THE CLADOCERANS
GENERAL DESCRIPTION
REPRODUCTION
FEEDING
CULTIVATION
GENERAL DESCRIPTION

Cladocerans are tiny aquatic crustaceans also known as water fleas. In contrast to the prosperity of cladocerans in freshwater systems, with more than 600 recorded species, marine cladocerans show a very low diversity, with only eight truly marine species. These 8 species can be divided in two distinct groups, the Podonidae (represented by the genera Evadne, Pleopis, Podon and Pseudevadne), and the Sididae (with only one species, Penilia avirostris). In spite of this low diversity, they have a cosmopolitan distribution and are common in warm and temperate waters all around the world.

 
Marine cladocerans typically inhabit surface neritic waters, although can also be found offshore due to advective transport. Characteristic reproductive strategies (parthenogenesis) and short generation times lead, under the right conditions, to fast population blooms. Once fully developed, these populations are dependent on favorable environmental factors, usually linked to vertical stability (driven either by high surface temperatures and summer stratification, or by freshwater dilution). In the Catalan coast (NW Mediterranean) periods of high abundance of cladocera have been reported during the summer stratification period, their biomass and importance in biogeochemical cycles eclipsing that of copepods.

Podon leuckarti

 
 
REPRODUCTION
Marine cladocerans have a very complicated reproductive cycle due to the alternation of diploid parthenogenesis (i.e. development of a diploid individual from a female gamete without fertilization by a male gamete) and sexual reproduction (i.e. with the fertilization of a female gamete by a male gamete).
Marine podonids exhibit also paedogenesis (i.e. the initiation of reproduction by young individuals before adulthood), which further accelerates population growth. Their development is direct, with no metamorphosis.
Parthenogenetic females of cladocerans carry their embryos on the back. On the left, Penilia avirostris; on the right, Podon leuckarti. Notice the developed eyes in the embryos of Podon.
Under favourable conditions, cladocerans typically reproduce by parthenogenesis. Parthenogenic females produce eggs that are carried in a brood chamber, where they develop into new parthenogenetic females and so on. At certain times, usually at the end of their season and very likely timed with a deterioration in optimal growth conditions, females change their reproductive pattern and start producing two different categories of eggs. Some of them will develop into diploid males, while others (haploid) will eventually be fertilized by the males after copulation. These fertilized eggs begin their cleavage, but soon they arrest their development and become resting eggs. Resting eggs are released when the female moults and usually sink to the bottom.They remain in the sediment until development resumes, typically at the start of the next season, producing again parthenogenetic females. Thus parthenogenetic reproduction seems to dominate under optimal growth conditions and is responsible for the bloom of cladoceran populations, whereas sexual reproduction appears in advance to decline in population numbers, under disadvantageous conditions, and ensures genetic variability for the next season.
Male of the cladoceran Penilia avirostris. Males are rare in cladocerans, and appear in populations to allow, through sexual reproduction, the formation of resting eggs.
Female of Penilia avirostris. After sexual reproduction, cladoceran females carry a resting egg instead of a developing embryo. The black mass on the back is the resting egg.
 
FEEDING

The actual knowledge on the feeding behaviour of marine cladocerans is scarce and presents serious gaps. Aspects like the composition of their diet in the field or the rate at which food is ingested are still poorly known. From an ecological point of view, their trophic impact on microbial communities is not ascertained yet.

Podonids seem to be raptorial feeders, i.e. perceive, attack and seize individual prey in a size range of 20-170 µm. Their well developed eyes may help to distinguish food items, and suggest a behaviour based on visual predation.

Composed eye of Podon leuckarti
 
On the contrary, Penilia is a suspension feeder. Suspension feeders create, by the movement of their feeding appendages, a water current (feeding current) to entrain and transport food items towards them. Particles as small as 2 µm can be cleared from the feeding current by Penilia.
On the left picture, the feeding appendages or phyllopodial limbs of Penilia avirostris, feather-like, can be seen by transparency. On the right picture a detail of the appendages is shown. They appear comb-like, carrying arrays of setulae. Notice the narrow space between the setulae.
 
CULTIVATION
Compared to freshwater cladocerans, the maintenance and cultivation of marine cladocerans in the laboratory is a much more complex subject. Marine cladocerans are fragile organisms, difficult to be collected alive and in good shape. Another problem is their hydrophobic carapace, which makes them prone to get trapped in the air/water interfase (e.g. in the laboratory) and eventually die. Finally, the lack of knowldge on their diet has made it impossible to find adequate food to raise them in culture. Even when provided with fresh supplies of seawater, they cannot be maintained in the laboratory for longer than a few weeks. Progress in rearing marine cladocerans under laboratory conditions will help to explore more precisely the ecophysiology and ecology of this group of plankters, and determine their role in the trophodynamic pathways of the pelagic realm.
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