Face to Face with a Daphnia
| Polish version is here |
The following article was originally published in the journal for educators Biologia w Szkole (eng. Biology in School) (5/2015):
The natural environment can be understood as the sum of all components of nature, both living and non-living. It includes living organisms as well as geological structures, landforms, water systems, climate, and soil types. All of these elements are closely interconnected and depend on one another. Natural balance is one of the most important features of the environment, occurring when the exchange of energy and matter in nature remains in equilibrium.
For a biologist, the main focus of interest is naturally the living world, its structure and functioning. In a broader sense, this also raises deeper questions about the origins of life itself, its evolution, and its future in a changing environment [1].
It is important to remember that, unlike other exact sciences, biology rarely describes its subjects in terms of fixed physical laws expressed through mathematics. The behavior of biological systems can only be predicted statistically, and such tendencies are usually far less precise than those studied in physics or chemistry. Still, we must not forget that biological systems obey the very same laws that govern the non-living world around us.
In my view, inspiring curiosity about the biological world among all people, and especially among students, is one of the most important responsibilities of anyone who seeks to share knowledge with others. It offers a chance to spark wonder at the diversity of living organisms, their structures, and the interactions that occur both among themselves and between them and their environment. After all, we are part of this world too!
Biology and nature classes in schools should naturally encourage questions about the living world. Unfortunately, as we all know, that is not always the case. Biology is, at its core, a science based on experience and observation. Yet today it is often taught almost entirely from books or, in a more modern version, through films and multimedia presentations. The lack of hands-on activities is still frequently justified by their supposed high cost or the time required to prepare them. But that kind of reasoning borders on educational sabotage and cannot be accepted as an excuse.
Carrying out engaging experiments and observations does not require a large budget and is entirely possible even in a moderately equipped school biology lab. In this article, I will describe a series of observations that can be made using easy-to-maintain yet fascinating organisms belonging to the genus Daphnia, commonly known as water fleas.
Observations
In searching for an interesting yet relatively easy organism to culture, I decided to use Daphnia. These small freshwater crustaceans, often referred to as water fleas, can play a valuable role in education and in sparking curiosity about biology, not only among children and students but also in anyone who takes the time to observe the natural world.
Members of the genus Daphnia belong to the phylum Arthropoda and the suborder Cladocera [2]. They inhabit ponds, temporary pools that may dry up seasonally, and even puddles.
These organisms are filter feeders that consume bacteria, algae, and organic particles suspended in the water [3].
Where can they be found? During the summer months, Daphnia can easily be collected from woodland ponds or puddles. Because they are a favored food source for fish, they can also be purchased at many pet stores, especially those that specialize in aquatic life.
These crustaceans can be kept in an aquarium or even in a large jar. However, it is important to provide them with an adequate food source. A good diet for Daphnia consists of water rich in algae. The most convenient setup is to prepare two containers, one for keeping the water fleas and another as a food reservoir. Naturally, it is best to start growing algae some time before introducing the animals. For this purpose, water from any reasonably clean natural pond can be used. In favorable conditions, with moderate temperature and sufficient light exposure such as on a windowsill, algae grow quickly and the water soon turns green. Once this happens, you can safely begin your culture. To feed the crustaceans, simply transfer some of the algae-rich water into their container and refill the second one with fresh water. They are efficient filter feeders and should be fed each time the water in their tank becomes clear again. Water fleas thrive in well-lit conditions but should be protected from excessive heating of the water in their container.
These minute creatures typically measure between 1 and 6 mm depending on the species (Photo 1). Watching them move in short, jerky hops powered by their antennae quickly reveals why they are so aptly nicknamed water fleas.
A particularly rewarding subject for observation is the common water flea, Daphnia pulex, whose carapace is transparent. This makes it easy to study not only its external appearance but also the structure and functioning of its internal organs (Photo 2).
A – second pair of antennae, B – compound eye, C – first pair of antennae, D – carapace, E – thoracic limbs, F – anus, G – digestive tract, H – brood chamber, I – heart
The transparent carapace of these small crustaceans, like that of other members of the class Crustacea, is made of chitin. It curves toward the ventral side, giving the impression of being two-lobed (Photo 2D). At the rear, it extends into a pointed spine.
The body is short and laterally compressed. It bears a single compound eye formed by the fusion of two simpler ones (Photo 2B). This movable eye helps the organism navigate its surroundings, at least in response to light.
The first pair of antennae is greatly reduced and often difficult to spot (Photo 2C), while the second pair (Photo 2A) is much larger and distinctly branched. These serve as the primary organs of locomotion. On the thorax, beneath the carapace, there are five pairs of short, flattened limbs that primarily function as filters for collecting food particles from the water (Photo 2E). The digestive tract (Photo 2G) and brood chamber (Photo 2H) are also clearly visible. As in other arthropods, the heart lies along the dorsal side of the body (Photo 2I) [4].
A – compound eye, B – naupliar eye, C – first antennae with sensory setae, D – midgut diverticulum
Examining the head under higher magnification reveals even more fascinating details. The most prominent feature is the spherical compound eye, composed of relatively few ommatidia (Photo 3A). Watching this organ move in response to changing light is truly remarkable. Interestingly, the animal also possesses a vestigial naupliar eye located just below the compound one (Photo 3B). At greater magnifications, one can observe the eye muscles and the nerves connecting the eye to the cerebral ganglion. The first pair of antennae, though very small, becomes visible and, thanks to its fine sensory setae, functions as a tactile organ (Photo 3C). A distinctive paired diverticulum of the midgut (Photo 3D) acts as a digestive gland [5].
A – post-abdominal claws, B – anus, C – abdominal setae
The posterior part of the body is elongated and curves ventrally. In addition to the digestive opening (Photo 4B), this region bears chitinous claws and setae (Photo 4A and 4C, respectively). These structures may serve to clean the carapace or fulfill other functions [4] [5].
The circulatory system of these crustaceans is open, meaning that hemolymph is not entirely contained within blood vessels but is pumped from the heart into open cavities called sinuses, which directly bathe the internal organs. The heart of Daphnia has a simple structure. Photo 5 shows it in both phases of the cardiac cycle. It beats fairly rapidly, with about 180 contractions per minute under normal conditions.
A – diastolic phase, B – systolic phase; the asterisk marks the heart
The ability to observe the heartbeat so clearly makes these tiny organisms perfect for simple yet fascinating experiments. For instance, students can measure heart rate in relation to water temperature or different light conditions. It is also possible to examine how certain chemicals influence their physiology. Like many other organisms, Daphnia are sensitive to alcohol. This can be demonstrated by adding a small amount of ethyl alcohol to the water and observing the resulting change in heart rate. At higher concentrations the animals die, but at lower doses they survive and recover once placed in clean water. Similar experiments using caffeine or nicotine can effectively demonstrate the harmful effects of these substances on the human body.
The circulatory system of these crustaceans is open, meaning that hemolymph is not entirely contained within blood vessels but is pumped from the heart into open cavities called sinuses, which directly bathe the internal organs. The heart of Daphnia has a simple structure. Photo 5 shows it in both phases of the cardiac cycle. It beats quite rapidly, with about 180 contractions per minute under normal conditions.
The life cycle and reproductive strategy of these crustaceans are equally fascinating, and the processes involved are not difficult to follow.
Under favorable environmental conditions, a population of water fleas consists almost entirely of females. In such situations, Daphnia reproduce by parthenogenesis, or asexual reproduction. The females produce large numbers of unfertilized eggs, known as “summer eggs”, which develop immediately. These eggs mature within the brood chamber, where the oviducts open. The juveniles remain there until they become independent, as shown in Photo 6. At this stage, they are already active and soon leave the mother’s body. This process produces successive generations of genetically identical females. Such a mechanism allows for rapid population growth and efficient colonization of the environment, provided that conditions remain favorable and food is abundant.
A – juveniles developing inside the brood chamber, B – compound eyes of the juveniles
These favorable conditions, however, can deteriorate for various reasons. The change may result from water that is too cold or too warm, a drop in oxygen levels, or simply a shortage of food. When this happens, some eggs begin to develop into males, which are smaller than the females. Their role is to fertilize the so-called winter eggs. These eggs are protected by a relatively thick chitinous shell and form a structure known as the ephippium, or “saddle” (Photo 7). The ephippium is released into the environment when the female molts. Before further development can occur, it must undergo a resting period. The ephippium is resistant to both freezing and drying, serving as a dormant survival stage. In the spring, new generations of parthenogenetic females hatch from the ephippia, completing the cycle. This phenomenon is known as heterogony [4].
The observations presented here, of course, do not cover all possibilities. One can also explore how these organisms respond to light of different intensities or colors. The results of such experiments can be surprisingly engaging for both students and hobbyists. I encourage readers to conduct their own investigations, especially since no specialized equipment is required. A simple microscope or even a good magnifying glass is enough to reveal fascinating details. Excellent results can also be achieved using the simple digital microscope I described in a previous issue of “Biologia…”, built from an inexpensive webcam [6].
References:
- [1] Jura Cz., Krzanowska H., Leksykon biologiczny, Wiedza Powszechna, Warszawa, 1992 powrót
- [2] Rybak J. I., Błędzki L., Słodkowodne skorupiaki planktonowe. Klucz do oznaczania gatunków, Wydawnictwo Uniwersytetu Warszawskiego, Warszawa, 2010 powrót
- [3] Gliwicz Z.M., Zooplankton, jako rozdział w: O'Sullivan P., Reynolds C. S., The Lakes Handbook: Limnology and Limnetic Ecology, John Wiley & Sons, 2008 powrót
- [4] Jura Cz., Bezkręgowce. Podstawy morfologii funkcjonalnej, systematyki i filogenezy, Wydawnictwo Naukowe PWN, Warszawa, 2007 powrót
- [5] Ebert D., Introduction to Daphnia Biology. Ecology, Epidemiology, and Evolution of Parasitism in Daphnia, Bethesda: National Center for Biotechnology Information, 2005 powrót
- [6] Ples M., Nieprzyzwoicie tani mikroskop (eng. Incredibly Cheap Microscope), Biologia w Szkole (eng. Biology in school), 4 (2015), Forum Media Polska Sp. z o.o., pp. 55-60 powrót
All photographs and illustrations were created by the author.
Addendum
As a supplement to the article above, I’d like to share a video:
Marek Ples