Why isn’t the Giant Isopod larger?

isopodWhat I want to discuss, and I use this word specifically as after 10 years contemplation I seem no closer to an answer, is why the Giant Isopod is, well, giant?

Mosely noted in 1880

Other [animals] attain under them gigantic proportions. It is especially certain crustacea which exhibit this latter peculiarity, but not all crustacea, for the crayfish like forms in the deep sea are of ordinary size. I have already referred to a gigantic Pycnogonid [sea spider] dredged by us. Mr. Agassiz dredged a gigantic Isopod eleven inches in length. We also dredged a gigantic Ostracod.

For over a 125 years, scientists have contemplated the extreme size of Bathynomus giganteus. Do other isopods attain these sizes? Gigantism is also known in the isopod Serolis but enlargement comes from flattening. B. giganteus appears unique in its extreme gain in bulk.

isopodcoverWhy the increase in size? Timofeev (2001) proposed that deep-sea gigantism, for all crustaceans, reflects colder temperatures leading to longer lifespans and thus larger sizes as these beasties continue to grow . However, despite little changes in temperature beyond the thermocline, deep-sea invertebrates including isopods continue to show changes in body size.  Alternatively, Chapelle and Peck (1999 and 2004) demonstrated  size was related to oxygen concentrations. It is suggested this relationship arises because the amount of oxygen available controls the amount of sustainable tissue. This has been shown experimentally in which cell size and cell number both increase with increasing oxygen concentration (Frazier et al. 2001). Larger sizes in snails are also found at more oxygenated sites in the deep sea (McClain and Rex 2001). However, giant isopods are known from the Gulf of Mexico deep where oxygen concentrations are low.

Kevin Zelnio in an old post also brought up another interesting point….

B. giganteus is a scavenger (3, 5, 6), but some suggest it is also a facultative predator (3, 6). Specimens in aquaria have survived 8 weeks between feedings (5) and it speculated that this may be an adaptation for carrying its brood, which would be severely impacted by a full stomach (3). Further support for this hypothesis are the large quantities of lipid reserves in the hepatopancreas (14) and fat bodies (2) of this isopod.

lloydbathynomusAlternatively, the larger size also increases fasting potential because greater fat reserves can be maintained. Larger size also confers a greater area for feeding, important for either a scavenger or a predator.  Both of these are important adaptations in the food-limited deep sea.

Of course all of this is speculative and it remains unclear why Bathynomus is unique among arthropods. Perhaps is size is simply a random walk in evolution and is nonadaptive. Gould noted in reference to another body size pattern, Cope’s Rule…

One would think that issues so fundamental, and so eminently testable, had been conclusively resolved long ago-except for a perverse trait of human psyche. We tend to pick most ‘notable’ cases out of general pools, often for idiosyncratic reasons that can only distort a proper scientific investigation

Is this case for the Giant Isopod? But perhaps the most interesting question is why the Giant Isopod is not larger?bathynomus

Dr. M (1801 Posts)

Craig McClain is the Executive Director of the Lousiana University Marine Consortium. He has conducted deep-sea research for 20 years and published over 50 papers in the area. He has participated in and led dozens of oceanographic expeditions taken him to the Antarctic and the most remote regions of the Pacific and Atlantic. Craig’s research focuses on how energy drives the biology of marine invertebrates from individuals to ecosystems, specifically, seeking to uncover how organisms are adapted to different levels of carbon availability, i.e. food, and how this determines the kinds and number of species in different parts of the oceans. Additionally, Craig is obsessed with the size of things. Sometimes this translated into actually scientific research. Craig’s research has been featured on National Public Radio, Discovery Channel, Fox News, National Geographic and ABC News. In addition to his scientific research, Craig also advocates the need for scientists to connect with the public and is the founder and chief editor of the acclaimed Deep-Sea News (http://deepseanews.com/), a popular ocean-themed blog that has won numerous awards. His writing has been featured in Cosmos, Science Illustrated, American Scientist, Wired, Mental Floss, and the Open Lab: The Best Science Writing on the Web.

One Reply to “Why isn’t the Giant Isopod larger?”

  1. Maybe there *are* larger isopods – either we just haven’t run across them (it’s not like we’ve thoroughly surveyed the abyssal plains – we haven’t), or, the people who have found The Big Ones were killed and eaten by the huge beasts before they could get word out. Either way, it is quite likely that we have not seen the biggest isopods Down There yet.

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