Editing Nudibranch (section)

==Defence mechanisms==
In the course of their evolution, nudibranchs have lost their shells, while developing alternative defence mechanisms. Some species evolved an external anatomy with textures and colours that mimicked surrounding sessile invertebrate animals (often their prey sponges or soft corals) to avoid predators with [[camouflage]]. Other nudibranchs, as seen especially well on [[Chromodoris quadricolor|''Chromodoris'' ''quadricolor'']], have an intensely bright and contrasting colour pattern that makes them especially conspicuous in their surroundings. Nudibranch molluscs are the most commonly cited examples of [[aposematism]] in marine ecosystems, but the evidence for this has been contested,<ref>{{Cite journal |title=Does warning colouration occur in nudibranchs? |last=Edmunds |first=M. |date=1991 |journal=Malacologia |volume=32 |pages=241–255}}</ref> mostly because few examples of [[mimicry]] are seen among species, many species are nocturnal or cryptic, and bright colours at the red end of the spectrum are rapidly attenuated as a function of water depth. For example, the Spanish dancer nudibranch (genus ''[[Hexabranchus]]''), among the largest of tropical marine slugs, potently chemically defended, and brilliantly red and white, is nocturnal and has no known mimics.<ref name="Hexabranchus">{{Cite journal|title = Defensive chemicals of the Spanish Dancer nudibranch, Hexabranchus sanguineus, and its egg ribbons: Macrolides derived from a sponge diet|last = Pawlik|first = JR|date = 1988|journal = Journal of Experimental Marine Biology and Ecology|doi = 10.1016/0022-0981(88)90225-0|volume = 119|issue = 2|pages = 99–109| bibcode=1988JEMBE.119...99P |display-authors=etal}}</ref> Other studies of nudibranch molluscs have concluded they are aposematically coloured, for example, the slugs of the family Phylidiidae from Indo-Pacific coral reefs.<ref>{{Cite journal |title=Marine benthic invertebrates use multimodal cues for defence against reef fish |journal=Marine Ecology Progress Series |date=2007 |pages=29–39 |volume=340 |doi=10.3354/meps340029 |first1=R. |last1=Ritson-Williams |first2=VJ |last2=Paul |bibcode=2007MEPS..340...29R |doi-access=free}}</ref>

Nudibranchs that feed on hydrozoids can store the hydrozoids' [[nematocyst]]s (stinging cells) in the [[dorsum (biology)|dorsal]] body wall, the [[cerata]].<ref>{{cite journal |author=Frick, K |title=Predator Suites and Flabellinid Nudibranch Nematocyst Complements in the Gulf of Maine |journal=In: SF Norton (Ed). Diving for Science...2003. |volume=Proceedings of the American Academy of Underwater Sciences |issue=22nd Annual Scientific Diving Symposium |year=2003 |url=http://archive.rubicon-foundation.org/4744 |archive-url=https://web.archive.org/web/20090129210100/http://archive.rubicon-foundation.org/4744 |url-status=usurped |archive-date=January 29, 2009 |access-date=2008-07-03 }}</ref> These stolen nematocysts, called [[kleptocnidae]], wander through the [[alimentary tract]] without harming the nudibranch. Once further into the organ, the cells are assimilated by intestinal protuberances and brought to specific placements on the creature's hind body. The specific mechanism by which nudibranchs protect themselves from the hydrozoids and their nematocysts is yet unknown, but special cells with large [[vacuole]]s probably play an important role. Similarly, some nudibranchs can also take in plant cells (symbiotic algae from soft corals) and reuse these to make food for themselves. The related group of [[sacoglossa]]n sea slugs feed on algae and retain just the chloroplasts for their own photosynthetic use, a process known as [[kleptoplasty]]. Some of these species have been observed practising [[autotomy]], severing portions of their body to remove parasites, and have been observed to regrow their whole body from their head if decapitated.<ref name="urlExtreme autotomy and whole-body regeneration in photosynthetic sea slugs: Current Biology">{{cite web | url = https://www.cell.com/current-biology/fulltext/S0960-9822(21)00047-6?_returnURL=https%3A%2F%2Flinkinghub.elsevier.com%2Fretrieve%2Fpii%2FS0960982221000476%3Fshowall%3Dtrue | title = Extreme autotomy and whole-body regeneration in photosynthetic sea slugs: Current Biology | format = | accessdate = }}</ref>

Nudibranchs use a variety of chemical defences to aid in protection,<ref name="Karuso 1987">{{cite book |title=Bioorganic Marine Chemistry |volume=1 |last=Karuso |first=P. |publisher=Springer-Verlag |year=1987 |isbn=978-3-642-72728-3 |pages=31–60 |editor-last=Scheuer |editor-first=PJ |doi=10.1007/978-3-642-72726-9_2 |chapter=Chemical Ecology of the Nudibranchs }}, a comprehensive review of the chemical ecology of the nudibranchs</ref> but the strategy need not be lethal to be effective; in fact, good arguments exist that chemical defences should evolve to be distasteful rather than toxic.<ref>{{Cite book |title=Antipredatory defensive roles of natural products from marine invertebrates. |last=Pawlik |first=JR |publisher=Springer Science |year=2012 |location=NY |pages=677–710 |work=Handbook of Marine Natural Products |editor-last=Fattorusso |editor-first=E. |display-editors=etal}}</ref> Some sponge-eating nudibranchs concentrate the chemical defences from their prey sponge in their bodies, rendering themselves distasteful to predators.<ref name="Hexabranchus" /><ref name="Gosliner 1987">{{cite book |last=Gosliner |first=T. M. |year=1987 |title=Nudibranchs of Southern Africa |publisher=Sea Challengers |isbn=978-0-930118-13-6 }}</ref> One method of chemical defense used by nudibranchs are secondary metabolites, which play an important role in mediating relationships among marine communities.<ref name="auto">{{Cite journal |last1=Avila |first1=C |last2=Iken |first2=K |last3=Fontana |first3=A |last4=Cimino |first4=G |date=2000-09-05 |title=Chemical ecology of the Antarctic nudibranch Bathydoris hodgsoni Eliot, 1907: defensive role and origin of its natural products |url=https://www.sciencedirect.com/science/article/pii/S0022098100002276 |journal=Journal of Experimental Marine Biology and Ecology |language=en |volume=252 |issue=1 |pages=27–44 |doi=10.1016/S0022-0981(00)00227-6 |pmid=10962063 |bibcode=2000JEMBE.252...27A |issn=0022-0981|url-access=subscription }}</ref> The evidence that suggests the chemical compounds used by dorid nudibranchs do in fact come from dietary sponges lies in the similarities between the metabolites of prey and nudibranchs, respectively. Furthermore, nudibranchs contain a mixture of sponge chemicals when they are in the presence of multiple food sources, as well as change defence chemicals with a concurrent change in diet.<ref name="Faulkner and Ghiselin 1983">{{cite journal |last1=Faulkner |first1=D. J. |first2=M. T. |last2=Ghiselin |year=1983 |title=Chemical defence and evolutionary ecology of dorid nudibranchs and some other opisthobranch gastropods |journal=Marine Ecology Progress Series |volume=13 |pages=295–301 |doi=10.3354/meps013295 |bibcode=1983MEPS...13..295F |doi-access=free }}</ref> This, however, is not the only way for nudibranchs to develop chemical defences. Certain Antarctic marine species defense mechanisms are believed to be controlled by biological factors like predation, competition, and selective pressures.<ref name="auto"/> Certain species can produce their own chemicals ''de novo'' without dietary influence. Evidence for the different chemical production methods comes with the characteristic uniformity of chemical composition across drastically different environments and geographic locations found throughout ''de novo'' production species compared to the wide variety of dietary and environmentally dependent chemical composition in sequestering species.<ref name="Barsby et al. 2002">{{cite journal |last1=Barsby |first1=T. |first2=R. G. |last2=Linington |first3=R. J. |last3=Andersen |year=2002 |title=De Novo terpenoid biosynthesis by the dendronotid nudibranch Melibe leonina |journal=Chemoecology |volume=12 |issue=4 |pages=199–202 |doi=10.1007/PL00012669 |bibcode=2002Checo..12..199B |s2cid=35384332 }}</ref>

Another protection method is releasing the [[ugdon acid]] from the skin.<ref name="Edmunds 1968">{{cite journal |last=Edmunds |first=M. |year=1968 |title=Acid secretion in some species of Doridacea (Mollusca, Nudibranchia) |journal=Proceedings of the Malacological Society of London |volume=38 |issue=2 |pages=121–133 |url=http://mollus.oxfordjournals.org/content/38/2/121.extract |archive-url=https://archive.today/20130415132743/http://mollus.oxfordjournals.org/content/38/2/121.extract |url-status=dead |archive-date=2013-04-15 }}</ref> Once the specimen is physically irritated or touched by another creature, it will release the mucus automatically, eating the animal from the inside out.

===Apparent production of sound===
In 1884, [[Philip Henry Gosse]] reported observations by "Professor Grant" (possibly [[Robert Edmond Grant]]) that two species of nudibranchs emit sounds that are audible to humans.<ref>P.H. Gosse, ''Evenings at the Microscope'', 1884 edition,[https://archive.org/stream/eveningsatmicros00goss#page/56/mode/2up/search/Professor+Grant] p57</ref>

<blockquote>Two very elegant species of Sea-slug, viz., ''Eolis punctata'' [i.e. ''[[Facelina annulicornis]]''], and ''Tritonia arborescens'' [i.e. ''[[Dendronotus frondosus]]''], certainly produce audible sounds. Professor Grant, who first observed the interesting fact in some specimens of the latter, which he was keeping in an aquarium, says of the sounds that 'they resemble very much the clink of a steel wire on the side of the jar, one stroke only been given at a time, and repeated at intervals of a minute or two; when placed in a large basin of water, the sound is much obscured and is like that of a watch, one stroke being repeated, as before, at intervals. The sound is longest and most often repeated when the ''Tritonia'' are lively and moving about and is not heard when they are cold and without any motion; in the dark, I have not observed any light emitted at the time of the stroke; no globule of air escapes to the surface of the water, nor is any ripple produced on the surface at the instant of the stroke; the sound, when in a glass vessel, is mellow and distinct.' The Professor has kept these ''Tritonia'' alive in his room for a month. During the whole period of their confinement, they have continued to produce the sounds with very little diminution of their original intensity. In a small apartment, they are audible at a distance of twelve feet. The sounds obviously proceed from the mouth of the animal, and at the instant of the stroke, we observe the lips suddenly separate as if to allow the water to rush into a small vacuum formed within. As these animals are hermaphrodites, requiring mutual impregnation, the sounds may possibly be a means of communication between them, or, if they are of an electric nature, they may be the means of defending from foreign enemies, one of the most delicate, defenceless, and beautiful Gasteropods that inhabit the deep.</blockquote>