by Johannes Dürbaum

The reproductive behaviour of one of the suborders of copepods (Copepoda harpacticoida) is the focus of a research project by the Zoomorphology working group at the Department of Biology. Until now, it had been assumed that all species of this suborder of these animals, which are only millimetres in size, follow a so-called precopulation strategy, i.e. that the males accompany the juvenile females before copulation. The Oldenburg study has now shown that a post-copulatory strategy also exists in some species. In such a case, males exclusively accompany adult females after copulation for a certain period of time, thus avoiding sperm competition.

It was July 1995 when I was sitting in the full sun on one of the most beautiful beaches in the world in eastern Indonesia, looking at small marine crustaceans with a microscope I had brought with me. What I saw swarming there in my seawater-filled shells in a high density of species were mainly copepods, some in the most dazzling colours. With the naked eye, the copepods can just be made out as bouncing dots if you look closely, and it is only under the microscope that the appearance and life expressions of the animals, which are often less than a millimetre in size, can be studied in more detail. Simple tools such as buckets and plankton nets are sufficient to catch the benthic species, i.e. those that live close to the bottom. In addition to washing out coral sand, it is also very rewarding to shake algae in the bucket and concentrate the washed-off animals in the plankton net. They are simply everywhere and in huge quantities. It seems strange to two Moluccans what I'm doing, and after they stare at me for a long time, the inevitable question comes, which I also hear so often at home in Oldenburg: "Untuk apa?" (What for, why?) At first, and just like most of the other enquiring locals, Ramlee and Singh believe that I am looking for small fish because fish is one of the most important sources of food in the subsistence economy practised there. The fact that this is not true and that I am interested in the behaviour of small inedible crustaceans makes me appear strange in their eyes, to say the least.

I had undertaken this trip to collect representatives of the Copepoda harpacticoida, which do not occur here, and to make live observations of species of which little more than their external appearance is known. Another important aim of this trip was to obtain live material for laboratory breeding, which I have been doing successfully for several years and which provides the basic material for my investigations. The main focus of my live observations is probably the most complex behaviour of these small crustaceans, namely their mating behaviour.

If you look at a live sample of such harpacticoid copepods, it doesn't take long to find two animals that seem to hang behind each other and swim around together. It is always the front antennae, with which the adult males cling to the females, that swim ahead in such pairs. Males of different species and families grab their females by different parts of the body. Males are most frequently found holding on to their own females by their tail bristles or by attachments at the rear end of the body, the furca. Males of other species anchor themselves to the dorsal shield or to the rear thoracic or abdominal segments of their sexual partners. Others always hold on to the fourth pair of legs of the females. If such pairs are observed in the shells, no active sexual behaviour can usually be observed. The association of males and females appears to be rather static and can last several weeks in some species. This raises the question of why the males spend so much time accompanying the females.

Already in the last century, some of the pioneering copepod researchers had noticed that females in such pairs are often much smaller than the males that accompany them. It is therefore understandable that cannibalism was initially considered, but the females are not eaten.

Today, the phenomenon of accompanying juvenile females in copepods is referred to as the precopulatory phase, as it precedes copulation itself. The reason for this behaviour of the males is their competition for females, which are not available in unlimited numbers. If adult, as yet unmated females are not to be found, there is only one chance for the males to get their chance. They have to secure a young female and accompany her until she is an adult. Immediately after the adult moult, the females are ready to mate and copulation takes place. Shortly afterwards, the male leaves the female to look for a new juvenile female. This competitive conflict between males is exacerbated by their ability to mate with several females in succession, whereas the females of many species only mate once in their lifetime, as the semen they receive in the process can be stored by them and is sufficient to fertilise all the eggs they produce in their lifetime.

It was previously believed that this reproductive behaviour with precopula was the rule in the Harpacticoida, but later long-lasting pair formation between exclusively adult partners was also found in other species. In order to be able to interpret the differences, the brief moment of mating, during which the male attaches a sperm packet, the so-called spermatophore, to the female, had to be observed more closely. My project therefore initially focussed on observing and documenting the actual transfer of this spermatophore to the female. To this end, animals were reared separately in minipetri dishes that had been prepared for a long time and, after reaching sexual maturity, were put together with a partner in film chambers. What was finally recorded over a period of one and a half years with the help of a sophisticated video system mounted on a microscope was more than just a documentation of spermatophore transfer in various species. The video sequences analysed in slow motion led to the discovery of many previously unknown behaviours, which are also of great importance for population biology.

The mating behaviour of species that exhibit a precopulatory phase is relatively simple. Males of the species Tachidius discipes, which is widespread in the North Sea, can determine in laboratory experiments in which they are offered adult females whether a female has already been mated with or not within a few seconds. The advantage of offering adult but virgin females is that the mating behaviour, which otherwise immediately follows the female's two-minute moult and is therefore almost impossible to predict, can be observed immediately. The lightning-fast examination of the male is carried out with the help of the front antennae and the pairs of legs, which are rubbed over the sexual opening. If the female is already mated, the male immediately detaches himself from her and continues his search. If it finds another suitable adult female, it only separates from her again after copulation. Female choice behaviour has also not been found in other species with a precopulatory strategy. Males appear to be the sole determinant in mate choice, as females do not appear to have the ability to reject males. This is difficult because most females are still much smaller than males when they are young.

In contrast to Tachidius discipes with precopulatory behaviour are those species in which there are only associations between adults. In this case, males only seize adult females and carry out a complex courtship, at the end of which, if the males are not rejected by the female, the spermatophore is transferred. In these species, the partners do not separate immediately afterwards, as is the case with the precopulaceous strategists, but the males continue to accompany their females for a certain period of time. As soon as the spermatophore is attached to the female, a complicated emptying mechanism begins. This process, which can be observed under the microscope on living animals, takes between 35 and 60 minutes, depending on the species. During this time and often beyond, the males remain with the females and guard the emptying of the spermatophore (see illustration). In this way they prevent other males from having the opportunity to attach another spermatophore, which could lead to a mixing of the sperm. The purpose of such postcopulatory behaviour is thought to be to prevent possible competition between the sperm of two males in the female genital tract for fertilisation of the eggs. Genetic evidence for sperm competition is still lacking for copepods, but has already been provided for some insects, for example.

Presumably, the precopulatory strategy is the more recent strategy in evolutionary terms, despite the females having little say in the matter, and the postcopulatory strategy is the original one in phylogenetic terms, from which the precopulatory strategy has developed independently several times. There are several indications of this, of which only one should be mentioned: Depending on the family, the males, as already reported, grab the females by different parts of the body (see illustration). They do this with the front antennae, whose structure varies accordingly. These differences indicate that the precopula behaviour was not only "invented" once, but several times independently of each other.

In addition to the fact that there are two different strategies for securing paternity in the males of copepods, species-specific courtship patterns and species-specific postcopulatory behaviour were observed in the males with a postcopulatory strategy. This behaviour is complex and consists of individual sections, but always occurs stereotypically in the same species-specific sequence. Just like certain physical characteristics, these stereotypical behavioural patterns are the result of a long evolutionary process. Closely related species are therefore generally more similar in their courtship behaviour than distantly related species. A precise analysis of courtship patterns therefore also makes it possible to obtain meaningful indications of phylogenetic relationships. Studies carried out on eight species of the genus Tisbe revealed relationships that corresponded to those that had been reconstructed with the help of physical characteristics. As part of a doctoral thesis currently underway, the field has been broadened and a larger subgroup of the Harpacticoida has been selected in order to further advance the discussion of relationships within the Harpacticoida using behavioural traits.

I tried to explain some of this to Ramlee and Singh on that beach in Indonesia. They listened with interest, but I'm afraid they thought I was not only strange, but an "orang gila" (crazy person).

Johannes Dürbaum (33) initially studied biology at the University of Bielefeld, specialising in zoology. In order to be able to work on marine biology topics, he moved to Oldenburg in 1989 and joined Prof. Dr Horst Kurt Schminke (Department 7 Biology) in the Zoomorphology working group, which is dedicated to researching mostly marine copepods. After receiving his Diplom in 1993, he was a research assistant in the working group. Dürbaum is currently working on his dissertation on the behavioural biology and ecology of Harpacticoida in tropical to polar latitudes.