Copepods And Other Small And Abundant Animals

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Copepods account for most of the macroscopic zooplankton in the world's estuaries and oceans (over 9000 species). Copepods are the archetypal zoo-plankter, growing from an egg, through six larval (nauplius) stages and to a further six juvenile (copepodite) stages before finally becoming a sexually reproducing adult (see Chapter 2). The nauplius larva is common to all Crustacea; it is around 0.5 mm in length sometimes with a single compound eye (Figure 8.4, A1-A6). Nauplii have only two or three pairs of limbs -typically the antennae and the feeding limbs with long setae extending out.

Zooplankton Identification

Figure 8.4 Smaller crustacean zooplankton line drawings showing (A1-A6) various nauplii, (B1-B3) calanoid copepods, (C1-C3) cyclopoid copepods, (D) barnacle cyprid larva, (E1-E3) harpacticoid copepods, (F1-F2) ostracods, (G1-G3) cladocerans Podon, Evadne, Penilia (Sources: Dakin and Colefax 1940; Wickstead 1965).

Figure 8.4 Smaller crustacean zooplankton line drawings showing (A1-A6) various nauplii, (B1-B3) calanoid copepods, (C1-C3) cyclopoid copepods, (D) barnacle cyprid larva, (E1-E3) harpacticoid copepods, (F1-F2) ostracods, (G1-G3) cladocerans Podon, Evadne, Penilia (Sources: Dakin and Colefax 1940; Wickstead 1965).

Juvenile and adult copepods are small - being 1 to 8 mm in length and with no carapace and a compound, sessile eye. There are no limbs on the abdomen, which is a distinctively thinner 'tail' compared to the thorax. The three pairs of thoracic limbs are well developed for swimming and feeding using comb-like rows of setae, and sinking is controlled by their fat content and extension of the antennae.

Copepods are classified into three major groups (or orders, with three other minor orders) - calanoid cyclopoid and harpacticoid copepods (Box 8.1).

Calanoids are usually larger and have long first antennae that almost reach the length of the animal and a thinner abdomen (for example, Acartia,

BOX 8.1 THREE KEY STEPS TO IDENTIFYING COPEPODS

1) Is it a calanoid, cyclopoid, harpaticoid or something else? Calanoids have long antennae and are larger, while cyclopoids tend to be smaller with very short antennae.

a) Does it have a movable articulation between the 5th and 6th thoracic segments? Usually with long, strong antennae, nearly as long as body. = Calanoid E.g. Acartia, Paracalanus, Undinula b) Does it have a moveable articulation behind 4th thoracic segment and the metasome is much wider than the urosome? Usually smaller than calanoids with short antennae. = Cyclopoid

E.g. Oithona, Oncoea, Corycella c) Does it have a movable articulation behind the 4th thoracic segment, a slightly wider metasome than urosome and both are more or less cylindrical. Usually with a long furcula or setae from the rear, almost as long as itself. = Harpacticoid E.g. Euterpina, Microsetella, Macrosetella.

d) None of the above (very rare and parasitic on fish: Thaumaleus, Monstrilloida, Caligus).

2) Is it from estuarine or oceanic waters?

Estuarine samples often contain smaller individuals and are less species rich. E.g. Oithona, Euterpina, Paracalanus, Acartia, Gippslandia, Gladioferens (especially at night)

3) To identify a copepod to genus or species when, based on size, shape, general appearance and habitat information, a number of possibilities exist then it is necessary to look at the shape of the 5th legs. To do this, dissect the copepod under the compound or dissecting microscope using a pair of tungsten needles (Box 4.8).

Calanus, Temora and Gladioferens, Figure 8.4B). They scatter their eggs into the water, or retain them in a sac until they hatch (Box 8.2). The first stage in identifying them is the number of segments behind the head (three, four or five, Figure 8.4B).

Cyclopoid copepods are often smaller, with distinctively shorter antennae. Females often retain eggs in an ovisac. Cyclopoid copepods include some carnivorous species (such as Oncaea, Oithona and Sapphiri-na, Figure 8.4C). Sometimes looking over the side of a boat offshore on a still day, the red and purple iridescent glint off the flattened body form of Sapphirina may be seen.

Harpacticoid copepods are smaller still, elongate and with no difference in width between the thorax and abdomen. They have short antennae, egg sacs and are typically benthic - although they may be found in the plankton at night or on drift algae (Macrosetella, Microsetella, Figure 8.4E). Some harpacticoids have distinctive very long tail setae - almost as long as the animal.

A related group of small crustaceans are the ostracods (>8000 species) and cladocerans (400 species) - sometimes known as seed shrimps or clam shrimps - which have their vastly reduced bodies and limbs contained within a bivalved carapace. Of the two, ostracods are smaller and often benthic, with the head and eye completely contained within the carapace

BOX 8.2 THE ECOLOGY AND AQUACULTURE OF A DOMINANT ESTUARINE COPEPOD

Gladioferens is a genus of calanoid copepods containing around five species, found abundantly in the estuaries of Australia and New Zealand over a wide range of salinities. It is described as a pioneer herbivore, exploiting the phytoplankton blooms after rainfall (Bayly 1965; Rippingale and Payne 2001). Their abundance is in part regulated by other copepods including the omnivorous predators Sulcanus (a cyclopoid) and Acartiura (a calanoid). Calanoids seemingly glide through the water, typically upside down, propelled by rapid beating of their second antennae. Jerky swimming may also occur when they rapidly swim with the five pairs of swimming legs. Adult male Gladioferens imparipes have a bent left first antennae (that is, they are asymmetric), which it uses to grasp the female and attach a sperm packet near her genital opening. The female releases the fertilised eggs into a sac until the free swimming nauplii hatch. They may complete all six naupliar moults and all six copepodite moults to become a mature adult in 10-12 days at 25°C (Payne and Rippingale 2001). By thriving in estuaries, G. imparipes has many natural attributes for aquaculture and as food for larval fish.

(for example, Pyrocypris and Euconchaoecia, Figure 8.4F). They swim by twirling a powerful pair of antennae that they can retract safely within the halves of the carapace. The Cladocera (Branchiopoda) are best known by the freshwater Daphnia or water fleas, which have a head and antennae that are frequently proud of the carapace. The marine equivalent is Penilia (Figure 8.4G3), which when dead in the sorting dish, lay on their backs and the two halves of the carapace relax wide open (like the wings of a small butterfly), exposing the limbs. Two other related species, Evadne and Podon, seem to be 'all eyes and a few limbs' showing remarkable simplification from the basic crustacean form (Figure 8.4G1-G2).

A related group are the larval cirripedes (barnacles), which may be found as dark, dense cyprid larvae ready to settle (Figure 8.4D). Sometimes the large translucent exoskeletons (exuvia) of adult barnacles occur, when they moult en masse during warm summer months.

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Responses

  • MEHARI
    Which plankton have shorter setae?
    2 months ago

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