Editing Copepod (section)

==Characteristics==
[[File:Haeckel Copepoda.jpg|thumb|left|upright|Copepods from [[Ernst Haeckel]]'s ''[[Kunstformen der Natur]]'']]
[[File:Corycaeus sp..png|thumb|upright|Two-eyed copepod of genus ''Corycaeus'']]

Copepods vary considerably, but are typically {{convert|1|to|2|mm|in|frac=32|abbr=on}} long, with a teardrop-shaped body and large [[Antenna (biology)|antennae]]. Like other crustaceans, they have an armoured [[exoskeleton]], but they are so small that in most species, this thin armour and the entire body is almost totally transparent. Some polar copepods reach {{convert|1|cm|in|frac=4|abbr=on}}. Most copepods have a single median [[compound eye]], usually bright red and in the centre of the transparent head. Subterranean species may be eyeless, and members of the genera ''Copilia'' and ''Corycaeus'' possess two eyes, each of which has a large anterior [[cuticular]] lens paired with a posterior internal lens to form a telescope.<ref>{{cite book |author=Ivan R. Schwab |url=https://books.google.com/books?id=8_U-DwAAQBAJ&pg=PA231 |title=Evolution's Witness: How Eyes Evolved |publisher=[[Oxford University Press]] |year=2012 |isbn=9780195369748 |page=231}}</ref><ref>{{cite book |author=Charles B. Miller |url=https://books.google.com/books?id=ltxNZJP5Ej4C&pg=PA122 |title=Biological Oceanography |publisher=[[John Wiley & Sons]] |year=2004 |isbn=9780632055364 |page=122}}</ref><ref>{{cite journal |author=R. L. Gregory, H. E. Ross & N. Moray |year=1964 |title=The curious eye of ''Copilia'' |url=http://www.richardgregory.org/papers/copilia/curious-eye-copilia.pdf |url-status=live |journal=[[Nature (journal)|Nature]] |volume=201 |issue=4925 |pages=1166–1168 |bibcode=1964Natur.201.1166G |doi=10.1038/2011166a0 |pmid=14151358 |archive-url=https://web.archive.org/web/20190712095721/http://www.richardgregory.org/papers/copilia/curious-eye-copilia.pdf |archive-date=2019-07-12 |access-date=2018-06-15 |s2cid=4157061}}</ref> Like other crustaceans, copepods possess two pairs of antennae; the first pair is often long and conspicuous.

Free-living copepods of the orders Calanoida, Cyclopoida, and Harpacticoida typically have a short, cylindrical body, with a rounded or beaked head, although considerable variation exists in this pattern. The head is fused with the first one or two [[thorax|thoracic]] segments, while the remainder of the thorax has three to five segments, each with limbs. The first pair of thoracic appendages is modified to form [[maxilliped]]s, which assist in feeding. The [[abdomen]] is typically narrower than the thorax, and contains five segments without any appendages, except for some tail-like "rami" at the tip.<ref name=IZ>{{cite book |author=Robert D. Barnes |year=1982 |title= Invertebrate Zoology |publisher= Holt-Saunders International |location=[[Philadelphia, Pennsylvania]] |pages=683–692 |isbn=978-0-03-056747-6}}</ref> Parasitic copepods (the other seven orders) vary widely in morphology and no generalizations are possible.

Because of their small size, copepods have no need of any [[heart]] or circulatory system (the members of the order Calanoida have a heart, but no [[blood vessel]]s), and most also lack [[gill]]s. Instead, they absorb oxygen directly into their bodies. Their excretory system consists of maxillary glands.

===Behavior===
The second pair of cephalic appendages in free-living copepods is usually the main time-averaged source of propulsion, beating like oars to pull the animal through the water. However, different groups have different modes of feeding and locomotion, ranging from almost immotile for several minutes (e.g. some [[Harpacticoida|harpacticoid copepods]]) to intermittent motion (e.g., some [[Cyclopoida|cyclopoid copepods]]) and continuous displacements with some escape reactions (e.g. most [[Calanoida|calanoid copepods]]).

[[File:Juvenile Clupea harengus feeding on copepods macrophotography video.gif|frame|Slow-motion macrophotography video (50%), taken using [[ecoSCOPE]], of juvenile [[Atlantic herring]] (38&nbsp;mm) feeding on copepods – the fish approach from below and catch each copepod individually. In the middle of the image, a copepod escapes successfully to the left.]]

Some copepods have extremely fast [[escape response]]s when a predator is sensed, and can jump with high speed over a few millimetres. Many species have [[neuron]]s surrounded by [[myelin]] (for increased conduction speed), which is very rare among [[invertebrate]]s (other examples are some [[annelid]]s and [[malacostraca]]n crustaceans like [[Palaemonidae|palaemonid]] shrimp and [[Penaeidae|penaeids]]). Even rarer, the myelin is highly organized, resembling the well-organized wrapping found in vertebrates ([[Gnathostomata]]). Despite their fast escape response, copepods are successfully hunted by slow-swimming [[seahorse]]s, which approach their prey so gradually, it senses no turbulence, then suck the copepod into their snout too suddenly for the copepod to escape.<ref>{{cite news | url=https://www.bbc.co.uk/news/science-environment-25103455 | work=BBC News | title=Seahorses stalk their prey by stealth | date=November 26, 2013 | access-date=June 20, 2018 | archive-url=https://web.archive.org/web/20171122144106/http://www.bbc.co.uk/news/science-environment-25103455 | archive-date=November 22, 2017 | url-status=live }}</ref>

Several species are [[Bioluminescence|bioluminescent]] and able to produce light. It is assumed this is an antipredatory defense mechanism.<ref>[https://academic.oup.com/plankt/article/39/3/369/3111267?login=false A light in the dark: ecology, evolution and molecular basis of copepod bioluminescence]</ref>

Finding a mate in the [[three-dimensional space]] of open water is challenging. Some copepod females solve the problem by emitting [[pheromone]]s, which leave a trail in the water that the male can follow.<ref>{{cite book |author=David B. Dusenbery |year=2009 |title=Living at Micro Scale |page=306 |publisher=[[Harvard University Press]] |location=[[Cambridge, Massachusetts]] |isbn=978-0-674-03116-6}}</ref> Copepods experience a low [[Reynolds number]] and therefore a high relative viscosity. One foraging strategy involves chemical detection of sinking [[marine snow]] aggregates and taking advantage of nearby low-pressure gradients to swim quickly towards food sources.<ref>{{cite journal|last1=Lombard|first1=F.|last2=Koski|first2=M.|last3=Kiørboe|first3=T.|title=Copepods use chemical trails to find sinking marine snow aggregates|journal=Limnology and Oceanography|date=January 2013|volume=58|issue=1|pages=185–192|doi=10.4319/lo.2013.58.1.0185|bibcode=2013LimOc..58..185L|s2cid=55896867 |url=https://backend.orbit.dtu.dk/ws/files/53238583/Lombard_et_al_final_for_print.pdf}}</ref>

===Diet===
Most free-living copepods feed directly on [[phytoplankton]], catching cells individually. A single copepod can consume up to 373,000 phytoplankton per day.<ref>{{Cite web |url=https://vineyardgazette.com/news/2018/04/26/small-beautiful-especially-copepods |title=Small Is Beautiful, Especially for Copepods - The Vineyard Gazette |access-date=2018-09-07 |archive-url=https://web.archive.org/web/20180907183250/https://vineyardgazette.com/news/2018/04/26/small-beautiful-especially-copepods |archive-date=2018-09-07 |url-status=live }}</ref> They generally have to clear the equivalent to about a million times their own body volume of water every day to cover their nutritional needs.<ref>{{Cite journal |url=https://academic.oup.com/plankt/article/33/5/677/1482868 |title=What makes pelagic copepods so successful? - Oxford Journals |journal=Journal of Plankton Research |date=May 2011 |volume=33 |issue=5 |pages=677–685 |doi=10.1093/plankt/fbq159 |access-date=2018-09-02 |archive-url=https://web.archive.org/web/20180902084531/https://academic.oup.com/plankt/article/33/5/677/1482868 |archive-date=2018-09-02 |url-status=live |last1=Kiørboe |first1=Thomas }}</ref> Some of the larger species are predators of their smaller relatives. Many benthic copepods eat organic detritus or the bacteria that grow in it, and their mouth parts are adapted for scraping and biting. Herbivorous copepods, particularly those in rich, cold seas, store up energy from their food as oil droplets while they feed in the spring and summer on [[Algal bloom|plankton blooms]]. These droplets may take up over half of the volume of their bodies in polar species.  Many copepods (e.g., fish lice like the [[Siphonostomatoida]]) are parasites, and feed on their host organisms. In fact, three of the 10 known orders of copepods are wholly or largely parasitic, with another three comprising most of the free-living species.<ref>{{cite journal|last1=Bernot|first1=J.|last2=Boxshall|first2=G.|last3=Crandall|first3=L.|title=A synthesis tree of the Copepoda: integrating phylogenetic and taxonomic data reveals multiple origins of parasitism|journal=PeerJ|date=August 18, 2021|volume=9|pages=e12034 |pmid=34466296|doi=10.7717/peerj.12034| pmc=8380027|doi-access=free }}</ref>

===Life cycle===
[[File:Mikrofoto.de-ruderfusskrebs10.jpg|thumb|left|Egg sac of a copepod]]
Most nonparasitic copepods are holoplanktonic, meaning they stay planktonic for all of their lifecycles, although harpacticoids, although free-living, tend to be benthic rather than planktonic.
During mating, the male copepod grips the female with his first pair of antennae, which is sometimes modified for this purpose. The male then produces an adhesive [[spermatophore|package of sperm]] and transfers it to the female's genital opening with his thoracic limbs. Eggs are sometimes laid directly into the water, but many species enclose them within a sac attached to the female's body until they hatch. In some pond-dwelling species, the eggs have a tough shell and can lie dormant for extended periods if the pond dries up.<ref name=IZ/>

Eggs hatch into nauplius larvae, which consist of a head with a small [[telson|tail]], but no thorax or true abdomen. The nauplius moults five or six times, before emerging as a "copepodid larva". This stage resembles the adult, but has a simple, unsegmented abdomen and only three pairs of thoracic limbs. After a further five moults, the copepod takes on the adult form. The entire process from hatching to adulthood can take a week to a year, depending on the species and environmental conditions such as temperature and nutrition (e.g., egg-to-adult time in the calanoid ''Parvocalanus crassirostris'' is ~7 days at {{convert|25|C|F}} but 19 days at {{convert|15|C|F}}.<ref>{{cite thesis |first=Thomas D. |last=Johnson |year=1987 |title=Growth and regulation of a population of ''Parvocalanus'' ''crassirostris'' in Long Island, New York |type=Ph.D. Diss |publisher=SUNY Stony Brook |oclc=19047124 }}</ref>

===Biophysics===
Copepods jump out of the water - porpoising. The biophysics of this motion has been described by Waggett and Buskey 2007 and Kim et al 2015.<ref name="Kim-et-al-2017">{{cite journal | last1=Kim | first1=Ho-Young | last2=Amauger | first2=Juliette | last3=Jeong | first3=Han-Bi | last4=Lee | first4=Duck-Gyu | last5=Yang | first5=Eunjin | last6=Jablonski | first6=Piotr G. | title=Mechanics of jumping on water | journal=Physical Review Fluids | publisher=American Physical Society (APS) | volume=2 | issue=10 | date=2017-10-17 | issn=2469-990X | doi=10.1103/physrevfluids.2.100505 | page=100505| bibcode=2017PhRvF...2j0505K }}</ref>