Question

How do biologists use the parsimony method when constructing a phylogenetic tree? If there is only one true evolutionary history for any group of species, discuss the possible reasons why the taxonomists analyzing genetic data (e.g., DNA sequence data) versus those analyzing morphological data may suggest different phylogenetic relationships among the same exact group of species?

4) Describe how the agnathans differ from other fish? Compare the morphology and natural history of hagfishes and lampreys. Your answer should include (at least) similarities and differences in the following: body shape, skeleton, circulation, feeding behavior, reproduction, and habitat.

5) Compared to air, what difficulties do fish have in obtaining oxygen from water? Describe the basic structure of a fish gill and how does gill structure vary among species based on activity level. What is a countercurrent exchange mechanism? How does countercurrent exchange in the gills of a shark increase the efficiency of O2 and CO2 exchange between the animal and its environment?

Answer 1

1) Compared to air, what difficulties do fish have in obtaining oxygen from water?

Normally the dissolved amount of oxygen in the water is low compared to the amount of oxygen in the atmosphere.in the water , the carbon dioxide released from the fish can easily dissolve in water,but the problem is that it is very difficult to extract oxygen from the water.

In order to remove oxygen from the water, they rely on special organs called "gills." Gills are feathery organs full of blood vessels. A fish breathes by taking water into its mouth and forcing it out through the gill passages.

2)Describe the basic structure of a fish gill and how does gill structure vary among species based on activity level.?

Each gill is supported by a gill arch – a bony structure that is oriented vertically on the side of a fish, just behind its head. The gill arch provides the support to hold a number of comb-like structures called gill filaments. Gill filaments extend out horizontally from the gill arches.

3)What is a countercurrent exchange mechanism?

Countercurrent exchange is a mechanism occurring in nature , in which there is a crossover of some property, usually heat or some chemical, between two flowing bodies flowing in opposite directions to each other.

4)How does countercurrent exchange in the gills of a shark increase the efficiency of O2 and CO2 exchange between the animal and its environment?

Fish gills use a design called ‘countercurrent oxygen exchange’ to maximize the amount of oxygen that their blood can pick up. They achieve this by maximizing the amount of time their blood is exposed to water that has a higher oxygen level, even as the blood takes on more oxygen.

Countercurrent oxygen exchange means the blood flows through the gills in the opposite direction as the water flowing over the gills. This flow pattern ensures that as the blood progresses through the gills and gains oxygen from the water, it encounters increasingly fresh water with a higher oxygen concentration that is able to continuously offload oxygen into the blood. The low-oxygen blood, which is just entering the gill, meets low-oxygen water.

Since there is more oxygen in the water, the oxygen can flow from water to blood. Likewise, the high-oxygen blood, which has nearly passed the entire length of the gill, meets fresh, high-oxygen water, and oxygen continues to flow from water to blood.