Bicamculoides vs. bimaculatus

Oktoputeao said:
What is the difference between this two octos?

Bimaculoides is the smaller of the two species, and generally lives near shore and can easily be found in tide pools.

Bimaculatus gets twice as big as bimaculoides, and generally lives in deeper water.

The two are often confused. Bimaculoides has chain link eye spots. Bimaculatus has sunburst-like eyespots.

I think Nancy had a Bimaculatus and thought it was a Bimaculoides and towards the end it out-grew her 40-something gallon tank.

Hehe :lol:
I have found no evidence that O. bimaculatus grows to an average size larger than its sympatric sister. The reference I'm looking at (Ambrose 1988) is a longitudinal, six year study of wild populations of O. bimaculatus south of Los Angeles using SCUBA. They list the average wet adult weight of 260 g and mantle length of 71 mm.

In contrast, Forsythe and Hanlon--in the same issue of the Malacologia--report male tank-raised specimens of O. bimaculoides with a mantle length of around 65 mm. The mean weight of mature females was about a half a kilogram and a mantle length of 110 mm.

F&H do report numerous wild populations of O. bimaculoides that are highly variable with respect to size. Predicting the maximum size a bimac is going to get in an aquarium thus is likely dependant on its genetic stock, something with which an aquaculturer might be limited with to begin with but may overcome by finding new wild populations.

What are the chances a vertebrate paleontology student might have this stuff just sitting around his room? :smile:


Refs, for those interested

Ambrose, R.F., 1988. Population dynamics of Octopus bimaculatus: influence of life history patterns, synchronous reproduction and recruitment. Malacologia, 29(1) 23-39

Forsythe, J.W. & R.T. Hanlon, 1988. Behavior, body patterning and reproductive biology of Octopus bimaculoides from California. Malacologia, 29(1) 41-55.

That's fascinating! Having dealt exclusively with bimaculoides, I was always under the impression they were the "runts" of the two similiar species.

See for example: octo size?

And web snippet:

Bimaculatus: Body plus longest arm 30" (76 cm) long. Arms 4–5 times mantle length.

Bimaculoides: In 1949 it was discovered that there were two closely related species of two-spotted octopods instead of one. The second was named the Mud Flat Octopus (O. bimaculoides). This is the species most commonly found between high- and low-tide lines, on mudflats as well as among rocks. Its arms are shorter, 2 1/2–3 1/2 times mantle length, and its eggs are larger.

Perhaps the articles you referenced are only referring to mantle length and not tentacle length? Either way definitely interesting information, as if that's the case, it appears bimac size between the two species is widely variable and more a factor of genetics than environment?
Black96WS6 said:
Perhaps the articles you referenced are only referring to mantle length and not tentacle length? Either way definitely interesting information, as if that's the case, it appears bimac size between the two species is widely variable and more a factor of genetics than environment?

Mantle length seems to be a better measurement than arm length. I think two octos with the same ML but an inch difference in arms are going to be a much more similar than two octos with the same arms but an inch difference in ML!

And, as our own Dr. O'Shea pointed out recently in another thread, any length value is going to be arbitrary to some degree because these are stretchy-shrinky animals. Weight is the best way to measure them.

I should confess I don't think these articles are completely definitive about size. They both deal more with reproductive biology. In F&H octopuses and eggs were collected throughout the range of O. bimaculoides, but few adult measurements were given. Ambrose was working in one particular locality. It may well be true that the average O. bimaculatus is larger than O. bimaculoides if we looked throughout their range, but at the very least these studies do tell us we can't count on it!

The two species are closely related and there is certainly individual variation.

I encountered this personally when my O. bimaculoides Ollie grew large enough to fall into the bimaculatus range. I then had people try to convince me that she was a bimaculatus, but I would point to her perfect ring and the fact that she laid large eggs. In fact, I found severeal other equally large bimaculoides among our members and I had reports of larger bimaculoides from Octopets.

Which one has the unbroken "sunburst-like" eyespots?

I’m reading contradictory information in this thread about how to use the eyespots to distinguish between O. Bimaculoides and O. Bimaculatus. Which one has a a solid (‘sunburst-like”?) blue ring, and which has a broken (“chain link”) eye spot?

Black96WS6 said:
“Bimaculoides has chain link eye spots. Bimaculatus has sunburst-like eyespots.”

Nancy said:
“my O. bimaculoides Ollie … her perfect ring …”

Here’s a picture of the ring on a little Bimac I caught in a tidepool recently. I would call this ring solid, unbroken, and “sunburst-like”, as opposed to “chain-link”. So is it a Bimaculatus or a Bimaculoides?


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He was caught in the morning, but not because he was "active". I (mostly MarineBoy) turned over the rock he was hiding under in a shallow tidepool, and he tried to swim away. We caught two in about an hour that way. We also broke open a muscle and set it in front of a crevice in a huge rock, also in a shallow tide pool, to see if an adult octopus would reach out and grab it, which one did within a few minutes. So while they are not out walking around much in direct sunlight, they are at least watching, and willing to grab an easy meal.
I should add that the little bimac I caught that day laid large eggs when it grew up, so it was definately bimaculoides. That means that if it is true that the solid vs. chain-like apearance of the blue ring is a species difference and not just an individual difference, then solid ring means bimaculoides, and chain-like ring means bimaculatus. Again, that's only conclusive if the blue ring apearance is truely distinct between the two species.
I found this on another thread, and I think it helps clear up the question:

CaptFish;164736 said:
from "Cephalopods: A World Guide"
Bimaculatus - The ocellus contains an iridescent blue ring in the form of broken chain links with spokes extending to the outer edge of the false eye spot. - small egged- size body to 20cm and arms 80cm

Bimaculoides- The iridescent ring with in the ocellus is in the form of unbroken chain links. - large egged- body 12cm and arms 35cm

pics of Ollie here: Cephalopod Care

So it sounds like they BOTH have blue rings that are chain-like, but that the chain on bimaculoides is unbroken, and does not have spokes (sunburst) radiating out, while bimaculatus has a broken blue chain, and spokes (sunburst) radiating out.
This thread was extremely helpful. I am going to try to analyze my dive video and see if I can get a better view of the eye-spots to determine which species I saw. Thank you!
Chances are you won't be able to tell from your images as the two are almost indistinguishable visually. The bimaculatus are typically a deeper water, larger animal but your depth of 55' is kind of in between. The animals we typically keep have been captured in shallow tide pools and are all bimaculoides, mostly juveniles. These are the large egg, benthic born version where the bimaculatus hatches out more, small, planktonic hatchlings.

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