Jawless fish long time survivors

Know your islands

The 25,000 known species of fish are divided into three main groups.

They are the jawless, the cartilaginous and the bony fish.

Jawless fish are the last survivors of the world’s first vertebrate animals, which means “back-boned”.

They lack both scales and jaws. Dating from over 5000 million years ago, only the hagfish and lampreys remain.

Cartilaginous fish developed about 100 million years later, ancestors of today’s sharks.

The skeleton of these fish is made of cartilage, which is not as hard as bone.

These fish have jaws, as well as teeth which are usually hard and sharp. Their bodies are covered with hard scales.

Bony fish – fish with bony skeletons – appeared at the same time as cartilaginous fish.

They are the largest group, with about 20,000 species.

These fish have an organ called a swim bladder which gives the animal buoyancy, the ability to float. All fish live in water and breathe with gills. Fish are cold-blooded, which means their internal body temperature changes as the surrounding temperature changes.

Visitors to the reef quickly learn to appreciate that these various fish groups do have personalities and different interesting behaviours.

Blennies are great subjects for behavioural study.

They are easily found in shallow water allowing observation in natural surroundings and they exhibit some interesting conduct!

Blennies are small, shallow water coastal fish with an extensive distribution.

They tend to be bold, curious little fish. They dart rapidly around the reef while actively surveying their environment.

They are openly aggressive and chase other blenny and when they do dare to go forth, it is only to immediately return or travel to another secure location.

The Yellowface Pikeblenny (Chaenopsis limbaughi) is an entertaining fish to watch for while diving or snorkeling. This pikeblenny is extremely elongated and the fore dorsal fin is quite high when extended.

It is often found in sea grass areas and inhabits worm tubes.

This species feeds mainly on planktonic copepods, but unidentified fish remains, shrimps and worms have also been examined within their stomach content.

They are highly territorial and this territorial aggression involves both real confrontation and mere acts of display by opening their large mouths, erecting the dorsal fin in aggressive threats, and the body is curved as if in a shell.

The accompanying photograph illustrates the behaviour of the blenny when met with its own reflection! Photograph generously provided by Nick Buckley.

The Redlip Blenny (Ophioblennius atlanticus) is another pretty little fish to observe.

It has a blunt head and large red lips. This blenny is common along the reef off South Sound where it prefers rocky inshore areas and shallow coral reefs.

These blennies spend the day nibbling on filmamentous algae and detritus within their small territories.

It is usually found resting on the bottom and is very territorial.

It will come out to meet a diver and dart around the reef until the diver swims away.

It is definitely not shy and can be closely approached. Both females and males patrol and defend their territories which average five square feet. Nesting sites are prepared by removing sand, rock and debris from gaps or openings in the reef.

The small blenny eggs (less than one millimeter) hatch five days after fertilization, one of the shortest incubation periods known for blennies.

Blennies that are able to set up a territory have a life expectancy of about one year.

Protect Cayman’s Marine Wildlife! For more information, to share your knowledge or if you would like to get involved with the many activities in the National Trust’s Know Your Islands Program, please visit www.nationaltrust.org.ky or call 949-0121. The weekly column from the National Trust is submitted by Marnie Laing, Education Programs Manager at the Trust.

Last week’s answer: The Green Anole is the endemic (found nowhere else in the world!) anole on Little Cayman.

Trivia question: What bird is most often associated with cattle in pasture?

Look for the answer in next week’s feature!

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