Editing Jellyfish (section)

=== Box jellyfish eye ===
The study of jellyfish eye evolution is an intermediary to a better understanding of how visual systems evolved on Earth.<ref name="Nilsson-2013">{{cite journal|author=Nilsson, DE |year=2013 |title=Eye evolution and its functional basis |journal=Visual Neuroscience |volume=30 |issue=1–2 |pages=5–20 |doi=10.1017/S0952523813000035|pmid=23578808 |pmc=3632888 }}</ref> Jellyfish exhibit immense variation in visual systems ranging from photoreceptive cell patches seen in simple photoreceptive systems to more derived complex eyes seen in box jellyfish.<ref name="Nilsson-2013" /> Major topics of jellyfish visual system research (with an emphasis on box jellyfish) include: the evolution of jellyfish vision from simple to complex visual systems), the eye morphology and molecular structures of box jellyfish (including comparisons to vertebrate eyes), and various uses of vision including task-guided behaviors and niche specialization.

==== 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.

==== Utility as a model organism ====
Box jellyfish eyes are a visual system that is sophisticated in numerous ways. These intricacies include the considerable variation within the [[Morphology (biology)|morphology]] of box jellyfishes' eyes (including their task/behavior specification), and the [[Molecule|molecular]] makeup of their eyes including: photoreceptors, [[opsin]]s, lenses, and [[synapse]]s.<ref name="Garm-2010" /> The comparison of these attributes to more derived visual systems can allow for a further understanding of how the evolution of more derived visual systems may have occurred, and puts into perspective how box jellyfish can play the role as an [[Evolutionary developmental biology|evolutionary/developmental]] model for all visual systems.<ref name="Piatigorsky-2004">{{cite journal|vauthors=Piatigorsky J, Kozmik Z |title=Cubozoan jellyfish: an Evo/Devo model for eyes and other sensory systems |journal=Int. J. Dev. Biol. |volume=48 |pages=719–729 |year=2004 |issue=8–9 |doi=10.1387/ijdb.041851jp|pmid=15558464 |doi-access=free }}</ref>

===== Characteristics =====
Box jellyfish visual systems are both diverse and complex, comprising multiple [[photosystem]]s.<ref name="Garm-2010" /> There is likely considerable variation in visual properties between species of box jellyfish given the significant [[Hybrid (biology)|inter-species]] morphological and [[Physiology|physiological]] variation. Eyes tend to differ in size and shape, along with number of receptors (including [[opsin]]s), and physiology across species of box jellyfish.<ref name="Garm-2010" />

Box jellyfish have a series of intricate lensed eyes that are similar to those of more derived multicellular organisms such as vertebrates. Their 24 eyes fit into four different morphological categories.<ref name="Gray-2009">{{cite journal|vauthors=Gray GC, Martin VJ, Satterlie RA |title=Ultrastructure of the retinal synapses in cubozoans |journal=Biol Bull |date= Aug 2009 |volume=217 |issue=1 |pages=35–49 |doi=10.1086/BBLv217n1p35 |pmid=19679721|s2cid=24400231 |url=http://libres.uncg.edu/ir/uncw/f/grayg2007-1.pdf }}</ref> These categories consist of two large, morphologically different medial eyes (a lower and upper lensed eye) containing spherical lenses, a lateral pair of pigment slit eyes, and a lateral pair of pigment pit eyes.<ref name="O'Connor-2009" /> The eyes are situated on [[Rhopalium|rhopalia]] (small sensory structures) which serve sensory functions of the box jellyfish and arise from the cavities of the exumbrella (the surface of the body) on the side of the bells of the jellyfish.<ref name="Garm-2010" /> The two large eyes are located on the mid-line of the club and are considered complex because they contain lenses. The four remaining eyes lie laterally on either side of each rhopalia and are considered simple. The simple eyes are observed as small invaginated cups of [[epithelium]] that have developed [[pigment]]ation.<ref name="Berger-1898">{{cite journal|author=Berger, Edward W |title=The Histological Structure of the Eyes of Cubomedusae |journal=Journal of Comparative Neurology |volume=8 |issue=3 |year=1898 |pages=223–230 |doi=10.1002/cne.910080317|s2cid=85422599 |url=https://zenodo.org/record/2067109 }}</ref> The larger of the complex eyes contains a cellular [[cornea]] created by a mono ciliated epithelium, cellular lens, homogenous capsule to the lens, [[vitreous body]] with prismatic elements, and a [[retina]] of pigmented cells. The smaller of the complex eyes is said to be slightly less complex given that it lacks a capsule but otherwise contains the same structure as the larger eye.<ref name="Berger-1898" />

Box jellyfish have multiple photosystems that comprise different sets of eyes.<ref name="Garm-2010" /> Evidence includes [[Immunocytochemistry|immunocytochemical]] and molecular data that show [[photopigment]] differences among the different morphological eye types, and physiological experiments done on box jellyfish to suggest behavioral differences among photosystems. Each individual eye type constitutes photosystems that work collectively to control visually guided behaviors.<ref name="Garm-2010" />

Box jellyfish eyes primarily use c-PRCs (ciliary photoreceptor cells) similar to that of vertebrate eyes. These cells undergo [[phototransduction]] cascades (process of light absorption by photoreceptors) that are triggered by c-opsins.<ref>{{cite journal|first1=Hiroshi|last1=Suga|first2=Volker|last2=Schmid|first3=Walter J.|last3=Gehring |title=Evolution and Functional Diversity of Jellyfish Opsins |journal=Current Biology |volume=18 |issue=1 |year=2008 |pages=51–55 |issn=0960-9822 |doi=10.1016/j.cub.2007.11.059|pmid=18160295 |s2cid=13344739 |doi-access=free |bibcode=2008CBio...18...51S }}</ref> Available opsin sequences suggest that there are two types of opsins possessed by all cnidarians including an ancient [[Phylogenetics|phylogenetic]] opsin, and a sister ciliary opsin to the c-opsins group. Box jellyfish could have both ciliary and cnidops (cnidarian opsins), which is something not previously believed to appear in the same retina.<ref name="Garm-2010" /> Nevertheless, it is not entirely evident whether cnidarians possess multiple opsins that are capable of having distinctive [[Spectral sensitivity|spectral sensitivities]].<ref name="Garm-2010" />

===== Comparison with other organisms =====
Comparative research on genetic and molecular makeup of box jellyfishes' eyes versus more derived eyes seen in vertebrates and cephalopods focuses on: lenses and [[crystallin]] composition, [[synapse]]s, and [[Pax genes]] and their implied evidence for shared primordial (ancestral) genes in eye evolution.<ref name="Piatigorsky-1989">{{cite journal|vauthors=Piatigorsky J, Horwitz J, Kuwabara T, Cutress C |title=The Cellular Eye Lens and Crystallins of Cubomedusan Jellyfish |journal=Journal of Comparative Physiology A |volume=164 |issue=5 |year=1989 |pages=577–587 |doi=10.1007/bf00614500|pmid=2565398 |s2cid=19797109 }}</ref>

Box jellyfish eyes are said to be an evolutionary/developmental model of all eyes based on their evolutionary recruitment of crystallins and Pax genes.<ref name="Piatigorsky-2004" /> Research done on box jellyfish including ''[[Tripedalia cystophora]]'' has suggested that they possess a single Pax gene, PaxB. PaxB functions by binding to crystallin promoters and activating them. PaxB [[in situ hybridization]] resulted in PaxB expression in the lens, retina, and [[statocyst]]s.<ref name="Piatigorsky-2004" /> These results and the rejection of the prior hypothesis that Pax6 was an ancestral Pax gene in eyes has led to the conclusion that PaxB was a primordial gene in eye evolution, and that the eyes of all organisms likely share a common ancestor.<ref name="Piatigorsky-2004" />

The lens structure of box jellyfish appears very similar to those of other organisms, but the crystallins are distinct in both function and appearance.<ref name="Piatigorsky-1989" /> Weak reactions were seen within the sera and there were very weak sequence similarities within the crystallins among vertebrate and invertebrate lenses.<ref name="Piatigorsky-1989" /> This is likely due to differences in lower molecular weight proteins and the subsequent lack of [[Immunology|immunological]] reactions with [[Antiserum|antisera]] that other organisms' lenses exhibit.<ref name="Piatigorsky-1989" />

All four of the visual systems of box jellyfish species investigated with detail (''[[Carybdea marsupialis]], [[Chiropsalmus quadrumanus]], [[Tamoya haplonema]] and Tripedalia cystophora'') have invaginated synapses, but only in the upper and lower lensed eyes. Different densities were found between the upper and lower lenses, and between species.<ref name="Gray-2009" /> Four types of chemical synapses have been discovered within the rhopalia which could help in understanding neural organization including: clear unidirectional, dense-core unidirectional, clear bidirectional, and clear and dense-core bidirectional. The synapses of the lensed eyes could be useful as markers to learn more about the neural circuit in box jellyfish retinal areas.<ref name="Gray-2009" />

==== Evolution as a response to natural stimuli ====
The primary adaptive responses to environmental variation observed in box jellyfish eyes include pupillary constriction speeds in response to light environments, as well as photoreceptor tuning and lens [[adaptation]]s to better respond to shifts between light environments and darkness. Some box jellyfish species' eyes appear to have evolved more focused vision in response to their habitat.<ref name="Seymour-2020">{{cite journal|last1=Seymour|first1=Jamie E.|first2=Emily P.|last2=O'Hara |title=Pupillary Response to Light in Three Species of Cubozoa (Box Jellyfish) |journal=Plankton and Benthos Research |volume=15 |issue=2 |year=2020 |pages=73–77 |doi=10.3800/pbr.15.73|s2cid=219759193 |doi-access=free }}</ref>

Pupillary contraction appears to have evolved in response to variation in the light environment across [[ecological niche]]s across three species of box jellyfish (''[[Chironex fleckeri]]'', ''[[Chiropsella bronzie]]'', and ''[[Carukia barnesi]]''). Behavioral studies suggest that faster pupil contraction rates allow for greater object avoidance,<ref name="Seymour-2020" /> and in fact, species with more complex habitats exhibit faster rates. ''Ch. bronzie'' inhabit shallow beach fronts that have low visibility and very few obstacles, thus, faster pupil contraction in response to objects in their environment is not important. ''Ca. barnesi'' and ''Ch. fleckeri'' are found in more three-dimensionally complex environments like [[mangrove]]s with an abundance of natural obstacles, where faster pupil contraction is more adaptive.<ref name="Seymour-2020" /> Behavioral studies support the idea that faster pupillary contraction rates assist with obstacle avoidance as well as depth adjustments in response to differing light intensities.

Light/dark adaptation via pupillary light reflexes is an additional form of an evolutionary response to the light environment. This relates to the pupil's response to shifts between light intensity (generally from sunlight to darkness). In the process of light/dark adaptation, the upper and lower lens eyes of different box jellyfish species vary in specific function.<ref name="O'Connor-2009" /> The lower lens-eyes contain pigmented photoreceptors and long pigment cells with dark pigments that migrate on light/dark adaptation, while the upper-lens eyes play a concentrated role in light direction and phototaxis given that they face upward towards the water surface (towards the sun or moon).<ref name="O'Connor-2009" /> The upper lens of ''Ch. bronzie'' does not exhibit any considerable optical power while ''Tr. cystophora'' (a box jellyfish species that tends to live in mangroves) does. The ability to use light to visually guide behavior is not of as much importance to ''Ch. bronzie'' as it is to species in more obstacle-filled environments.<ref name="O'Connor-2009" /> Differences in visually guided behavior serve as evidence that species that share the same number and structure of eyes can exhibit differences in how they control behavior.