2. INTRODUCTION
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If one follows the food web from man through fishes one may eventually reach the plankton of fresh- and brackish waters. Of the plankton organisms the zooplanktons are important for many fishes, including some of the species that are cultivated in ponds, pens and cages all over the Philippines (Aquino, 1982, Aquino et Nielsen, 1983, Fernando, 2002a, Hartmut, 2003, Nielsen, 1983, SOGREAH-Report, 1974, Papa, 2008a) - just take a view over Laguna de Bay and bee convinced

Zooplankton in the food chain.

Zooplankton constitutes the main food of most fish species when these are at their young, post-larval stage. Insects and other organisms from the littoral zone supplement the zooplankton. Some fishes like the herbivorous cichlids switch from being zooplanktivory to plant diet at a very early age (Fernando, 2002a). . The invasive Mosquito fish, Gambusia affinis, is widespread and common in canals, creeks, swamps, ponds and shallow areas of lakes. It feeds on zooplanktons, small insects and detritus (Joshi, 2006). One of the major reasons why larger fish abandon this food source is that it becomes too small relative to their body size and is too dispersed in the water to be profitable. Practically the only way for larger fish to obtain sufficient quantities is by filter feeding (e.g. bighead carp). Nevertheless, the larger plankters are normally scarce in or absent from the diets of phytoplanktivorous fish such as milkfish and tilapia. These fish move to slowly through the water when filtering than zooplanktivorous fish do, allowing the larger plankton to escape (Richter, 2001).

Sustainable fisheries development thus partly depends on the availability of adequate zooplankton as principal food items of early life history stages of economically important fish species as well as of the adults of some species (Mamaril, 2001a). The cultured bighead carp, Hypophthalmichthys nobilis (Richardson, 1845) feed on zooplankton mechanical filtering of the water through the gillrakers with an interrraker distance of 60-100 µm, it is able to select especially copepodits from the water in Laguna de Bay, shown by Petersen (1981e) and Santiago (2004). The catadromous milkfish Chanos chanos (Forsskål, 1775), that breed in the sea, feed on the abundant zooplankton there and then enter freshwater to take advantage directly or indirectly of the year round food supply and the high primary production (Gross et al. in Fernando, 2002a).

In Lake Taal in Batangas the clupeid freshwater sardine Sardinella tawilis (HERRE 1927) exhibit surface filter feeding with particulate filtering capacity. Periodically it has a planktivorous diet of zooplankton such as ostracods, cladocerans (Bosmina sp., Moina sp., Diaphanosoma sp., Ceriodaphnia sp. and Simocephalus sp.) copepods (Cyclopoids and Calanoids), and rotifers (Brachionus spp.). In a more recent study Bosmina sp. had the highest occurrence items in the stomachs of tawilis purchased from the market. Planktivorous fishes may have a direct effect on these large zooplankton groups (Papa, 2008a) and the cladocerans even almost eliminated by fish (Fernando, 2002a). The freshwater sardine is thus an important example of the importance of zooplankton as a trophic link between phytoplankton and fishes. Visually guided planktivorous fish can thus exert strong predation pressure, which zooplankton avoid by migrating downwards to a depth where low irradiance prevents detection (Papa, 2008a, Carlos 1982.). In at study of vertical distribution during daylight, copepods with eggs where thus primarily found near the bottom of the shallow, but turbid lake Laguna de Bay, invisible for the zooplanktivorous fishes (Petersen, 1981e). The high predation from zooplanktivorous fishes and fish-larvae, the competition from phytoplanktivorous fishes are reducing the diversity of zooplankton of tropical freshwater (compared with temperate waters), the size of zooplankton animals and the total biomass. As a consequence the zooplankton of the tropical lakes does not control phytoplankton biomass (Fernando, 2002a) as seen in many temperate lakes. In the Danish Lake Esrom, Daphnia sp., which is rare in the tropical freshwaters, controlled the phytoplankton-biomass in the warm summer period (Petersen, 1983).

Other factors can influence the composition of zooplankton in a lake. The most dominate zooplankton in Laguna de Bay before 1980 was cladocerans followed by copepods and then rotifers. This was about the time when the total fish pen area was at its maximum (ca. 30,000 ha). Declining zooplankton biomass has been marked between 1982 and 1983. The expanding fish pen industry, the operation of the Hydraulic Control Structure (HCS) across the Napindan channel had become operational in regulating the flow of water in and out of the lake via the Pasig River, and lake pollution had separate adverse effects on the zooplankton. Shifts in the structural features of zooplankton communities became apparent seven year after the operation of the HCS, or 4 to 5 yr after the demolition of fish pens. Copepods became dominant over the cladocerans and rotifers in that order (Nielsen, 1983, Tamayo-Zafaralla et al., 2002).

There is no doubt that zooplankton enters into the diet of different species and stages of fish, but the role of Cladocera, Copepoda and Rotifera in the food of these fishes is only recently being investigated. (Fernando, 2002a, Papa 2008a, Petersen, 1981e, Richter, H. 2001, Santiago et al., 2004). It is the hope of the author that this key will help in intensifying the research about this problem, finding the per capita ration of zooplankton (and other animals) necessary for raising a fish to recruitment stage in fishery or to fingerling stage for fish culture, as on of the most important ways of getting more animal protein at less cost.

Studies have shown that physico-chemical parameters can be correlated to zooplankton abundance, species number and diversity. The study by the Papa and his group (Lazo, 2009) has revealed a moderate to high correlation to rotifers population in Pasig River to the dissolved oxygen in the river.

Systematic studies.

Since the earliest report of a Philippine ROTIFER zooplankton species (Semper, 1872, 1875), the first publications about systematics of about Philippine freshwater zooplankton species are mainly written in the German language (Brehm 1933a, 1937, 1938, Kiefer 1928b, 1930a, 1938a, 1938b, Woltereck 1941) and a few in English (Marsh 1932, Wright 1928a, 1928c, 1937). Woltereck's paper (1941) is very extensive, including zooplankton as well as records of phytoplankton, molluscs, some macro crustaceans, and fishes from major lakes in the Philippines, together with Hydrobiological notes. Ueno (1966) included further 6 species to the list from his collections in Luzon.

The first comprehensive papers are the studies by Mamaril and Fernando (1978a,b, 1986a) who noted that there are around 125 species of freshwater zooplankton belonging to Rotifera, Cladocera and Copepoda. Of the total, 61 species belong to Rotifera, 49 to Cladocera and 15 to Copepoda. Mamaril 's papers mainly describe species collected from the shorelines of the sampling localities. In this way of sampling, primarily species found in ponds and the littoral zone of lakes have been recorded and some pelagical zooplankton species was not included.

Lai, Mamaril and Fernando (1979a) had revised the freshwater calanoid copepods of the Philippines and created a new genus (Filipinodiaptomus) for an endemic species of the Philippines. My own studies revealed a few more records for the Philippines through a one-year ecological study of Laguna de Bay, mainly (Petersen 1981e, Petersen and Carlos 1984a).

Recently the Rotifers have been further studied (Tuyor and Segers, 1999). In a study by Berbano et al (2001c) 6 species of Rotifera, 8 species of cladocerans and 6 species of copepods were found in Lake Taal where the Keratella procurva, Diaphanosoma sarsi and Eucyclops "serrulatus" were the most abundant rotifer, cladoceran and copepod, respectively. Further the composition, abundance and distribution in space and in time of the rotifers of Pasig River has been investigated by Papa and his team (Lazo et al, 2009). Factors as dissolved oxygen, temperature and salinity affected the abundance of the species, among which pollution tolerant and indicator species from Brachionus (e.g. B. caudatus), Keratella (e.g. K. tropica) and Filinia were among the most common rotifers. The Philippine species of Diaphanosoma are investigated and revised by Korovchinsky (1986, 1991, 1998a, 2000a+b)). The mainly marine Calanoid copepods of the genus Pseudodiaptomus was studied by Walter (1986b) and Walter et al (2006).

The key

The aim of this work is to give the researcher of the Philippines a key is to overcome one of the main constraints in doing ecological-aquaculture studies of zooplankton in the Philippine freshwaters - the key literature for determination of species.

The key will hopefully be useful in zooplankton studies, which might be planned in ecological and limnological studies in connection with aquaculture. Although primarily a systematic paper, rather it is intended as a handbook of the freshwater zooplankton ecologist, and is though included some ecological notes. For further reading se Fernando (2002a).

The key include information from the published literature to which I have access to (see list of literature) and personal observations about the zooplankton done during own ecological studies in the Philippines, primarily at SEAFDEC BRS in 1979-81 sponsored by the Danish Foreign Ministry, DANIDA-department.

The list of species of zooplankton in the Philippines is still expanded with new records for the Philippines. Further research will, without doubt, recover more species to be added to the list. This is said to warn against not being open-minded for the possibility of finding still unknown species of zooplankton for the Philippines. In other words do not try to fit not described species into the key. Some benthos species may also occasionally be sampled. See Fernando, 2002a for worldwide keys of tropical zooplankton.

Rey Donne S. Papa studying zooplankton in microscope.

Sampling of zooplankton.

Included in the key are more than 80 species of euzooplankton, holoplankton or true zooplankton adapted to live in the limnetic or pelagic zone (Pelagic zone refers to the free water mass in the sea and in lakes, while limnetic zone refers to lakes only) of the lakes and those littoral forms, which are often found among the euzooplankton as visitors or tychozooplankton.

The recent paper by Papa and Mamaril (2008c): Methods in Zooplankton Sampling, Ecology and Identification for General Biology Teachers included this paper; gives a review of the basic methods of sampling zooplankton quantitatively.

In ecological studies of zooplankton a Sedgwick-Rafter counting chamber is a must, or better (and more expensive) settling chambers used with an inverted microscope. Only if very large species (e.g. Diaphanosoma) are studied, stereo lupe will be sufficient in counting and measurements (Petersen 1983).

In order to determine the species, the animals have to be studied under microscope. Rotifers should be studied alive. They can be kept alive in a refrigerator for considerable periods of time. Some species, however, can be determined when preserved in 5% formalin (those with stiff shell). The diagnosis of some species is based on the detailed structure of the trophi (jaws). To study the trophi the specimen is irrigated with a dilute sodium hypochlorite under a cover slip on a glass microscope slide. The other tissues are dissolved away and the trophi can be conveniently studied.

The cladocerans, which are transparent, can be studied without dissection, while the copepods has to be dissected to determine the species. The 5th leg from the adult copepod (male or female) is an important morphological character. The use of the biological reagent “Polyvinyl Lactophenol” coloured red with the biological stain Lignin Pink is often used for preparation of short time objects for determination.
Se Brandl (2002a) for more information about methodology.

How to use the key.

The presented key is ordinary "two-choose" (dichotomous) - rarely three-choose - key and is arranged, as much as possible, systematically according to the literature. The main morphological and some ecological information are included in the key. It has to be said that most investigations of the ecology of zooplankton has taken place outside the tropical region, and can only be applied tentatively to the Philippine region. Exception is first of all the Lewis’ (1979) opus on Lake Lanao zooplankton, a comprehensive analysis of the zooplankton community of a tropical lake. It is advised as inspiration in the planning of any ecological zooplankton study. Petersen (1981e) estimated secondary productions of all major zooplanktons through one year in Laguna de Bay. Papa et al. (2007a, 2008c) studied in a five-month monitoring of Paoay Lake the Impact of blooms of the colonial green algae Botryococcus braunii on the Zooplankton of Paoay Lake founding that both the composition and the abundance of zooplankton in Paoay Lake were negatively affected by the occurrence of B. braunii.

The localities of occurrence are primarily only mentioned for major lakes. A figure of the location of some major Philippine freshwater lakes is modified from (1984a):

More information can be found in the literature (Woltereck 1941, Mamaril and Fernando 1978b). The illustrations, following the key, are from two sources: Firstly by copying from the literature by simple tracing technique (Flössner 1972, Kiefer 1978, Mamaril 1978a, Pontin 1978c, Ruttner-Kolisko 1972 (R-K)) and secondly from the original drawings by the author (1980, marked FP₈₀). The black bars in the illustrations equal 100 microns or 100 μm or 0.1 mm if no other measure is noted. Most of the original drawings are copied in the same scale in order to give the "beginner" in zooplankton studies a good chance to recognize the species and compare the dimensions relatively to the other species found in the samples. In some figures preserved specimens are included to show how they appear in fixed condition (e.g. 76a) and some are shown at different angles as might be observed in the counting chambers. Finally some photos are added to the key to give a more visual impression of the animal as seen in the croscope. These photos are all original photos.

The numbers in the key (la, lb, 2a, 2b, etc.) correspond to the numbers in the illustrations. The key and the illustrations, therefore, can be crosschecked together.

The literature quoted in the key (after the species name) is given only as the year of publication. To find the reference it is necessary to find the author name in the chronological list of literature, and there upon enter the alphabetical list of literature.

The index gives key and illustration numbers for all species and groups. It has to be noted that information about a species also can be found in the key for the genera. The Internet can often give further information about the species – use the scientific name as key words for the search.

Acknowledgements
The Binangonan Freshwater Research Station (SEAFDEC) at Tapao Point, Binangonan, Rizal, has given me a good opportunity to study the zooplankton populations in Laguna de Bay and other lakes (se 1984a) and made me realize the importance of zooplankton studies in aquaculture research.

Thanks to Dr. Santiago, Jr. (Former chief of SEAFDEC Aquaculture Department) for full-hearted support and to Mr. Andy Santiago (M.S.) for organization of the ecological/limnological teamwork. A special thank to Mr. Manuel Carlos for being a patient and inspiring counterpart. Mr. H. Segers (Belgium) for personal correspondence about the Philippine rotifers.

Further thanks to following persons being involved in different ways of the publishing of the key: late Dr. J. B. Pantastico (SEAFDEC), Director Roger S. V. Pullin (ICLARM), Rey Donne S. Papa (University of Santo Tomas), Hanne Kamstrup, Mr. B. H. Nielsen (Denmark), Ole P. Udvang (Norway). This edition has not been, if not for the interest, discussions and comments from Rey Donne S. Papa (MS), a new generation researcher in Philippine zooplankton.Finally a very special thanks to Professor A. Mamaril (University of the Philippines) for reviewing the manuscript of the 1^st edition of the key and for giving many good and profes-sional advices and suggestions. This work would not be without his great expertise on what he calls "the little beasts" of the Philippine Lakes. They are lovely.


Augustus Mamaril, Sr.	Rey Donne S. Papa –2008	May 1979 - Philippines

Flemming Petersen

Denmark, 2009

[1] Coordinates of BRS laboratories at SEAFDEC: 14^o24' 44'' N, 121^o 12' 57'' E