Other Large Zooplankton

There are only about 100 species of ctenophores and nearly all are holoplanktonic (there are a few benthic species). They are major predators of copepods and larval fish, using sticky cells on their pair of tentacles, or lobes around the mouth, to catch their prey. Typical ctenophores are globular - ranging in size from a pea to a golf-ball (Figure 8.6A). They have eight longitudinal rows of cilia (ctenes, or fine hairs), which can be iridescent, giving the illusion of a spinning top. Unlike the true jellyfish, they have bilateral symmetry on top of their radial symmetry, and have sticky - not stinging - cells (known as colloblasts). Like jellyfish, cteno-phores have only two basic tissues, inner and outer, separated by a large layer of jelly (mesoglea).

Local estuarine ctenophores are either tentaculate, with a pair of one metre long tentacles (Figure 8.6A, Pleurobrachia and Hormiphora), or softer-bodied lobate forms without tentacles, but with two large oral lobes (Figure 8.6A2, Beroe, Bolinopsis and Leucothea). Tentacles of ctenophores may be retracted into sheaths within the body, especially after being caught in a plankton net. Ctenophores release eggs that hatch into (cydippid) larvae, which are less than 0.2 mm long and similar to the adult. Lobate ctenophores are the largest of ctenophores (90 mm bell height) and exceedingly fragile,

Figure 8.6 Other, larger zooplankton showing (A1) tentaculate and (A2) lobate ctenophore, (B1, B3) doliolids, (B2, B4) salps, (C1-C3) larvaceans, (C3) a sketch of a larvacean inside its house, (D1-D2) chaetognaths with (D3) detail of head, (E1, E2) fish eggs, (F1, F2) larval fish, (G) tadpole larva of a sea squirt or ascidian (Sources: Dakin and Colefax 1940; Wickstead 1965).

Figure 8.6 Other, larger zooplankton showing (A1) tentaculate and (A2) lobate ctenophore, (B1, B3) doliolids, (B2, B4) salps, (C1-C3) larvaceans, (C3) a sketch of a larvacean inside its house, (D1-D2) chaetognaths with (D3) detail of head, (E1, E2) fish eggs, (F1, F2) larval fish, (G) tadpole larva of a sea squirt or ascidian (Sources: Dakin and Colefax 1940; Wickstead 1965).

sometimes resulting in a puzzling plankton sample of clear amorphous jelly (Box 8.3). They do not preserve well.

Salps, doliolids and the larvaceans (or appendicularians) are the third group of gelatinous zooplankton. They are the specialised pelagic relatives of benthic sea squirts, and indeed ourselves (because these animals possess a notochord - the precursor to a 'backbone', at least during larval development, they are all within the Phylum Chordata). Salps and doliolids are similar, but the former have discontinuous muscle bands around their gelatinous barrel shaped body, while the latter have continuous muscle bands around the 1-2 cm long animal, containing the opaque gut and gonad (Figure 8.6B). At one end of the animal is an inhalant siphon leading to a filtering basket for removing bacteria and very small phytoplankton, with an exhalent siphon at the other end. They have no limbs, tentacles or eyes. Following the phytoplankton bloom, salps tend to bloom during the early spring months by asexual budding. Delicate chains of these animals may be seen in situ - composed of two to dozens of individuals (examples are Salpa, Pegea and Doliolum). The iridescent cyclopoid copepod Sapphirina is often found inside salps (Dakin and Colefax 1940).

Related to the salps are colonies of free floating sea squirts (Pyrosoma). They appear as cigar-sized cones or up to 3 m long tubes of 'orange eggs' in shallow waters from southern NSW and Tasmania (but usually occur in very deep water). They are bioluminescent at night.

Larvaceans are even more specialised sea squirts, with only around 60 species. They consist of a tiny spongy ball containing the head, mouth, gut and gonad, which seems barely attached to a very flat fibrous looking tail (1-2 mm in length, Oikopleura, Fritillaria, Figure 8.6C). In an undisturbed state, the larvacean constructs a delicate gelatinous house around itself,

BOX 8.3 CTENOPHORE BLOOMS

Ctenophores may sometimes bloom (up to one per litre) and may fill a plankton net making it difficult to retrieve into the boat. Ctenophores are voracious predators, eating over 10 times their body weight in crustacean zooplankton per day, despite their body composed of 96% water. The feeding rate seems to be related to tidal turbulence, bringing zooplankton into contact with the sticky cells on the tentacles or lobes. Once Pleurobrachia senses it has 'fly-papered' a copepod onto a tentacle, it spins its body to rapidly wrap its tentacles around the body, somehow wiping the copepod across the single body opening to the central gut. Their abundance varies seasonally and is not necessarily indicative of any environmental concern.

which support very fine primary and secondary filters built into its wall (Figure 8.6C3). The tail generates a filter-feeding current, but eventually the filters clog and the tail helps to inflate a new house from under its mouth. The discarded house may sink to the sea floor and - because six or more houses may be made per day - they are regarded as important components of the global carbon transport, from the atmosphere to the deep ocean. The growth rates of larvaceans are phenomenal, and have been described as the fastest growing animals on the planet. Their central role in the microbial loop and the global carbon flow (Box 8.4) is only surmised as we know very little about this important group.

Chaetognaths, or arrow worms, are holoplanktonic worm-like animals that are placed in their own phylum (Chaetognatha, about 100 species). They are 1-2 cm long, have fins and may initially appear like larval fish without eyes (for example, Sagitta, Figure 8.6D). They are predatory, with a row of bristles or spines either side of the mouth, and may sometimes be found grasping another animal. Some oceanographers use them as indicators of a particular water mass.

Fish eggs are usually perfectly spherical, each containing a ball of yolk or embryo delicately suspended inside (Figure 8.6E). An exception is the elliptical anchovy egg. The eggs hatch into larvae with large and distinctive eyes and only fin folds (Figure 8.6F, Section 8.8). The larvae of

BOX 8.4 SALPS, LARVACEANS AND CLIMATE CHANGE

Salps and the appendicularians have been described as the fastest growing metazoans (multi-cellular animals) on the planet (Hopcroft and Roff 1995). They consume tiny phytoplankton and bacteria that are many orders of magnitude smaller than themselves (a much greater size difference than the copepod diet), and produce dense fecal pellets that rapidly sink. Therefore salps have the potential to alter regional food-webs and even global fluxes of carbon via their fecal pellets (Madin et al. 2006; Andersen 1998). The most common salp of south-east Australia, Thalia democratica, can reproduce both sexually and asexually. An individual may produce a chain of individual clones, resulting in the population doubling or more per day (Heron 1972). Salps compete with other zooplankton such as copepods and krill. In the Southern Ocean, for example, a decrease in krill populations over the last 50 years has been accompanied by an increase in salp populations (Atkinson et al. 2004). In sub-tropical waters, the relative abundance of salps in the zooplankton community could alter the balance between those predator species that avoid salps and those fish for which salps are an important component of their diet.

sea-squirts are very small, delicate little fish-like creatures without eyes (Figure 8.6G).

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