Clunio-pedia
Hands-on information in a nutshell
This is an introduction to Clunio midges, which may help you to find these moonstruck critters on your coast. In case you are interested in further details, you can consult our bibliography of scientific publications on Clunio (including e.g. all species descriptions).
Contents
Habitat
Clunio midges are usually found in the intertidal zone (i.e. the area between the low and high tide marks) on rocky coasts. The larvae live in patches of turfing algae which are only exposed at low tide. The adults also only swarm and reproduce at low tide. You will find them hovering on the water surface or crawling on the exposed algae and rocks.
While this is true for the well-studied Clunio marinus (Europe) and Clunio tsushimensis (Japan), there are notable exceptions:
- Clunio marshalli from Florida (USA) was reported from a tidal mud flat, where the larvae lived on conches and barnacles.
- An Arctic population of Clunio marinus was also reported from a mud flat in Tromso (Norway).
- In the tide-free Baltic Sea, Clunio adults swarm on the open water (often kilometres offshore) and the larvae live in red algae down to a depth of 20 metres.
- There is a single observation of Clunio from a freshwater stream in Hawaii, several kilometres from the coast.
So while your chances of finding Clunio are best in the rocky intertidal, you may also find them in other marine habitats.
Life Cycle
Clunio larvae are marine and want to be permanently submerged. Therefore they usually live at the lowest possible levels of the intertidal or at the edges of rock pools. In Clunio marinus, development takes 6-16 weeks in the laboratory, but depending on climate and latitude this may differ in the field - it could be faster in southers places, and it is known to last one year in Arctic Clunio marinus populations. It may also differ between species.
The circalunar clock (see rhythms) ensures that pupae ready to emerge are only present during spring tide days.
On these days, adults then emerge into one of the two daily low tides. Their life-span is only a few hours. They reproduce immediately, lay their eggs and often die before the tide returns. The eggs are layed in a gelatinous tube (with 50-150 eggs) and these are glued to algae. At 20°C the embryos take about 4 days to develop before the new larvae hatch.
Rhythms
We assume that in most species of the genus Clunio the presence of adults is strongly rhythmic - adults will usually only be found around full moon or new moon, i.e. during the time when the low tides are loewst (“spring tide days”). Only then the adult midges can reach the lowest level of the intertidal for oviposition. This lunar rhythm is controlled by a biological clock, meaning the midges can anticipate lunar phase for months in advance, which is one of the major reasons we study these midges. Notably, some populations emerge just at new moon, some just at full moon and some at both.
On spring tide days, the midges usually emerge only during one of the two daily low tides. If a place has uneaqual tides, the midges usually emerge during the lower low tide. This has been described for Clunio tsushimensis in Japan, and we could show the same phenomenon for Clunio californiensis - only based on iNaturalist records.
There can be many exceptions to these generalisations:
- In some places in Brittany (France) several strains of Clunio marinus were found to co-exist, so that midges could be found during both daily low tides.
- Clunio purpureus from Japan and Arctic populations of Clunio marinus are reported to emerge and reproduce during every low tide, that is in a tidal rhythm.
- Clunio balticus from the tide-free baltic sea emerges every day at dusk.
So depending on the conditions at your local coast, Clunio rhythms could be very different.
Maybe you can discover yet another local adaptation!??
If you want to know more about adult emergence rhythms in Clunio marinus, please have a look at the following video (English subtitles available, audio track in German):
Explanatory video on adult emergence rhythms in Clunio marinus
© Neumann, Dietrich: Semilunarperiodische Fortpflanzung von Clunio marinus - Biologische Zeitmessung in der Gezeitenzone. IWF (Göttingen), 1972. https://doi.org/10.3203/IWF/C-1091
Sexual Dimorphism & Mating
Sexual dimorphism means that males and females of the same species differ in certain characteristics. Humans, for example, differ not only in their genitals, but many other characteristics, such as average height or average hip proportions.
Clunio species are strongly sexually dimorphic. Females have very much reduced appendages: They are wingless, and have small legs, antennae and eyes.
This is reflected in the mating behaviour: The males usually emerge 30-60 minutes before the females (yet another instance of sexual dimorphism). They patrol the water surface for emerging female pupae. The females are usually unable to shed the pupal skin on their own. Instead, the males use the forceps-like structures (called “gonostyli”) at their well developped rear end (called “hypopygium”) to shed the female pupal skin and then mate immediately. The males carry the females during a short nuptial flight (see watergliding) and then deposit them on emerged algae for oviposition.
Below:
Slow motion video of mating behaviour in Clunio marinus (before 1:56 you can also see eclosion of the male)
© Neumann, Dietrich; Dordel, Hans-Jürgen: Clunio marinus (Chironomidae) - Schlüpfen und Kopulation. IWF (Göttingen), 1971. https://doi.org/10.3203/IWF/E-1798
Watergliding
Clunio males can fly - but usually they don’t! Instead, they glide on the water surface. Their legs continuously touch the water and the wings are only used as propellers to move forward. Clunio males can be quite fast - we estimate up to 1 m/s (but this has not been measured). During mating, the females are carried by the males in what is called a “nuptial flight”, but this also happens exclusively hovering on the water surface. These swarming copulae can be identified in the field by the fact that they are much slower than the males alone.