Water Fleas

(For informative captions click on photographs.)

Standing beside the pond in the 40° C Okanagan heat, I was dreading the thought of climbing into my heavy and very hot chest waders. Not a fan of wading barefoot through eight inches of mud, sharp sticks and the occasional broken bottle, I reluctantly donned the waders and proceeded into the pond to begin making the collections I had come for. I would repeat this activity many times in the following week using dip nets, plankton nets and an Eckman grab to scoop up mud, vegetation and, the object of my quest, the myriad of tiny organisms that call ponds, marshes and lakes their home. In particular I was looking for a group of tiny but extremely abundant animals known as cladocerans or water fleas.

Called water fleas because of their small size, general body shape and jerky swimming motions, cladocerans are not fleas at all, but tiny crustaceans that inhabit almost any kind of freshwater habitat from large lakes to ponds, and even small ditches, puddles and water-filled tire ruts. Cladocerans have even been found in moss growing on trees in rain forests several metres above the ground. There are a few estuarine species but this group has not been successful in the oceans. Some species are planktonic, living in the open water of lakes, while others live on or near the bottom or on aquatic vegetation. Some species are found primarily in small ponds or saline lakes that lack fish predators.

Cladocerans are extremely abundant in most standing water, sometimes occurring in densities of many thousands of animals per square metre of lake bottom. Often they are the most abundant bottom-dwelling animals in lakes. There are approximately 150 species known in North America, and it is not uncommon to collect 15 or more species in one small area of lake bottom. The greatest abundance of species is found in the vegetation at margins of lakes and ponds

Cladocerans have a single compound eye and often a smaller secondary eye, the ocellus. A carapace covers the body, including the 4 to 6 pairs of thoracic appendages. The carapace is often transparent and opens ventrally and posteriorly. A brood chamber is located in the dorsal region of the carapace; here eggs are held until they develop into fully-formed juveniles. The abdomen and postabdomen are generally bent forward under the thorax. The postabdomen bears two large claws used primarily for cleaning debris out of the carapace. Swimming is accomplished by downward strokes of the large second antennae.

A few species are predacious but most are herbivores or detritivores, feeding on phytoplankton, attached vegetation or decaying organic material. In most planktonic species complex movements of the thoracic appendages produce a constant current of water between the valves. Small particles in the water are filtered out by fine setae on the thoracic legs and moved along a groove at the base of legs to the mouth. Although there is some evidence that certain types of food, such as particular types of algae, Protozoa, or bacteria may be selected by some species, it is generally believed that all organic particles of suitable size are ingested without any selective mechanism. When undesirable material or large tangled masses are introduced between the mandibles, they are removed by spines on the first legs and then kicked out of the carapace by the postabdomen.

An important link in food chains of virtually every inland body of water, cladocerans convert phytoplankton, benthic plants and decaying organic matter into animal tissue. Bottom-dwelling species are prime converters of decaying organic matter with its associated micro-organisms into a form usable by fish and larger invertebrates. In large lakes planktonic cladocerans are a major food source for many kinds of fish such as sticklebacks, minnows and young Sockeye salmon. Many aquatic insect larvae and other invertebrates also feed on cladocerans. Most fresh water ecosystems would collapse quickly if cladocerans were removed.

Because of their large numbers, cladocerans consume vast amounts of organic material, and by reducing the amount of phytoplankton are important in maintaining water clarity in many lakes.

Most cladocerans can reproduce by parthenogenesis; females producing eggs that develop into young without fertilization by males. As a result, males are not present in the population for much of the year. As long as environmental conditions remain favourable, females will continue to reproduce in this manner, producing only female offspring capable of asexual reproduction. If environmental conditions deteriorate due to overcrowding, lack of food or oxygen depletion, eggs are produced that develop into males, and females capable of sexual reproduction. These females have modified carapaces that are thicker and darker dorsally than a regular carapace and produce haploid eggs that must be fertilized by males. The fertilized egg goes through several cell divisions, then enters a resting stage and cell division stops. The thick dorsal portion of the carapace, the ephippium, containing the resting embryos, is shed when the female next moults. These embryos protected by the ephippium are capable of surviving extended periods of unfavourable conditions such as drying or freezing. In this manner cladoceran populations survive through winter and periods of drought. When environmental conditions are again favourable the embryos develop and break free of the ephippia.

The ephippia are also used as a means of dispersal for many species, being carried by the wind or in the fur, feathers or digestive tracts of animals to new habitats. In this way cladoceran ephippia end up in small depressions such as tire ruts or small puddles that eventually fill with water. Once water is present and conditions favourable the embryos develop quickly into adults and produce eggs before the puddle dries.

Like the shell of other crustaceans, that of a cladoceran does not grow. Growth can only take place through the moulting of the shell or carapace and the formation of a new larger one. Because cladocerans need their carapace as protection, and to act as a skeleton for muscle attachment, a new carapace is grown under the old one before the old one is shed. Once the old carapace is moulted the cladoceran takes in large amounts of water swelling its body and stretching the new carapace. The increase in volume takes place in the first few minutes after moulting due to the absorption of water. Cladocerans thus grow in spurts with each stage called an instar. Juvenile instars increase in size significantly after each moult, sometimes doubling in size from one instar to the next. Between moults the absorbed water is gradually released and is replaced by tissue as the cladoceran grows.

Each time an adult female moults she releases developed juveniles from the brood pouch into the surrounding water, and a new clutch of eggs is released from the ovary into the brood chamber of the newly formed carapace. Cladocerans generally have 10 to 25 adult instars in their lifetime, each instar lasting one day to several weeks, depending on the environmental conditions.

This group, although extremely abundant and important in the ecology of virtually every body of water larger than your kitchen sink, has not received much taxonomic study in British Columbia. Even in other areas of the world where studies have been conducted only select families have been examined. As a result there is not even an up-to-date species list for British Columbia and no identification key to BC species.

During environmental studies, collections are made in various lakes and streams throughout British Columbia. These collections inevitably contain many cladocerans. By monitoring the community composition of a lake over time, changes in the conditions of the lake can be detected by changes in the species present. The task of identifying these animals is made difficult by the fact that there is no identification guide or even up-to-date species list for cladocerans occurring in the province. People attempting to identify cladocerans from BC must use old keys, usually written for other geographic areas. This leads to misidentification and possibly, subsequent misinterpretation of results. Past studies have reported species in central BC that have been previously found only in the southeastern United States; others record species from the province that are not known to occur in North America. It is highly unlikely that these species actually occur in BC. Since specimens from these studies are rarely available for re-examination their indentity cannot be confirmed. The known environmental requirements of these species are then used to gauge the health of a lake in BC, where the species do not even exist.

Since 1992 my research at the Royal British Columbia Museum has focused on the taxonomy of this diverse and fascinating group of crustaceans. The aim of my research was to sort out the long and confusing list of cladocerans reported from BC, determine which species actually occur in the province, and write an identification key for those species.

The small size (average 1 - 1.5 mm, with many species under 1 mm) and great morphological variation that often occurs within some species, makes the taxonomy of cladocerans a time-consuming and frustrating task. Morphological variation within a species is particularly confounding. A single species can have a number of different head shapes; tail spines may be different lengths, or head spines may vary in number and arrangement, all within a single year, and in the same water body. These morphological changes may occur due to time of year, temperature of the water, geographic location, or the presence in the habitat of certain predators. Sometimes there appears to be more variation within a species than between two closely related species.

The genus Daphnia shows a very high degree of intraspecific variation. Many studies have been carried out in an attempt to establish morphological characteristics that can be used to distinguish consistently between closely related species of this genus. Although many authors have suggested differences, few of these have withstood scrutiny when specimens are examined from a wide geographic area. Even genetic studies have failed to establish whether some species complexes contain many closely related species or a few extremely variable species.

Breeding studies are difficult because parthenogenic reproduction means males are often unavailable, especially in larger lakes where seasonal variations in water conditions are not as radical as in smaller bodies of water. Hybrids also present a problem. Hybrids are usually not capable of breeding and producing viable offspring. Using asexual reproduction however, cladoceran hybrids can quickly produce a large population of animals with characteristics of both parent species, further muddying the already turbid waters of cladoceran taxonomy.

Members of some families such as the Macrothricidae and Moinidae are rare, so collecting sufficient specimens from a wide geographic area, for taxonomic comparisons, can take years of intensive collecting.

Museum collections are extremely valuable for the study of the taxonomy of rare or morphologically variable organisms. Using collections from the RBCM and museums throughout North America, a number of specimens can be assembled from a large geographic area in a relatively short period of time. This allows examination of the largest possible number of specimens from different geographic locations, seasons, and types of water bodies, and makes it possible to account for intraspecific variation caused by these factors.

Although considerable research has been done on cladocerans in some regions of the world we do not yet know how many species occur in British Columbia, or most other parts of the world. New species are constantly being found, and for most of the known species we do not possess the most basic information on their ecology, feeding habits or behaviour.

The Thompson-Okanagan is an ideal region for the study of aquatic invertebrates because of the variety of water body types in the area. A range of sizes from small ponds to large lakes and a great variety of water chemistry including saline ponds and alkali ponds make for a large number of different habitat types and thus a great diverstiy of aquatic life.

Using RBCM specimens collected primarily from the Thompson-Okanagan region, and collections from the Ministry of Environment Lands and Parks from across BC, I have recently written a key for species in the families Daphniidae, Sididae, Bosminidae, Holopediidae, Leptodoridae and Polypehemidae. These families contain the majority of planktonic cladocerans. Hopefully, at least for a while, this key will eliminate some of the confusion of European names and exotic species finding their way onto lists of the British Columbia fauna. Keys are still needed for three families, the Macrothricidae, Moinidae and Chydoridae. These families contain mostly small, benthic forms.

Cladoceran taxonomy will probably always cause lively debate among taxonomists and perhaps will never be resolved to everyone's satisfaction. But it is important that we learn as much as we can about cladocerans; they are an integral part of every freshwater ecosystem on earth. Without them, most of these ecosystems would surely collapse.

Gordon Green
RBCM