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

Tiny snails on cockle shells

My daughter found some tiny snails under a cockle shell in a pot yesterday. I took some photos but I could not identify them so today I went to find them again and try and take some shots of the more informative mouth of the shell. There were five or six of them, that were now active after last night's rain.

I looked in the pot and I found dozens of them. I had never seen them before and we had a whole pot population of these 2 mm or so snails happily living in the garden! Fauna, from WAB, identified my photos of the shell mouth today as Lauria cylindracea, a very common and widespread snail in the U.K. A favourite habitat is on stone walls covered on Ivy, but also lives in moss, leaf litter and crevices of old trees, habitats not particularly wet. It feeds on fungus growing on the dead leaves.
 As many snails, individuals of L. cyclindracea are hermaphrodites, which mate with each other to fertilize their eggs. This species, however, is relatively unusual for a snail in that it is ovoviviparous. It retains its eggs - 4 to 6 -inside its body until they hatch, and they are born into miniature snails, even tinier than their parents. Some of these juveniles were also present under the shell near the adults. The snails reach maturity at two years old and can live over 5 years, reaching a maximum size of a mighty 3.5 mm.

 In their study of life history and reproduction of L. cylindracea in Israel, Heller and collaborators discussed what evolutionary forces might have led to the evolution of ovoviviparity in this species. They hypothesize that a combination of small size - which limits the total number and the size of the eggs that can be produced - and high egg mortality, for example due to drought or lack of appropriate egg laying sites or inability to bury the eggs, are the key selective pressures. That way, offspring are born right next to their parents at the conditions most suitable for survival. The snails can also determine up to some point when the birth happen, retaining the juveniles longer when the conditions are very dry, and quickly giving birth when favourable moisture conditions return, that way maximising the chances of their offsprings survival. Heller and collaborators state:

Active hatchlings are advantageous, as compared to eggs, in that they can immediately start to feed, grow, fight off fungi, cope with brief periods of desiccation by moving into deeper layers of litter or temporarily retreating into their shell, and avoid drowning (or being washed away) as a result of occasional flooding of the riparian litter habitat, by moving into higher layers.

They compared this reproductive strategy in minute snails with the situation in larger species:

Large snails can produce eggs in vast numbers. Further, lack of size constraints enables the large adult to lay large eggs, generously coated with thick layers of albumin and mucus to tide the developing embryos over varying periods of drought and hunger. In Helix texfa, a species of 40 mm and 4.3 g (found in Israel very close to our present study site), each single egg averages 5.5 mm in size and 70 mg in wet weight (bigger and heavier than an entire adult Lauria), an adult produces 60 eggs per clutch, and clutch weight represents about 10% of body weight (Heller & Ittiel, 1990). In large snails with large clutches also sib cannibalism may occur, thereby increasing survival chances of the first individual to hatch (Baur, 1994). Further, large oviparous snails frequently deposit their eggs into pockets in moist soil. A deep cavity lined with mucus can protect the eggs against bacteria and fungi (Baur, 1994); it also enables the deposition of eggs in soil layers that remain damp for longer periods of time, and thereby reduces risks of desiccation. The cavity is dug out by the snail’s foot, so its depth is broadly correlated to foot size. Helix can dig 5cm into moist soil and Arclzachatina can dig a 15 cm cavity in one night (Tompa, 1984; Baur, 1994). Minute snails, however, do not have the potential to dig much beyond 0.2cm (except for subterranean genera, see Heller, Pimstein & Vaginsky, 1991). They can either dig minute cavities, or deposit their eggs in damp litter, where they would be exposed to fungi. Ovoviviparity may thus be a mode of parental care for snails that can neither lay many eggs, nor big eggs, nor dig deep egg cavities.

I will keep looking for these snails to see how widespread they are in the garden. I wouldn't be surprised to have overlooked these fascinating creatures due to their minute size. Were not for the white background of the shell where we found them we would have probably missed them altogether.

Reference
Heller, J., Sivan, N. & Hodgson, A. (1997). Reproductive biology and population dynamics of an ovoviviparous land snail (Pupillidae). Journal of Zoology, 243 (2), 263-280 DOI: 10.1111/j.1469-7998.1997.tb02781.x

The fast woodlouse

I have featured two common woodlice species before. There is a third common garden species, the fast woodlouse, Philoscia muscorum. As it name implies, it is quite fast, with long legs that keep its body well above the ground. Its dark, rounded head, and a longitudinal dark stripe along its shiny, marbled body, make it easy to identify (for a simple key to ID British woodlice click here). The fast woodlouse is a native European species which is also found in the U.S. It is of medium size (around 1 cm.) and inhabits a range of habitats from sand dunes to woodland. A P. muscorum population living in the dunes of Spurn Head investigated by Grundy and Sutton showed the phenomenon of year-class splitting. This means that there are two reproductive strategies amongst individuals born each particular season. The 'fast growers' reproduce when they are one year old and die, the 'slow growers' breed for the first and only time when they are in their second year.

Figure from Grundy and Sutton 1989. Note that the dark (slow) growers are joined in their second year by a new cohort (in grey).

Grundy and Sutton's experiments showed that females only reproduced with they reach large size and they have one or two broods (rarely three) with an interbrood period of 5.4 weeks. This and the positive relationship between temperature and growth rate means that most individuals born in the first brood are able to reach a large size before winter sets in and start growing faster when the breeding season arrives. Most individuals born in the second brood (a bit more than a month later) miss out on the benign growing conditions and are too small after the winter and when the next breeding season arrives they are still too small and waiting until the following breeding period makes sense to maximize their chances of reproducing successfully.
More information
Grundy, A. & Sutton, S. (1989). Year class splitting in the woodlouse Philoscia muscorum explained through studies of growth and survivorship Ecography, 12 (2), 112-119 DOI: 10.1111/j.1600-0587.1989.tb00829.x