View source for Daphnia

== Appearance and characteristics ==
[[File:Daphnia Heartbeat.ogg|thumb|left|The beating heart of ''Daphnia'' under the microscope]]
The body of a ''Daphnia'' species is usually {{convert|1|-|5|mm|abbr=on}} long,<ref name="Ebert">{{cite book |author=Dieter Ebert |year=2005 |title=Ecology, Epidemiology, and Evolution of Parasitism in ''Daphnia'' |chapter=Introduction to ''Daphnia'' biology |location=Bethesda, MD |publisher=National Center for Biotechnology Information |isbn=978-1-932811-06-3 |url=https://www.ncbi.nlm.nih.gov/books/NBK2042/}}</ref> and is divided into [[Segmentation (biology)|segments]], although this division is not visible.<ref name="Oneida">{{cite web |url=http://www.seagrant.sunysb.edu/OLI/olei-daphnia.htm |title=''Daphnia'' |work=Oneida Lake Education Initiative |publisher=[[Stony Brook University]] |access-date=October 9, 2013}}</ref> The head is fused, and is generally bent down towards the body with a visible notch separating the two. In most species, the rest of the body is covered by a [[carapace]], with a ventral gap in which the five or six pairs of legs lie.<ref name="Oneida"/> The most prominent features are the [[compound eye]]s, the second [[antenna (biology)|antennae]], and a pair of abdominal [[seta]]e.<ref name="Oneida"/> In many species, the carapace is translucent or nearly so, so they make excellent subjects for the [[microscope]], as one can observe the beating heart.<ref name="Oneida"/>
 
Even under relatively low-power [[microscopy]], the feeding mechanism can be observed, with immature young moving in the brood pouch; moreover, the eye being moved by the [[ciliary muscle]]s can be seen, as well as [[blood cell]]s being pumped around the [[circulatory system]] by the simple heart.<ref name="Oneida"/> The heart is at the top of the back, just behind the head, and the average heart rate is about 180&nbsp;bpm under normal conditions. ''Daphnia'' spp., like many animals, are prone to [[ethanol|alcohol]] intoxication as well as caffeine effects, which makes them excellent subjects for studying the effects of the depressant and stimulants on the [[nervous system]] due to the translucent exoskeleton and the visibly altered heart rate. They are tolerant of being observed live under a coverslip and appear to suffer no harm when returned to open water.<ref name="Oneida"/> This experiment can also be performed using [[nicotine]], or [[Epinephrine|adrenaline]], each producing an increase in the heart rate. {{Citation needed|reason=Literature with evidence required|date=October 2018}}

Due to its intermediate size, ''Daphnia'' spp. use both diffusion and circulatory methods, producing [[hemoglobin]] in low-oxygen environments.<ref name="Hemoglobin Sometimes">{{Cite journal |url=http://jeb.biologists.org/content/207/25/i.2 |last=van Bergen |first=Yfke |title=Total Recall |date=2004 |journal=Journal of Experimental Biology |volume=207 |issue=25 |page=i |doi=10.1242/jeb.01364 |bibcode=2004JExpB.207R...1V |s2cid=219208202 |access-date=2018-04-11|url-access=subscription }}</ref>