Home | Courses | Calendar | Graduate Credit | Registration | About Us | Contact Us
  • Easy-to-use online courses designed around your busy schedule
  • Access to cutting-edge research and world-renowned scientists
  • Affordable graduate credit available


Sample Discussion Thread
This discussion was selected from the AMNH online course The Diversity of Fishes. This is an excerpt from an actual course discussion, but learner names have been changed. The Diversity of Fishes is part of Seminars on Science, a program of online graduate-level professional development courses for K-12 educators.

Learn more about this course   Go back to learning online

Week 2 Discussion: Fish Evolution
After reading and reviewing the materials for Week 2, from your perspective, how has habitat most evidently shaped the evolution of fish?

Megan (initial post) 5 Jul 8:01 AM
I believe that fish have developed sensory apparatus which maximize survival in water. Their olfactory apparatus is located in their nostrils near the olfactory lobes of the brain. In our article it ascribes excellent olfaction to the presence of an olfactory rosette as well as the fact that the odorant taken in by the nose does not circulate through a respiratory tree. Olfaction is also enhanced by the solubility of molecules in water which increases the sensitivity of the olfactory apparatus. The lateral line is also fascinating because even without the eyes fish are able to navigate based upon the sensitivity of this structure on the sides of fish. I think that the because vibrations and sound are perpetuated through water more effectively than sound through air, this is an effective means of responding to the environment and to communication perhaps with other fish of the same species. I also learned about the structure of the lens of the eye and the layers which help to bend the image so that when it reaches the retina it is not distorted.

In evolution fish have also evolved in terms of their body shape, mouth and teeth, and internal organs. Fresh water and salt water fish have evolved different styles of osmoregulation. Salt water fish excrete salt through their gills and conserve water. Fresh water fish excrete water and conserve salt. In addition to exchange of O2 and CO2, the gills excrete nitrogenous wastes Nitrogen is eliminated in the form of ammonia which is very soluble in water facilitating the process.
Deep water fish have evolved means of adapting to great depths and pressures. They have also evolved light producing structures capable of emitting light ( photophores) at great depths.

The articles have inspired me to search more - so I will wait to read some other reactions to this topic.
I definitely think that I will include many of the ideas presented this week in both evolution and comparative vertebrate anatomy.

Didier (response to Megan) 5 Jul 2:00 PM
Well so far I agree with all that you both have said and it seems to me that to put it simply, form and function are created based upon a specific environment. If a fish is in murky water, such as the Amazon, they will rely more on the sensory signals and even electrical impulses than eyesight. As we read last week some of the Catfish in the Amazon are even blind. As far as body shape goes the streamlined shape of their bodies makes it easy to move in water and is a simple and effective design for any watery medium, muddy or clear, salty or fresh.

Fish seem to make great use of the water, not only in body design but in the fact that it can be used for heightened smell and "touch." I had no idea that fish really were able to smell. I have seen their nostrils and have guessed that they were nostrils but only by default and guessed that they were no longer in use. This touch and smell adaptation will really help survival rates in water that dark and murky. Even waters that are not supposed to be murky can be due to a storm.....these fish are ready. I just wonder though if prey can also sense a fish swimming toward it just as they can sense a predator that is close by.

Also in the first essay re locomotion and feeding...its quite brilliant the way they fish can utilize almost all the food in one area that they live in. If they open their mouths, anything that can fit in will be sucked right up. This is very different from the land creatures that have specialized mouths and tongues. The related essay about the Impact of Tropic Ecology brought out some interesting points about the mouth of fish and its expanding cone shape. Fish have no need to travel very far for food as they can eat almost anything that will fit into their mouths in their habitat and as the article called it ...diet switching. They really can make the most of the food in their habitat. The expanding cone model makes perfect sense. I also liked the idea of the hydrodynamic tongue that is able to move anything in the buccal cavity around.

One Question though.....I have seen the tongue like structure in a fishes mouth but is it really used like a terrestrial tongue??

Fish seem to really utilize the space around them and have a form that relates directly to their habitat. Even their eyes which are not always needed thanks to the highly specialized rosette for smelling and lateral line for sensory perception are perfect for the underwater world and have evolved to see under water. I found the size factor really interesting........and its due to oxygen content....that makes perfect sense. Maybe one day fish will evolve to something other than oxygen to "breathe" as there is so little of it in the waters. Another way that fish have adapted is with "antifreeze" in their blood if living in very cold waters.....

I could go on but will stop here and make sure that I am on the right track.... fish seem to adapt to where they live and do so quite perfectly. Fish are so diverse because of their adaptability to their surroundings. In conclusion I will say that the most evident adaptation is body shape. that is something that all fish share...the streamlined shape with the least amount of drag , makes their bodies perfect for propulsion. Rambling Didier

Didier (response to self) 5 Jul 2:03 PM
One more thing.......the fact that there is so much diversity among the species and that there are 200 new species found every year. is wild!!! That's like a fish every 2 days! Amazed Didier

Anne (response to Didier) 5 Jul 3:57 PM
Math and fish. Didier, you are right we could use fish biology in math studies. So Much of learning is interconnected I wish more schools would have inter grated learning. Anne

Krishna (response to Anne) 6 Jul 1:42 PM
Math and fish. My husband teaches aquaculture and has a workshop each summer for high school teachers to integrate aquaculture (growing fish) into math and science. You can calculate growth rates and feeding rates, and the students can see the fish grow throughout the year and then have a fish cookout at the end of the year. There are many measurements that can be done (including the measurements we did in week 1 assignment). I agree there are many disciplines that can be integrated to increase student interests. Krishna

Anne (response to Krishna) 7 Jul 7:01 AM
Aquaculture. Krishna, Tell me more about aquaculture. I have some good friends that are doing this in Fiji trying to cut down on the depletion of the fish populations in the reefs around the islands. I have heard mixed messages about the practices as far as population etc... Is this is full time job? Are the fish sold to markets or used to restock lakes? Interested, Anne

Krishna (response to Anne) 7 Jul 9:04 AM
Aquaculture is growing fish as opposed to catching wild caught fish. Aquaculture is on the rise due to over-fishing the natural populations and meeting the high demand for seafood in the world. The regulations for aquaculture vary from country to country. The US has stronger regulations than other countries. This, along with high land and labor cost in the US, is why there is more aquaculture in other countries, such as in China (the world's leading producer). From a global perspective, aquaculture is environmentally and economically sound and a net producer of seafood biomass. Approximately 30% of the seafood consumed globally is aquacultured (50% in the US).

My husband's program is a 2 year program at a community college. His full time job is as the program manager at the Aquaculture Education and Development Center at Gadsden State Community College and he is a part time PhD student. As I mentioned in the icebreaker, I am taking this course to improve my fish knowledge so that I can teach courses at his facility in the future. He trains technicians to work in the aquaculture industry or starts students on a track to move to a 4 yr school, such as Auburn (the leading 4 yr aquaculture program in the US). He mainly works with food fish (tilapia, freshwater shrimp, catfish), but also introduces his student to the marine ornamentals industry. I also mentioned previously that he trains K12 teachers to use aquaculture as a teaching tool in the classroom for math and science. The fish at his facility are mainly used for teaching although some are sold to farmers. He does not compete on the retail market.

I would be happy to answer any questions related to aquaculture and if I don't know the answer, I can always ask my husband. Krishna

Instructor: Shankar (response to Suzzane) 7 Jul 9:56 AM
Adaptations. Body shape is definitely is determined by how deep and in what type of water this fish lives in. Can you think of other adaptations that maybe your fish market fish has?

Helen (response to Shankar) 7 Jul 11:01 AM
Adaptations. Most of the fish are able to eat whatever is given to them unlike other animals. One of the articles said it is much easier to feed fish than all of the other animals in a zoo. You can't give a lion bananas to eat! Helen

Scientist: Denise (response to Didier) 6 Jul 3:13 PM
Hi "rambling Didier",
You make interesting comments based on the readings and previous postings on habitat as an "evolutionary tailor" of form and function of fish. I guess we can extend the idea to all living organisms.

I will add a couple of ideas to your thoughts that actually can be tied to our next week readings. You're surprised about the sense of smell among fishes. Well, olfaction seems to be among the oldest of the senses among vertebrates. We have inherited olfaction from our "fishy" ancestor. There's no reason why the other fish lineages (those that didn't lead to tetrapods) had to loose the smell sense. But don't worry about not having thought about this before: someone told me once that she never thought fish had a brain!).

You mention the feeding mechanism. Isn't it great? You write: "Fish have no need to travel very far for food as they can eat almost anything that will fit into their mouths in their habitat and as the article called it ...diet switching." This is correct, but there are differences across fishes: some of them are really flexible in terms of diet. Does any example comes to your mind?

As for your question about the tongue: good question! Actually the function of the tongue among fishes is still uncertain, and it doesn't seem to hold taste cells. Some propose that it helps food to get down but this hasn't been confirmed yet. The fish tongue is partially homologous to that of terrestrial vertebrates: its skeleton is formed by a bone called basihyal. In terrestrial vertebrates, this bone forms only the basal portion of the tongue. Another characteristic that is unique among some fishes is that the tongue can hold teeth (actually the presence of teeth on the palate and even the pharynx is not unusual among fish).

Following one of the previous postings you mention the fact that living in a habitat with a content of oxygen that is 20 times lower than in the atmosphere. However, where did vertebrates originate? Aren't we, terrestrial organisms, who have challenged history? Wouldn't be more amazing to think how on Earth terrestrial organisms have been able to inhabit and thrive without collapsing from an overdose of oxygen?

"Another way that fish have adapted is with "antifreeze" in their blood if living in very cold waters....." I will add to this perfect sentence: "some fish have adapted ...." "Fish are so diverse because of their adaptability to their surroundings." Remember: adaptability at the level of individual is different from adaptive radiation at the level of the evolution of the whole group .... Rambling Denise

Didier (response to Denise) 6 Jul 9:05 PM
Re: Rambling Didier - I enjoyed your response and now have more questions. About the brain...I have heard that fish have such a small brain that, for instance in a fish bowl, by the time they swim to one end and turn around they have forgotten where they are and swim to the other end. Basically they don't realize that they are in a small tank and are fine being confined......I have thought this silly but had to ask :) Also so cool about the tongue an the ability to hold teeth.... Amazing. Strange that it has no taste cells...do they enjoy anything that they eat? Didier

Scientist: Denise (response to Didier) 7 Jul 11:51 AM
For "enjoying" food you need to have self-consciousness and have the ability not only to develop the emotion "pleasure" but also to develop the feeling "pleasure."

Neurobiologists now separate emotion from feeling, the latter required a further step in the chain of cognitive processes. A little baby or a dog may have the emotion of fear but do they know they have the emotion? I mean, are they able to recognize themselves as having the emotion? i.e. "I feel afraid." The comprehension of the "mind of others" is one of the greatest challenges (even I don't have any direct way to "know" that YOU have a mind!). You can imagine how little we know about the mind of other species. But there's scientific evidence supporting that animals, including worms, mollusks, and definitely non-primate vertebrates, that there are patterns of reaction that can be "read" as fear, for instance. Now, enjoyment is something else. The same applies with sexual pleasure.

As for memory, fish have definitely memory. In the same way we do, they have short term memory (working memory) and long-term memory. One of the first studies on memory that helped us to understand how it works was done by a Kandel, from Rockefeller University, using a mollusk.

Now, "fishy" vertebrates' brain have a forebrain and prefrontal cortex very little developed, which means that they cognitive abilities are very limited. What is the long-term memory span? That's been definitely studied for example in salmon and how is it that they spawn going back to the same stream where they born, or even among armored catfish: individuals of one species can go back to the same stone over and over. What about those species that are monogamous? Again, I may be mixing different kinds of "memory," even information that may be programmed in their genes. Any further thoughts? Denise

Didier (response to Denise) 10 Jul 5:26 AM
Thanks.....now I am in deep thought and thinking of a project outline that may include some type of fish memory?????? that's so right about the Salmon returning to the same place...I had forgotten that. Neat. Thanks, Didier

Megan (response to Denise's rambling Didier post) 7 Jul 4:48 PM
Just a thought - Despite the fact that we are living in an environment with 20% oxygen - O2 in the air differs with altitude. When we breath in we are dependent upon moist mucous membranes in our alveoli for exchange of respiratory gases. Even though we are terrestrial we still require moisture to exchange gases. As is stressed in one of the articles - oxygen dissolved in water while existing in very low levels is effectively exchanged for CO2 by the gills.

Sam (response to Megan) 10 Jul 8:26 AM
A question... I don't know if this is the right place for this, but I've found myself wondering about what effect oxygen and feeding has on fish during red tide...or as I have recently discovered the preferred term is harmful algae bloom.

I recently went to Florida on the Gulf and they were in the midst of a wicked red tide...I saw a fish so big dead on the shore I couldn't believe it. I've done some internet reading about this algae bloom but can only find reference to the harmful effects the bloom has on shellfish and consequently people that eat the shellfish.

I'm just wondering if it is eating the algae, as I've learned fish have the ability to simply open their mouths and eat whatever is there, that causes the fish to die. O is there perhaps a link between the algae and the oxygen levels in the water? Just curious and in this course I find myself thinking about it a lot--especially after witnessing the most bizarre forms of dead fish on the shoreline in Florida. Sam

Scientist: Denise (response to Megan) 11 Jul 1:29 PM
RIGHT!!!! I'm glad that you've brought up that fact, Megan. Actually it goes along with my "theory" that we still are aquatic organisms that have found the way to go around the dryness of leaving outside water. We don't only need the wall of the lung's alveoli to be moist, but also the olfactory mucose, the eye pupil, our skin .... WE are around 75% of water, the placenta is basically an aquatic environment for the embryo. We may not be literally aquatic but certainly we are close ...