DIVE DRY WITH DR. BILL #771: FIVE, SIX, SEVEN GILLS
Three months ago I saw my oncologist after my 18th chemo treatment (going on 30 now!). He went over the results of an MRI I had and I asked if I could resume SCUBA diving. He said he wasn't very knowledgeable about the issues, but felt it would be okay. I told him I would check with DAN (Divers Alert Network) as they could give me sound medical advice. Until I get clearance from them, I'll continue to depend on several outstanding underwater imagers in SoCal for images.
The subject of today's column is a species of fish I've never even seen. Dive friends in La Jolla and "way up north" do get the opportunity though and San Diego diver and photographer Weiwei Gao had some excellent images of them that he kindly let me use for this column. Now I've dived with hundreds of sharks ranging from 6" horn shark babies to 18 ft great whites. Most sharks have five gill slits, but there are rather primitive groups that possess six or even seven. The subject this week is the broadnose sevengill shark (Notorynchus cepedianus). They were formerly known as a mud or cow shark and there are many common names for it reflecting its wide distribution.
The broadnose sevengill shark is the only living member of its genus. A living relative with seven gills is the sharpnose sevengill shark (Heptranchias perlo) which is in a different genus. Their ancient relatives have all gone extinct. They originated in the Jurassic Period 200-145 million years ago when dinosaurs roamed the Earth (but no cave man was around to see them).
This shark has a wide geographic range. Here in the eastern Pacific it is known from British Columbia to southern California and also down to Chile. It may also be found in the western Pacific western Pacific Ocean off China, Japan, Australia and New Zealand; as well as the southern Atlantic Ocean off Argentina and South Africa. They are common at the mouth of San Francisco Bay. They dwell just off the bottom in bays and channels along the coast but larger individuals may be found in deeper waters offshore.
It has a quite different appearance from most of its five gilled distant relatives. The body is thick with a single dorsal fin located far back on the body and a tail with the upper lobe longer than the lower one and notched. The coloration may be a silvery gray to brown, and the body is countershaded with a light belly. Thus it is camouflaged when viewed from above against the substrate or from below against the sky. There are also many small dark and light spots on the body. Mature males reach about five feet in length while females may exceed seven feet.
Its head is broad and blunt. There are two types of teeth in the jaws. The upper teeth are jagged and cusped. Wikipedia defines a cusp as "an occlusal or incisal eminence on a tooth." My canine teeth have a sharp cusp which is very helpful on nights of the full moon. Teeth in the lower jaw are comb-shaped. The former allow it to grasp and swallow smaller prey whole while the latter give it an edge in ripping and tearing into larger prey.
So what does this species eat? Researchers in one study found that marine mammals made up about 30-35% of its diet. It will also gladly chow down on other sharks, rays, and bony fish. Being an opportunistic feeder, it will also munch on other goodies it encounters including crustaceans, snails, shark eggs and carrion. The feed primarily at night. However, I'm willing to bet it doesn't eat broccoli or asparagus! In turn, these sharks may fall prey to cannibalistic compatriots and the great white shark. I was surprised to learn this species was targeted in a fishery back in the 1930s and 1940s along our coast. This commercial fishery petered out but sport anglers continued to fish for them.
Unlike humans, males are ready to reproduce at the tender age of 4-5 years while the girls wait until they are at least 11 and sometimes 21 years old! These sharks are ovoviviparous. The female carries the eggs internally and the young get their nourishment from the yolk rather than a connection to the mother as in mammals. During spring the female enters shallower coastal waters to give birth to litters numbering as high as 95 pups measuring about 16 to 18 inches in length. Good thing she doesn't have to find child care for them. Sevengills live at least 30 years with one source suggesting up to 49 years.
My friends Mike Bear and Barbara Lloyd of Ocean Sanctuaries down in San Diego set up a website to archive sevengill shark sightings. You can learn more about them there: Ocean Sanctuaries' Sevengill Shark Sightings.
© 2018 Dr. Bill Bushing. For the entire archived set of nearly 775 "Dive Dry" columns, visit my website Star Thrower Educational Multimedia (S.T.E.M.) Home Page
Image caption: Sevengill shark images from La Jolla (courtesy of Weiwei Gao).
Three months ago I saw my oncologist after my 18th chemo treatment (going on 30 now!). He went over the results of an MRI I had and I asked if I could resume SCUBA diving. He said he wasn't very knowledgeable about the issues, but felt it would be okay. I told him I would check with DAN (Divers Alert Network) as they could give me sound medical advice. Until I get clearance from them, I'll continue to depend on several outstanding underwater imagers in SoCal for images.
The subject of today's column is a species of fish I've never even seen. Dive friends in La Jolla and "way up north" do get the opportunity though and San Diego diver and photographer Weiwei Gao had some excellent images of them that he kindly let me use for this column. Now I've dived with hundreds of sharks ranging from 6" horn shark babies to 18 ft great whites. Most sharks have five gill slits, but there are rather primitive groups that possess six or even seven. The subject this week is the broadnose sevengill shark (Notorynchus cepedianus). They were formerly known as a mud or cow shark and there are many common names for it reflecting its wide distribution.
The broadnose sevengill shark is the only living member of its genus. A living relative with seven gills is the sharpnose sevengill shark (Heptranchias perlo) which is in a different genus. Their ancient relatives have all gone extinct. They originated in the Jurassic Period 200-145 million years ago when dinosaurs roamed the Earth (but no cave man was around to see them).
This shark has a wide geographic range. Here in the eastern Pacific it is known from British Columbia to southern California and also down to Chile. It may also be found in the western Pacific western Pacific Ocean off China, Japan, Australia and New Zealand; as well as the southern Atlantic Ocean off Argentina and South Africa. They are common at the mouth of San Francisco Bay. They dwell just off the bottom in bays and channels along the coast but larger individuals may be found in deeper waters offshore.
It has a quite different appearance from most of its five gilled distant relatives. The body is thick with a single dorsal fin located far back on the body and a tail with the upper lobe longer than the lower one and notched. The coloration may be a silvery gray to brown, and the body is countershaded with a light belly. Thus it is camouflaged when viewed from above against the substrate or from below against the sky. There are also many small dark and light spots on the body. Mature males reach about five feet in length while females may exceed seven feet.
Its head is broad and blunt. There are two types of teeth in the jaws. The upper teeth are jagged and cusped. Wikipedia defines a cusp as "an occlusal or incisal eminence on a tooth." My canine teeth have a sharp cusp which is very helpful on nights of the full moon. Teeth in the lower jaw are comb-shaped. The former allow it to grasp and swallow smaller prey whole while the latter give it an edge in ripping and tearing into larger prey.
So what does this species eat? Researchers in one study found that marine mammals made up about 30-35% of its diet. It will also gladly chow down on other sharks, rays, and bony fish. Being an opportunistic feeder, it will also munch on other goodies it encounters including crustaceans, snails, shark eggs and carrion. The feed primarily at night. However, I'm willing to bet it doesn't eat broccoli or asparagus! In turn, these sharks may fall prey to cannibalistic compatriots and the great white shark. I was surprised to learn this species was targeted in a fishery back in the 1930s and 1940s along our coast. This commercial fishery petered out but sport anglers continued to fish for them.
Unlike humans, males are ready to reproduce at the tender age of 4-5 years while the girls wait until they are at least 11 and sometimes 21 years old! These sharks are ovoviviparous. The female carries the eggs internally and the young get their nourishment from the yolk rather than a connection to the mother as in mammals. During spring the female enters shallower coastal waters to give birth to litters numbering as high as 95 pups measuring about 16 to 18 inches in length. Good thing she doesn't have to find child care for them. Sevengills live at least 30 years with one source suggesting up to 49 years.
My friends Mike Bear and Barbara Lloyd of Ocean Sanctuaries down in San Diego set up a website to archive sevengill shark sightings. You can learn more about them there: Ocean Sanctuaries' Sevengill Shark Sightings.
© 2018 Dr. Bill Bushing. For the entire archived set of nearly 775 "Dive Dry" columns, visit my website Star Thrower Educational Multimedia (S.T.E.M.) Home Page
Image caption: Sevengill shark images from La Jolla (courtesy of Weiwei Gao).
