EEB267 - Lecture 6

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Ecology & Evolutionary Biology
Deborah Mc Lennan

Welcome to Osteichthyes Cephalochordata Urochordata Haikouella Myxiniformes Petromyzontiformes Chondrichthyes Actinopterygii Sarcopterygii calcium salts in cartilaginous endoskeleton jaws skull proto-vertebrae, true gills, two eyes, olfactory lobes many molecular characters dorsal hollow nerve cord, notochord, post-anal tail Surprisingly, the synapomorphy for Osteichthyes is ... the presence of a gas filled structure called the gas bladder (the bladder looks silvery because it is covered with guanine crystals, which makes it practically impermeable to gas leaks). lymphocytes have unique antigen receptors gas bladder 1 What is bone? Another type of hard tissue composed of:
 a matrix of (mainly) collagen fibers and other proteins +
 hydroxyapatite crystals 3(Ca3PO )4 2a(OH) (~ 20% of the weight, rigid, hard)
 water (~ 5%) produced by specialized cells called osteoblasts. Blood vessels, nerve fibers run throughout the bone in canals Specialized cells called osteoclasts are responsible for breaking bone down. Osteoblasts then produce new bone matrix to replace the digested area (in other words, bone can repair itself). 2 The evolution of bone: it’s older than we thought bone is found in a number of extinct fishes that existed before Osteichthyes originated. This is why bone isn’t an autapomorphy for the “bony fishes”: †
 Cephalo. Urochor. Haikouella Myxin. [ -------- extinct jawless “fishes” --- ] [ -------- Gnathostomata --------------- ] Petromyz. † † † † † Chondrichthyes Osteichthyes = loss of ability to make bone cellular cartilage bone acellular cartilage The Actinopterygii (aktis [ray] + pteron [wing]) = ray-finned fishes Autapomorphy for the Actinopterygii: The presence of ganoine in scales. Ganoine is shiny, hypermineralized (~ 97% hydroxyapatite), acellular material laid down in thick, successive layers producing “growth rings” 3 Actinopterygians, their big evolutionary innovations ......... nothing major to begin with ......... but
 the ray-finned fishes are both incredibly species-rich (at approx. 32,000 species, they are the largest vertebrate group) and incredibly ecologically diverse. WHAT HAPPENED TO MAKE THEM SO DIVERSE? 1. How not to sink like a rock in water Simple physics: Density = mass/volume
 Specific gravity = density of object/density of water
 Pure water has a specific gravity of 1.0, while salt water has a s.g. of ~ 1.026 so
 If your specific gravity is > 1.0 you sink (in pure freshwater) If your specific gravity is < 1.0 you float (in pure freshwater) How to change your density: Chondrichthyes Solutions •cartilaginous endoskeleton (s.g. cartilage 1.1): all Chondrichthyes •simply stay on the bottom and don’t worry about it (some skates and rays) •liver full of lipids (s.g. 0.90 - 0.92; squalene: s.g. 0.86). For example, great white sharks have a large liver that is ~ 90% oil (and this makes up ~ 30% of their total body weight) = overall specific gravity close to 1.026 (almost neutrally buoyant in salt water) the benefit: oil does not compress like gas, so sharks can change depth very rapidly (extensive vertical range in the water) the cost: in order to change depth, you must swim, which is energetically costly 4 Osteichthyes has a new problem: the bony skeleton (s.g. of bone is ~ 2.0 so bony fishes are much heavier than water. They will sink unless ....) Solutions:  store lipids in skin, muscle, bones, liver   swim continuously (many big pelagic predators such as tuna, marlin) or stay on the bottom and don’t worry about it (benthic fishes like sole, flounder)   the most common solution: the gas bladder (Good thing #1) But pressure changes as you move up and down in the water, so how do you control the volume of gas in your bladder?  5 There are three general types of gas bladders in ray-finned fishes: 1. Physostomous (physa [bladder] + stoma [mouth]) type 1 (the ancestral gas bladder type) muscles around duct gas bladder pneumatic duct esophagus gut pneumatic duct mouth To fill the bladder (the fish move upwards): come to surface and gulp air, which travels from the mouth to the esophagus then through the pneumatic duct (red arrow points to it) into the gas bladder. To empty the bladder (the fish moves downwards): the muscles around the pneumatic duct contract and open the passage between the pneumatic duct and the bladder. Gas passes from the bladder into the esophagus and is spat out through the mouth (gass- puckreflex). Found in the oldest ray-finned fishes, e.g., sturgeon. Doesn’t allow you to live in very deep water because you need to come to the surface to fill you gas bladder. Not the most effective gas bladder but provides some reduction in density. 6 vein 2. Physostomous type 2 gas cells rete mirabile artery mouth esophagus
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