Interspecific relationship between a euryhaline rotifer
Rotifers are some of the most important zooplankton, and are the focus of much attention from aquaculture scientists. Rotifers have been used as a primary live food for the seedling production of many economically important marine animals, because rotifers can be easily cultured at high densities. Stable mass culture of rotifers is needed for successful fish larval rearing. A variety of mass culture methods should be devel-oped to facilitate successful aquatic animal culture through stable live food organism production. However, the instability or sudden crashes of rotifer mass cultures remain problematic. The causes of such culture failures are not fully understood, although it is predicted that one major cause is contamina-tion by other organisms such as protozoa (Takayama, 1979; Reguera, 1984; Chen et al., 1997), copepods (Fukusho et al., 1976) and bacteria (Yu et al., 1990).
Coexing populations of different taxonomic groups depend on the organisms that occur together in space and time, and interact with each other through the processes of mutualism, parasitism, predation and competition (Begon et al., 1990). The relationships between
Copepods and rotifers coexist in estuaries as well as in brackish water fish culture ponds in North Sulawesi, Indone-sia. Of several copepod species collected, the only one that could be adapted to laboratory culture was
Here, I observed, the survival strategies involved in the in-terspecific relationship between
Copepods and rotifers were collected from a milkfish pond in Bitung, 30 km east of Manado, North Sulawesi, Indonesia. The pond is separated from the adjacent sea by mangroves, but is connected through an inlet during high tide. Throughout the year, salinity of the pond varies from 12 to 25 psu and temperature ranges from 29℃ to 35℃.
The specimens were kept in darkness during a three-day acclimation culture to laboratory conditions before isolation. Various copepods were included in the sample, but only a cyclopoid copepod survived. The species was identified as
Experimental design and conditions used were the same as those described by Hagiwara et al. (1995b). Salinity, tempera-ture, and culture volume were 22 psu, 25℃ and 40 mL, re-spectively. The organisms were cultured in total darkness. The initial number of animals in mixed cultures was 20 females of the Bitung rotifer strain of
For the observation and calculation of the rotifer mixis rate (%), all individual non-egg bearing fe-males, amictic females, unfertilized and fertilized mictic females, males and resting eggs were counted, and the mixis rate was calculated (Hagiwara et al., 1988). The numbers of all individual nauplii, copepodites and egg-bearing females of the copepod
[Fig. 2.] Densities of remaining food on the every two days in rotifer Brachionus rotundiformis mono culture (A), copepod Apocyclops borneoensis mono culture (B) and two species (rotifer and copepod) mixed culture (C).
distinct correlation interactions between the two experimental organisms were observed (Fig. 1C).
The remnant food (
Population growth of rotifers in the mixed culture was sig-nificantly decreased (
In both experimental conditions, inductions of mixis rates (%) were observed. However, the mixis rate (%) of rotifers was notably changed by presence or absence of copepods. The difference between rotifer monoculture and mixed culture
with copepods was particularly notable on the 4th day (Fig. 4). The presence of copepods influenced the mixis rate (%) of co-existing rotifers (Fig. 4), as well as the production numbers of rotifer resting eggs. On day 4 of the observations, the highest values of mixis rates (%) was observed in both experimental conditions (Fig. 4), and the production of rotifer resting eggs was observed from the 6th day. However, there were differ-ences in the numbers of rotifer resting eggs produced. More resting eggs were formed in the copepod mixed culture than in the rotifer monoculture (
[Fig. 5.] Formed numbers of rotifer resting eggs in the rotifer Brachionus rotundiformis (Bitung strain) mono culture (white circles) and mixed culture with copepod Apocyclops borneoensis (black circles).
The production of rotifer resting eggs was 1.56 times higher in the mixed culture than in the rotifer monoculture (
Fig. 6 shows that copepod population growth did not differ between copepod monoculture and mixed culture with roti-fers during the 16 day experimental period. This trend was not changed with the separation of the developmental stages (nauplius, copepodid and egg-carrying females) of the cope-pods (Fig. 7).
Rotifer mass culture tanks comprise intricate relationships among the constituents of the small ecosystem. These small ecosystems are mainly composed of rotifers and contaminant micro-organisms, such as bacteria, microalgae, protozoa and copepods. Many of these contaminant micro-organisms affect rotifer population growth through interactions such as exploit-ative competition for food, conmensalism, ammensalism and physical interference competition (Hagiwara et al., 1995b; Jung et al., 1997).
[Fig. 7.] Comparison of the numbers of each developmental stages, nauplius, copepodid and egg carrying female of copepod Apocyclops borneoensis in the mono culture (blank circles) and mixed culture with rotifer Brachionus rotundiformis (solid circles).
No predator-prey relationship was observed between the two zooplankton species during the experimental period. However, interference between the two species was observed. It was noted that the copepod used a filter feeding mechanism in which it filtered the slowly swimming rotifers together with algae (food), but immediately rejected the rotifers, which started to swim. Rejected rotifers appeared to be unharmed, but may have suffered some physical damage or injury. The physical damage by copepods may have brought about the de-crease in rotifer population growth.
The presence of the rotifer did not influence the popula-tion growth of the copepods, likely because of the large size differential between
An analysis of the population structures revealed that the rotifer population growth decreased due to a 30% reduction in females without eggs and amictic females. In contrast, mictic females in mixed cultures were more abundant than those in monocultures. Thus, the presence of copepods stimulated the sexual reproduction of the rotifer (mixis rate or resting egg formation), which in turn caused decreased rotifer population growth. It is of interest to conduct further research to clarify this mechanism.
The dormant stage of the rotifer (resting egg or cyst) is very resistant to harsh environmental conditions and may be dis-persed over wide areas by wind, water or migrating animals. Sometimes, scientists induce the production of mixis repro-duction (cyst) as an easy method of storing and transport-ing for marine fish larvae culture or aquaculture study. The hatching of rotifer resting eggs is caused by stimulation from light, temperature, salinity and some chemicals (reviewed by Hagiwara, 1996), but there is no known method to artificially produce rotifer cysts.
The high mixis rate (70% or more) in this study could be related to the algal species used as food.
Population growth of