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Types of Mollusks

Describes gastropods, bivalves, and cephalopods.

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Wet Blankets
Teacher Contributed

Wet Blankets

Take My Arm! I Won't Be Using It.

Sexual dimorphism. What in the world is that? Well, let's dissect this phrase. Sexual means it must have something to do with sex. Dimorphism? Di means two and morph means form, so it must have something to do with two sex forms. If you followed all that, then you have the answer. Sexual dimporphism is when the male and female of a species have different forms, this doesn't just mean the two sexes having different sex organs, it means parts of the body not directly connected to sex have different forms. The most common version is when there is a size difference between the sexes. Homo sapiens are sexually dimorphic with the males being larger than the females but not by that much. The primates with the greatest sexual dimorphism are gorillas which are also the largest extant primates. In the case of gorillas, the males are much larger than the females, following a larger pattern seen with other species which have a "harem-type" mating system like the gorilla. This is a mating system where one male maintains a group of females and tries to maintain exclusive breeding access to them. I say try because studies have shown they aren't always that successful in achieving exclusivity, though they do mate more than other males. This form of sexual dimorphism where big males duke it out for females is certainly the best known type in popular culture, but the thing is natural selection doesn't really care about ego or who is the biggest or perceived baddest, it cares about results. There are many species where the females are bigger than the males. In fact, in the animal species with the greatest known degree of sexual dimorphism, it's the female that is larger. For the blanket octopus (Tremoctopus spp.) females can be up to 2M long while the males top out at about 2.5 cm. Here's a clip where you can see a female Tremoctopus.

Not a lot is known about Blanket Octopus as they are pelagic animals (meaning they live primarily in open water away from coastlines), and as such, they are not that easy to locate for study purposes. There are 4 species currently recognized in this genus. In this next clip, you'll see an individual in deeper water which is likely a different species than the individual in the first clip. What sex do you think this individual is? Notice both the similarities and differences between the individuals in the two clips.

Did you notice the color change and the jettison of part of the blanket? Watch the video again if you need to, look for the jet of ink, then the color change, then the jettison. What part of the blanket does it leave behind? It was theorized that the purpose of the blanket was some form of predator deterrent (you can read more about this theory here http://tolweb.org/notes/?note_id=3124) but this is some of the first visual evidence supporting the theory. Where was the predator it was seeking to avoid? That would be the ROV (Remotely Operated Vehicle) which was taking the video, notice how the behavior of the blanket octopus changes as the ROV pursues it and approaches closer.

But back to sexual dimorphism. Why is it so great in the blanket octopus? That isn't entirely clear, as I said this is a rarely-encountered, poorly studied genus. However, there is enough information to theorize. First off, the small males which have been collected were found to have fully formed testes, so they are sexually mature. This eliminates the possibility that there are bigger males out there which just haven't been found. The males also develop special arms which are used to deliver spermatophores (these are enclosed packets of sperm which allow females to save sperm until they can actually become pregnant). These arms have been found detached from the male in the mantle of the female, and females have been found with arms from multiple males. No males have been found with this special arm regrowing. Also, as these are solitary pelagic animals it is believed encounters between males and females are rare. How does this all fit together? Take a minute to think about these facts and try to put them together to form your own theory.

Are you finished? If so, here is how other people have put together these facts to explain the sexual dimorphism in this genus. It is believed that males see their highest reproductive success by developing rapidly, becoming sexually mature at a small size and producing lots of sperm. This maximizes the chance that if they encounter a female, they will have a spermatophore filled arm ready to give to her. Since the chance of encounter is low, and no males have been found regrowing their special arm, it is believed the males simply die after delivering their spermatophores. Not a very romantic life for a male blanket octopus. Does this theory explain the sexual dimorphism? Probably to some extent at least, but something else interesting about this genus is its members have a defense mechanism which works only at small size. They acquire nematocysts (stinging cells) from siphonophores (a type of Cnidarian), and it seems when they and their suckers grow large, they lose the ability to manipulate these nematocysts and use them as weapons (this defense mechanism has never been observed in individuals over 7 cm in length). So maybe the males stay small because this defense increases their chance of avoiding predation long enough to encounter a female, more than being able to move around more would increase their chance of encountering a female. Which reason is correct? At this time, we don't know for sure.

Now that you've thought about why the male blanket octopus are so small, why don't you think about why the females are so big?

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Use the below resources to answer the following questions

  1. In cases of sexual dimorphism, are males always larger than females? Why or why not?
  2. Why do you think sexual dimorphism exists in organisms? Consider both reproductive success and energetics in your answer.
  3. Are the theories of rapid development or predation avoidance to explain sexual dimorphism in blanket octopus mutually exclusive? Think carefully and explain your reasoning fully.
  4. Which theory do you think best explains sexual dimorphism in blanket octopus. Why do you favor this theory?
  5. Can you think of another possibility to explain a high degree of sexual dimorphism? Do not limit yourself to what is specifically known with this species. The reason you propose may be right for another species even if it doesn't work for this species.


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