Editing Jellyfish (section)

==== Evolution ====
Experimental evidence for [[photosensitivity]] and [[Photoreceptor cell|photoreception]] in [[cnidaria]]ns antecedes the mid 1900s, and a rich body of research has since covered evolution of visual systems in jellyfish.<ref name="Garm-2010">{{cite book|first1=Anders|last1=Garm|first2=Peter|last2=Ekström |title=Chapter 2 – Evidence for Multiple Photosystems in Jellyfish |chapter=Evidence for Multiple Photosystems in Jellyfish |journal=International Review of Cell and Molecular Biology |publisher=Academic Press |volume=280 |year=2010 |pages=41–78 |issn=1937-6448 |isbn=9780123812605 |doi=10.1016/S1937-6448(10)80002-4|pmid=20797681 }}</ref> Jellyfish visual systems range from simple [[Photoreceptor cell|photoreceptive cells]] to complex image-forming eyes. More ancestral visual systems incorporate extraocular vision (vision without eyes) that encompass numerous receptors dedicated to single-function behaviors. More derived visual systems comprise perception that is capable of multiple task-guided behaviors.

Although they lack a true brain, cnidarian jellyfish have a "ring" [[nervous system]] that plays a significant role in motor and sensory activity. This net of nerves is responsible for [[muscle contraction]] and movement and culminates the emergence of photosensitive structures.<ref name="Nilsson-2013" /> Across [[Cnidaria]], there is large variation in the systems that underlie photosensitivity. Photosensitive structures range from non-specialized groups of cells, to more "conventional" eyes similar to those of [[vertebrate]]s.<ref name="Garm-2010" /> The general evolutionary steps to develop complex vision include (from more ancestral to more derived states): non-directional photoreception, directional photoreception, low-resolution vision, and high-resolution vision.<ref name="Nilsson-2013" /> Increased habitat and task complexity has favored the high-resolution visual systems common in derived cnidarians such as [[box jellyfish]].<ref name="Nilsson-2013" />

[[Basal (phylogenetics)|Basal]] visual systems observed in various cnidarians exhibit photosensitivity representative of a single task or behavior. Extraocular photoreception (a form of non-directional photoreception), is the most basic form of light sensitivity and guides a variety of behaviors among cnidarians. It can function to regulate [[circadian rhythm]] (as seen in eyeless [[hydrozoa]]ns) and other light-guided behaviors responsive to the intensity and spectrum of light. Extraocular photoreception can function additionally in positive [[phototaxis]] (in [[planula]] larvae of hydrozoans),<ref name="Garm-2010" /> as well as in avoiding harmful amounts of [[Ultraviolet|UV radiation]] via [[negative phototaxis]]. Directional photoreception (the ability to perceive direction of incoming light) allows for more complex phototactic responses to light, and likely evolved by means of [[membrane]] stacking.<ref name="Nilsson-2013" /> The resulting behavioral responses can range from guided spawning events timed by moonlight to shadow responses for potential predator avoidance.<ref name="Garm-2010" /><ref>{{cite journal |vauthors=Suga H, Tschopp P, Graziussi DF, Stierwald M, Schmid V|display-authors=3|title=Flexibly deployed ''Pax'' genes in eye development at the early evolution of animals demonstrated by studies on a hydrozoan jellyfish |journal=PNAS |date=August 10, 2010 |volume=107 |issue=32 |pages=14263–14268 |doi=10.1073/pnas.1008389107|pmid=20660753|pmc=2922549|bibcode=2010PNAS..10714263S|doi-access=free}}</ref> Light-guided behaviors are observed in numerous [[scyphozoa]]ns including the common [[Aurelia (cnidarian)|moon jelly]], ''[[Aurelia aurita]]'', which migrates in response to changes in ambient light and solar position even though they lack proper eyes.<ref name="Garm-2010" />

The low-resolution visual system of box jellyfish is more derived than directional photoreception, and thus box jellyfish vision represents the most basic form of true vision in which multiple directional photoreceptors combine to create the first imaging and [[spatial resolution]]. This is different from the high-resolution vision that is observed in [[Camera eye|camera]] or [[compound eye]]s of vertebrates and [[cephalopod]]s that rely on focusing [[optics]].<ref name="Garm-2010" /> Critically, the visual systems of box jellyfish are responsible for guiding multiple tasks or behaviors in contrast to less derived visual systems in other jellyfish that guide single behavioral functions. These behaviors include phototaxis based on sunlight (positive) or shadows (negative), obstacle avoidance, and control of swim-pulse rate.<ref name="O'Connor-2009">{{cite journal|vauthors=O'Connor M, Garm A, Nilsson DE |title=Structure and Optics of the Eyes of the Box Jellyfish Chiropsella Bronzie |journal=Journal of Comparative Physiology A |volume=195 |issue=6 |year=2009 |pages=557–569 |doi=10.1007/s00359-009-0431-x|pmid=19347342 |s2cid=9563849 }}</ref>

Box jellyfish possess "proper eyes" (similar to vertebrates) that allow them to inhabit environments that lesser derived medusae cannot. In fact, they are considered the only class in the [[clade]] ''[[Medusozoa]]'' that have behaviors necessitating spatial resolution and genuine vision.<ref name="Garm-2010" /> However, the [[lens]] in their eyes are more functionally similar to cup-eyes exhibited in low-resolution organisms, and have very little to no focusing capability.<ref>{{cite journal|vauthors=Nilsson DE, Gislén L, Coates M, Skogh C, Garm A|display-authors=3 |title=Advanced optics in a jellyfish eye |journal=Nature |volume=435 |pages=201–205 |year=2005 |issue=7039 |doi=10.1038/nature03484|pmid=15889091 |bibcode=2005Natur.435..201N |s2cid=4418085 }}</ref><ref name="O'Connor-2009" /> The lack of the ability to focus is due to the focal length exceeding the distance to the [[retina]], thus generating unfocused images and limiting spatial resolution.<ref name="Garm-2010" /> The visual system is still sufficient for box jellyfish to produce an image to help with tasks such as object avoidance.