One of the most striking features of the sunfish, so full of expression, that makes it looks so surprised yet alert, are its big, wide eyes. What may initially seem like a cartoonish attempt at vision has been further revealed to have complex functions that have evolved to greatly increase the scope of this marine predator’s hunting ability!
It might sound strange, but yes ocean sunfish really do count as marine predators on benthic (seafloor) crustaceans and molluscs, and neritic (open water) jellies and gelatinous plankton. Mmmm tasty!!
Marine predators are often visual hunters, and many have evolved keen eyesight, even it seems the sunfish! (Well it is important to find your prey even if it doesn’t travel very fast!) It was recently discovered that the visual acuity of sunfish is similar or greater than some sharks and cetaceans. See Table 1. below.
Table 1. Comparative visual acuity of ocean sunfish with several other marine species.
In fact, the values for the ocean sunfish Table 1. were measured from juvenile specimens and visual acuity typically increases over the lifetime of an individual (Fritsches & Marshall, 2003), so we still do not know how good their vision might be in a full sized adult!
The large eye of the sunfish is another clue, as eyeball size is widely believed to reflect the importance of sight to each species (Walls, 1942). It seems relatively safe to conclude therefore that vision is very important to ocean sunfish, and perhaps they evolved such large eyes in response to the advantage these provided. Larger eyes could help identify prey over greater distances, and take advantage of lower light conditions enabling the sunfish to dive deeper to search for prey, such as siphonophores which a sunfish was recently filmed eating at 200 meters deep (Nakamura et al., 2015).
So it seems that one of the sunfishes (admittedly many) strange features offers a real strategic advantage, which we are only just beginning to fully comprehend.
If you find this topic interesting then check out the references below for more information. As always please get in touch if you want to chat or if you’ve seen a sunfish dead or alive! 🙂
Bozzano, A.A. & Collin, S.P (2000). Retinal Ganglion Cell Topography in Elasmobranchs. Brain Behavioural Evolution 55, 191–208.
Kino, M., Miayzaki, T., Iwami, T. & Kohbara, J. (2009). Retinal topography of ganglion cells in immature ocean sunfish, Mola mola. Environmental Biology of Fishes 85, 33–38.
Murayama, T. & Somiya, H. (1998). Distribution of ganglion cells and object localizing ability in the retina of three cetaceans. Fisheries Science 64, 27–30.
Nakamura, I., Goto, Y. & Sato, K. (2015). Ocean sunfish rewarm at the surface after deep excursions to forage for siphonophores. Journal of Animal Ecology, 1365–2656.
Walls, G.L. (1942). The vertebrate eye and its adaptive radiation. Optometry and Vision Science 20, 30–32.