Pufferfish inflate to prevent predators eating them, but how do they do it and why don't their ribs break when they do so?
Puffer fish inflate by rapidly pumping water into their stomach. Studies of the Long-spine porcupinefish (Diodon holacanthus) show it has evolved a stretchy stomach which has lost its digestive function to allow it to puff up.
The stomach, which has a special lining, is large and folds back on itself within an enlarged cavity in the peritoneum — a membrane within the abdominal cavity.
When the puffer is threatened, the stomach expands into the peritoneal space and the stomach unfolds to fill gaps beneath the head, dorsal, anal fin and caudal peduncle. The fish balloons and the spines that lie on the surface of its skin stick out, making it a highly unattractive meal!
These fish also lack some ribs and have no pelvis, allowing them to become ball-shaped without breaking any bones. Their skin is also adapted for stretching and the dermis layer contains lots of collagen fibres that allow it to expand by 40%. When the fish expands, these become hard and the fish becomes a stiff, tight sphere.
Although puffers have evolved to suck in water, if lifted out they can sometimes suck in air. They sometimes have difficulties expelling this from their stomach, so take extra care when catching them.
This item was first published in the October 2009 issue of Practical Fishkeeping. It may not be reproduced without written permission.
Blind cave fish show a remarkable ability to find their way around, despite their lack of eyes, as Rupert Collins explains.
The Astyanax mexicanus species comprises an eyed epigean (surface) form and at least 29 different pink, eyeless, hypogean (cave) forms, many of which are believed to have independent evolutionary origins over the last 10,000 years.
How the eyes were lost remains unclear; either as a result of random mutations in seemingly redundant eye-making genes, there being specific selective advantage in not having eyes — or that other cave-orientated adaptations have forced the indirect loss of them. Studies have indicated the latter idea as the best explanation so far.
Blind cave fish navigate, feed and reproduce with enhanced senses of smell, taste and feel. They do not shoal like surface cousins and do not feed in the same way either — by not feeding from the water column. They substrate feed at a different angle too, at 45 rather than 90°.
The cave form has also been shown to out-compete surface fish for food in complete darkness.
Keys adaptations for cave life are larger and increased neuromast (lateral line) cells around the head which sense pressure differences in water movement.
The lateral line organ senses pressure changes caused by underwater objects and the fish builds up and remembers a complicated spatial map.
These fishes have even been shown to respond to and ‘re-map’ a changing environment with an increased swimming rate.
This article was first published in the Christmas 2009 issue of Practical Fishkeeping magazine. It may not be reproduced without written permission.
Look at the colouration of your typical fish and you'll see that it has a paler belly and a darker back. But why is that?
This is called counter-shading and is an anti-predator adaptation. Most fish show this, having a dark-coloured dorsal surface, or back, and a paler ventral surface or underside.
Predators looking down from the surface will probably see a dark substrate and the fish should blend in because its back is a similarly colour. Bottom-dwelling predators looking up will see the sky and might not spot the fish because its belly is also pale.
Fish that spend their lives upside down, such as the Upside-down catfish (Synodontis nigriventris) have reverse counter-shading. Their bellies are dark and backs pale — the opposite of fish that swim the normal way up.
This item was first published in the October 2009 issue of Practical Fishkeeping magazine. It may not be reproduced without written permission.