, when
studied microscopically, show the same histological pattern. This
pattern includes three layers of tissue, the outermost of which contains
characteristic barrel-shaped groupings of cells called "microglands".
These anal fin glands are compared histologically to similar structures
on the dorsal fins of some blenniids and the anal fins of certain
clinids. The microscopic structure of the anal fin glands is distinctive
and characteristic.