13 COMMON FRESHWATER PLANKTONIC ANIMALS By J.D. Green* INTRODUCTION Although the occurrence of zooplankton in Auckland lakes and reservoirs has been discussed by Bayly (1962), no elementary descriptions of these species are available and the beginning limnologist often has trouble in identifying the animals he finds. To help overcome this difficulty, diagrams of the most common genera and species have been included in this paper together with short accounts of their biology. A. CRUSTACEA 1. COPEPODA: Copepods can be distinguished from the other common group of planktonic crustaceans, the Cladocera, by their elongate, clearly segmented bodies, divided into three regions; cephalothorax, thorax and abdomen (Fig. lc). Two groups of freeliving copepods are found in the plankton of lakes, the herbivorous Calanoida and the carnivorous or herbivorous Cyclopoida. (a) Calanoid Copepods: (Figs, la, b, c & d) Calanoid copepods have elongated bodies, often of slightly ovoid shape. The width of the cephalothorax is similar to that of the thorax, and the the adult, egg and naupliar stages, are often coloured bright red by stored fat deposits. Adult males can be distinguished from females by their geniculate right antennule (Fig. lb) which is modified for clasping the female during copulation, as is the fifth leg (Fig. lb & 3b). Males are also smaller than females (Fig. la & b). Both antennules of females are symmetrical, and the animal carries a single egg sac attached to the genital segment (Fig. la) which is often enlarged and highly coloured. Particularly in Boeckella (Fig. la) the postero- lateral edges of the last thoracic segment of females are often extended into long projections. (b) Cyclopoid Copepods: (Fig. le & f). Unlike calanoids, the body of cyclopoids is pear-shaped, the thorax being significantly narrower than the cephalothorax which has a broadly rounded anterior end. (Fig. If). The antennules are little more than half the body length. *Department of Zoology, University of Auckland.
FIG. 1. Copepoda. (a) Boeckella propinqua, female (1.5 2.0 mm). (b) B. propinqua, male (1.4 1.6 mm), (c) Calamoecia lucasi, female (0.7 mm), (d) C. lucasi, male (0.65 mm), (e) Mesocyclops leuckarti, male (0.8 mm). (f)m. leuckarti, female (1.2 mm), atl = antennule, c = cephalothorax, th = thorax, abd x abdomen, ga = geniculate antennule, gs = gential segment, es = eggsac.
15 FIG. 2. Naupliar stages of Boeckella propinqua (Calanoida) and Mesocyclops leuckarti (Cyclopoida), showing distinguishing features, (a) Dorsal view of body, (b) Anterior view, (c) Lateral view. M =Mesocyclops, B = Boeckella, at. = antennule, at = antenna, md = mandible, fs = furcal setae.
16 Males are very much smaller than females (Fig. lc) and have two geniculate antennules. Females carry paired egg sacs. Cyclopoid copepods are generally more "spiny" than calanoids. Copepod Life Cycles: The egg hatches into a small larval stage called the nauplius (Fig. 2) which has only the first few appendages. By a series of moults five more naupliar stages are gone through during which more appendages are added. Five copepodite stages (which are similar in body shape to the adult, but are smaller) then follow in a similar manner, each moult resulting in an increase in size and the addition or enlargement of a pair of legs until the adult stage is reached. Naupliar Stages: Although superficially similar, the nauplii of calanoid and cyclopoid copepods are fairly easy to tell apart. The following characteristics can be used to distinguish between the nauplii of Boeckella propinqua and Mesocyclops leuckarti. They are shown diagrammatically in Fig. 2. 1. Boeckella (Calanoid) a) Antennules considerably larger than antenna, with a flattened terminal segment. b) The coxa of the antenna is without large masticatory spines. c) The mandible possesses coxal masticatory processes. d) The length of the labrum is greater than the width when viewed ventrally. e) When viewed dorsally the body is elongate and flattened anteriorly (Fig. 2a). f) The body is rather laterally compressed (Fig. 2b). g) From the lateral aspect the greatest depth occurs midway along the body and the anterior portion of the head is bluntly flattened (Fig. 2c). h) The furcal setae are asymmetrical. 2. Mesocyclops (Cyclopoid) a) The antennule is not larger than the antenna, and is less robust. b) The coxa of the antenna has a large masticatory claw. c) The mandible has no coxal masticatory processes. d) When viewed ventrally, the labrum is as long as it is wide. It is larger in relation to body size and much more prominent than in Boeckella. e) When viewed dorsally, the body is more or less pear-shaped with a broadly rounded anterior end (Fig. 2a).
17 f) The body is not laterally compressed, rather it is somewhat flattened dorsoventrally (Fig. 2b). g) The greatest depth occurs towards the posterior end of the body where there is a distinct hump just anterior to the furcae (Fig. 2c). h) The furcal setae are symmetrical. Local Copepods: (Fig. 1) In the Auckland area, both the New Zealand genera of calanoid copepods, Boeckella and Calamoecia, are found. Calamoecia is represented by C. lucasi (Fig. lc & d) in Lake Pupuke in particular, while of the species of Boeckella, B. propinqua (Fig. la & b) which is found in the Waitakere reservoirs and various small ponds throughout the area, is perhaps the most common. These two species differ greatly in size (Fig. la & lc) and the females of Boeckella propinqua often carry much larger clutches of eggs than do those of Calamoecia lucasi. However correct identification can only be made by dissecting off the modified male fifth leg which has a distinctive structure in each calanoid species (Fig. 3). The most common planktonic cyclopoid is Mesocyclops leuckarti (Fig. le & f) although various other littoral cyclopoids are often found in plankton samples. Cyclopoid copepods are difficult to identify, and those interested should consult Ward and Whipple or Pennak. The fifth legs of some common pond species are shown in Barclay (1964). a b e FIG. 3. Fifth legs of Calanoid Copepods. (a) Female Boeckella propinqua. (b) Male B. propinqua. (c) Male Calamoecia lucasi.
18 2. CLADOCERA: The Cladocera lack any obvious segmentation, are laterally compressed and covered with a folded carapace (Fig. 4b). The second antennae are large and used for swimming, while the antennules are reduced. The large postabdomen (Fig. 4b) bears two terminal claws, lateral teeth and two large setae. It is an important aid in identification. Dorsal to the body and beneath the carapace is a brood pouch (Fig. 4a) where eggs are deposited and incubated after laying. Cladoceran Life Cycles: For most of the year only females are present in many cladoceran populations, and the eggs develop without fertilization (parthenogenesis). In many species males are unknown, but when they do occur they are most abundant at times of low food levels and population crowding. Males are smaller than females, have larger antennules and a conspicuous hook projecting from the first legs (Fig. 4c). Sexual reproduction results in the production of resting eggs able to survive adverse conditions. Local Planktonic Cladocera: In the Auckland area only three species of cladocera are regularly found in the plankton. These are Bosmina meridionalis (Fig. 4a), Ceriodaphnia dubia (Fig. 4e) and Daphnia carinata (Fig. 4b). Occasionally cladocerans which normally lead a littoral existence are found in the plankton, two of the most common of these being Simocephalus spp. (Fig. 4f) and the ubiquitous Chydorus sphaericus (Fig. 4d). Descriptions of other littoral cladocerans can be found in Barclay (1964). B. ROTATORIA (ROTIFERS) Rotifers, being small, are often overlooked, yet they are extremely common members of the planktonic community. Their most characteristic feature is the whorl of cilia on the head (Fig. 5b). Vortices in the water set up by this so called "wheel organ" capture food particles and deposit them in the region of the mouth. The jaws (mastax) are noticeable by their constant movement as food is ground up before entering the stomach. Many littoral rotifers possess a foot which contains adhesive organs for attachment to the substrate (Fig. 7c), but this is reduced or lost in many planktonic forms (Fig. 5a, 6a). Rotifers exhibit an enormous range of body form. Planktonic species are often saccomorph (Figs. 5 & 6) although many (as do littoral types) possess a hardened shell, or lorica, which often has characteristic markings (Fig. 7a).
FIG. 4. Cladocera. (a) Bosmina meridionalis, (0.2 0.6 mm). (b)daphnia carinata, (1.9-2.2 mm), (c) Male Ceriodaphnia dubia, note modified antennules and thoracic spine, (d) Chydorus sphaericus, (0.35 mm), (e) Ceriodaphnia dubia, female (0.3-1.15 mm), (f) Simocephalus exspinosus, (1.6 mm), bp = brood pouch, cp = carapace, pa = postabdomen, atl = antennule, sp = spine from 1st thoracic leg, ant = antenna.
20 FIG. 5. Rotifers, (a) Asplanchna brightwellii, (0.8 mm), (b) A. priodonta, lateral view, (0.6 mm), (c) mastax of A. priodonta, (d) mastax of A. brightwellii. (e) Conochiloides sp. (0.25 mm), (f) Conochilus sp. (0.25 mm). (Modified after Hudson & Gosse). c = whorl of cilia on head (= corona), mx = mastax, st = stomach, o = ovary, ant = antenna, e = egg.
21 eye Rotifers, (a) Polyarthra sp., (0.1 mm), (b) Synchaeta pectinata, (0.25 mm), (c)hexarthra mira, (0.15 mm), (d) Filinia terminalis, (0.4 mm), (a + b after Grasse. modified; c modified after Hudson & Gosse.) bl = lateral blades, ft = foot, Is = lateral spine, ts = terminal spine.
FIG. 7. Loricate Rotifers, (a) Keratella valga, (0.12 mm), (b) K. cochlearis, (0.1mm). (c) Platyais quadricornis, (0.3 mm), (d) Brachionus calyciflorus, (0.4 mm). (c)b. bidentatus, (0.4 mm).
23 Long spines are also common features. Rotifer Life Cycles: The life cycles of most rotifers are similar to those of the cladocera, with normal parthenogenetic development of eggs, and the production of small, reduced males during unfavourable conditions. Eggs are often carried till they hatch (Fig. 7d & e) but some species are viviparous (e.g. Asplanchna, Fig. 5) and the young can be seen inside the adult body cavity. Identification: If possible, rotifers should always be studied alive as on preservation most of the soft bodied species retract the wheel organ and foot, and in this state they are difficult to recognise. The mastax is an important aid in identification (e.g. compare Figs. 5, c & d) and can be removed by eroding the tissues of the animal away with sodium hypochlorite. Further information on techniques and specific identification of rotifers can be found in Ward and Whipple. Common Local Rotifers: Two species of the genus Asplanchna, a large, saccomorph, carnivorous rotifer, are found around Auckland. These differ both in the shape of the ovary (horseshoe shaped in A. brightwellii (Fig. 5a) and spherical in A. priodonta (Fig. 5b) and in the structure of the mastax (Fig. 5c & d). The colonial Conochilus sp. (Fig. 5f) is very abundant in the Waitakere reservoirs and can be distinguished from the similar, but solitary Conochiloides sp. (Fig. 5e) by the position of the dorsal antenna. This is located within the corona in Conochilus, and on the dorsal body surface, outside the cornona, in Conochiloides. Polyarthra (Fig. 6a), a small rotifer, has a number of lateral feather-like blades, while Hexarthra (Fig. 6c) as well as being somewhat larger, has six prominent, fleshy, lateral arms. Synchaeta (Fig. 6b) is another clear, soft bodied, carnivorous rotifer. It has a characteristic triangular shape and is found in more acid environments than Asplanchna. Filinia (Fig. 6d) is often very abundant, and in preserved samples it is usually in the retracted state, with the two lateral spines pointed anteriorly. It is often found sticking to the carapaces of any cladocera which are present. Perhaps the most common planktonic rotifer is Keratella cochlearis (Fig. 7b) which has one median posterior spine on the lorica. Keratella valga (Fig. 7a) which has two spines which are often unequal. A', quadrata also has two spines, but differs from K. valga in that the width of the body at the base of the posterior spines is as great as the width at the base of the anterior spines. In K. valga the body is narrower at the base of the posteroir spines.
24 Some littoral species of rotifers sometimes found in the plankton are also shown in Fig. 7. These are Brachionus calyciflorus (Fig. 7d), B. quadridentatus (Fig. 7e) and the related Platyais quadricornis (Fig. 7c). ACKNOWLEDGEMENTS 1 wish to thank Dr M.A. Chapman for critically reading the manuscript. REFERENCES BAYLY, I.A.E. 1962 BARCLAY, M.H. 1964 GRASSE,P.P. 1965 HUDSON, C.T. 1886 GOSSE, P.H. PENNAK, R.W. 1953 WARD, H.B. 1959 WHIPPLE, G.C. Ecological studies on New Zealand Lacustrine Zooplankton, with special reference to Boeckella propinqua Sars (Copepoda, calanoida). Aust. J. Mar. Freshwat. Res. 13(2): 143-197. Some common pond Arthropoda of the Auckland district. Tane 10: 40-48. "Traite de Zoologie, Tome IV." Paris. "The Rotifera; or wheel animalicules, both British and Foreign." 2 vols. Longmans, Green. London. "Freshwater Invertebrates of the United States." New York. 2nd cd: Edmonson W.T. ed. "Freshwater Biology" Wiley.