Two freshwater flatworms

The garden barrel is one place where some invertebrate activity is guaranteed any time of the year. Snails graze algae in the winter and Water Slaters, Asellus aquaticus, munch the decayed leaves. There are predators too: flatworms. These innocent looking, slug-like animals, covered in sticky mucus slither on the surfaces of leaves looking for prey. Today I found two species, a small, dark one, probably Dugesia lugubris (top) and a large white one, Dendrocoelum lacteum (bottom). Both species shown here have a pair of simple eye spots, other species have more eyespots.

The LED lights of the hand held microscope reflect on the water surface and bring out the colour of the branching gastrovascular system of Dendrocoelum lacteum. The white band in the middle of the body is the evertible pharynx, on one end of which is the mouth. Small organisms, including Water Slaters, are often prey of flatworms, and are subdued with the mucus and wrappen on the flexible bodies of the flatworm.

More information

Richard Fitter & Richard Manuel 1986 Collins field guide to Freshwater life. 

Helen Mellanby 1975 Animal life in freshwater life. Chapman & Hall.

Marsupial water slaters

 Water slaters or water-lice, Asellus aquaticus, scurry about the leaf litter of my indoor pond. They are fascinating pond inhabitants, always active, sometimes three individuals walking in a line across the tank walls. Most of the water slaters had broken antennae when I collected them, but they have now grown back, the new segments were still thin and transparent a few weeks ago (above), but have now fully developed. Unlike insects, which have very limited regeneration abilities, crustaceans can regenerate lost or broken appendages - as even when adults they carry on molting, and in each molt, they regenerate a bit more of the missing leg or antenna.

Water slaters tolerate a wide range of ecological conditions and are distributed across much of temperate Europe, Russia and North America. They are often abundant in polluted water and live in lakes, rivers, springs and even caves - where albino subspecies with smaller eyes have evolved. Being scavengers, they just need some detritus and fallen leaves and organic material to subsist.

In the last few days, a female has laid eggs into her brood pouch or marsupium. While she climbed over the leaves beside the tank wall, I took a photo where her yellow eggs are visible through the transparent brood pouch.

The marsupium is made of overlapping flat blades coming out of the female's anterior four pair of legs, and allows the mother to hold the brood under her body. This structure is a common feature of all Isopods - which include also terrestrial woodlice. The female not only carries the eggs but also the developing larvae. The larvae will emerge as miniature versions of the adults (see the figure below).

Fig. I. Marsupial stages of Asellus aquaticus. Top plate: A, stage A showing two membranes (arrows). B, stage B showing one membrane (arrow) and cleft yolk mass. C, stage B shedding membrane to reveal lateral outgrowth (arrow). D. stage C with its comma shape surrounded by a membrane (arrow). Bottom plate: A, stage C shedding membrane (arrow) to reveal stage D with free appendages. B, stage D with lateral outgrowth (arrow) and appendages covered i n a closely-fitting membrane. C, stage E showing setose body and appendages (arrows). It is at this stage that the juvenile leaves the marsupium to become free-living (from Holdich & Tolba 1981)

Occasionally females expel some larvae from the pouch - possibly due to physical limitations of how many she can carry as they grow - and larvae can also develop normally outside the marsupium from developmental stage B. Probably they need the ventilation and extra oxygen produced by the mother in the initial stage. Carrying the eggs and larvae is also bound to offer them some protection from predators, so it probably evolved as a form of maternal care.

Although water slaters normally stop reproducing during the coldest months of the year, my indoor pond is warmer than outside, and this might have triggered the start of reproduction; a situation that also occurs in wild populations downstream thermal power stations where water is warm all year round. 

More information

Vitagliano, G., Fano, E., Marchetti, E., Colangelo, M., & Vitagliano, E. (1991). Importance of longevity, growth, and diapause in the evolution of Asellus aquaticus Bolletino di zoologia, 58 (2), 113-117 DOI: 10.1080/11250009109355740

Holdich, D., & Tolba, M. (1981). The effect of temperature and water quality on the in vitro development and survival of Asellus aquaticus (Crustacea: Isopoda) eggs Hydrobiologia, 78 (3), 227-236 DOI: 10.1007/BF00008519

Water slater embrace

Our pond does not even qualify as a pond. It is half a wooden barrel, almost full of dead leaves and overgrown irises and marsh marigolds. Still, it's got some water snails and frogs have bred the last couple of years. A few days ago, I was watching the very active ramshorn snails when I noticed this paired water slaters, Asellus aquaticus. Water slaters are isopods, like woodlice, but unlike their terrestrial relatives they display a behaviour which is common in crustaceans, mate guarding, by which males spend some time holding onto the female before the actual copulation occurs. This mate guarding, or pre-copula, is thought to have evolved in response to a very short fertile time window in females. As in many crustaceans, females can only mate right after moulting, so this guarding behaviour provides males with the reward of a fertile female at the end, and is more effective than searching randomly for receptive females.
 Unlike most sexually dimorphic crustaceans, male water slaters are larger than females. This is thought to have evolved by sexual selection. Larger males are more efficient dislodging other males already guarding females, or avoiding being taken over by other males.  Females seem to offer little resistance to being guarded, so sexual selection by females can be discounted. However larger males also pair faster, suggesting they might be better at finding females, and experimental manipulation of antenna length has shown that it is their longer antennae which increases the rate of encounters of larger males with females.
 Mate guarding has also been suggested to have an antipredatory function in water slaters. Water slaters are often predated by newts, and research by Paul Verrell indicated that paired males had a lower predation risk than both unpaired males and females in laboratory trials. Female predation rates are unaffected by being paired (see Table 1 below). According to Verrell two factors explained these results, first, males are much more active when unpaired, as they move about searching for females, so they might be much more exposed to predation. Second, when they are paired, after an attack males have a quick escape response from the female which might reduce their predation risk.

(from Verrell, 1985)


References
Bertin, A. & Cezilly, F. (2003). Sexual selection, antennae length and the mating advantage of large males in Asellus aquaticus Journal of Evolutionary Biology, 16, 698-707 DOI: 10.1046/j.1420-9101.2003.00565.x
Verrell, P. (1985). Predation and the Evolution of Precopula in the Isopod Asellus aquaticus Behaviour, 95, 198-202 DOI: 10.1163/156853985X00127