How to distinguish a shark from other fish. |
To keep it simple, here are three tests to make
the identification. There are several more, but these three external
characteristics can be observed without dissecting the shark.
1.Sharks have 5,6,or 7 pair of gill openings.- Only a total of 7 shark
species out of over 450 species have 6 or 7 pair of gill openings, and they are not
in the New England area.
All sharks in New England
will have: 5 pair of gill openings
These gill openings will be located on the sides of the head.
(The flat bottom dwelling angel shark is an exception.)
Other fish that we normally encounter have one pair of gill openings. Skates and
rays will have their multiple gill openings on the underside.
Six gill sharks :
bluntnose sixgill - Hexanchus griseus
bigeye sixgill - Hexanchus nakamurai
frilled shark - Chlamydoselachus anguineus
southern african frilled shark-Chlamydoselachus africana
sixgill sawshark - Pliotrema warreni
Seven gill sharks:
Broadnose sevengill - Notorynchus cepedianus
Sharpnose sevengill - Heptranchias perlo
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2.
Rough sand papery skin (especially when rubbed
from tail to head.)
This roughness is caused by small tooth like scales called dermal denticles. (also called
placoid scales or skin teeth.)
The dermal denticles on a shark will not be transparent and flaky as they are on many
fish.
In the magnified image you can see the denticles on the sharks skin.
On some species the scales overlap, on others they do not overlap.
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 The degree of abrasiveness
varies from species to species. |
| 3. A New England
shark will have an upper tail lobe longer than the lower tail lobe. There
are shark species that lack a fork in their tail that would define an upper and lower
tail lobe, but those sharks are not in the New England area. Two examples would be
the zebra shark, and the nurse shark.
The bottom dwelling, flat, skate like angel sharks are
exceptional; their lower tails are longer than the upper tail. And their gill
openings are mostly on the underside.
There are 18 species of angel sharks. |
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Other fish like striped bass, tuna, bluefish, mackerel, have close to
100 % symmetrical tails. (homocercal)
In some specimens of the shortfin mako the lower tail lobe is close to 90 % as long
as the upper. That's about as close to being symmetrical as it gets in
sharkdom.
In most shark species the upper lobe is two or more times longer than the lower tail lobe.
The extreme case would be the thresher..
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As you can see on the left, many
different size and shaped fish qualify to be sharks because they have
multiple pairs of gills, and meet the other requirements of shark
classifications.
When scientists break down sharks into
categories they use the following methods.
They start with grouping by whether the shark has an anal fin or not. (The anal fin is a single fin closest to the tail on the bottom side.)
The other fin on the bottom side is the pelvic fin, which is paired, and
closer to the pectoral fins.
Then the classification branches off into two groups using several more criteria
to place the shark.
Ultimately, you end up with 34 families of sharks. Some families having only 1
member and others having many.
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Neither of the two New England species in the
photo will qualify as sharks : Capt. Tom, Homo sapiens, and the striped bass,
Morone saxatilis.
Both species lack 5-6-or 7 pairs of gills on the sides of
the head , and lack longer upper tail lobes. Neither species has dermal denticles on
their skin.
That's Boston Harbor in the background.
(And yes, I did release the striper) photo
by Rusty Barry |
How
to determine Male and Female Sharks.
It's easy. A male shark will have two claspers extending
from his pelvic fins.
The two rod like white claspers are visible in
the left side of the picture on the right.
A female will not have claspers as shown in the picture below. |
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Any fin on the back is a dorsal fin. Dorsal
fin characteristics on sharks can vary widely from species to species. Dorsal fin
size and location on sharks also varies. Examples: having only one dorsal fin (six
and seven gill sharks); or having two dorsal fins nearly the same size (sandtiger,
Carcharias taurus - lemon shark, Negaprion brevirostris, plus
many others). The most
common dorsal fin configuration for a shark in New England is having the first dorsal much larger than the
second. There are many species of sharks that have both dorsals
almost the same size.
The sandtiger and spiny dogfish are two New England examples of having
a fairly large second dorsal. Rigid pectoral fins are a characteristic most sharks share in common.
They don't fold back against the body as they do in other fish species. |
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Not all sharks
have eye protection called nictitating membranes
Of the 453 shark species worldwide, there are many shark species that lack this
eye protection.
Species in New England coastal waters without a nictitating membrane are:
the
white, shortfin mako, porbeagle, thresher, sand tiger, dogfish, and basking shark.
The mako on the photo left, which lacks a nictitating membrane, rarely rolls its
eyes for protection no matter what difficulty it is in.
The blue shark on the photo right has brought up it's nictitating membrane for protection.
Mako Photo by: Christopher S. Moore, NOAA; Commanding
Officer, NOAA Ship DAVID STARR
Blue shark photo by
Karen Solari |
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