Zooplankton of the Atlantic and Gulf Coasts: A Guide to Their Identification and Ecology

Introduction to Zooplankton

INTRODUCTION TO ZOOPLANKTON Plankton (from the Greek word for wanderer) are the small organisms suspended in the water, neither attached to the bottom (benthos) nor able to swim effectively against most currents (nekton). Phytoplankton include the photosynthetic unicellular protozoans and bacteria. Zooplankton include both unicellular and multicellular organisms. Most zooplankton are only a few millimeters in size, but some are much larger. The traditional definition of plankton evokes an image of organisms drifting passively as they are carried to and fro with the currents. Although they are at the mercy of currents, many zooplankton are accomplished swimmers capable of complex feeding and evasive maneuvers. Most zooplankton samples include a diverse mixture of holoplankton, organisms that spend their entire lives in the plankton, and meroplankton, zooplankton that spend only a part of their life cycle in the water column, usually as larval stages (Fig. 1). Demersal zooplankton spend much of their time on or near the bottom but periodically swim upward into the water column, especially at night. The neuston consist of organisms specifically associated with the uppermost layer of the water column, either at or just below the surface . The Portuguese man-of-war (Physalia) is an example of a surface drifter, but many other planktonic animals swim just below the surface. Plankton span a size range of more than eight orders of magnitude, as shown in Tables 1 and 2. Because pico- and nanoplankton pass through the finest plankton nets, we were long unaware of their abundance and ecological importance. In fact, these organisms may equal or exceed the more visible forms in productivity and energy flow. The term net phytoplankton refers to phytoplankton retained by a 20 µm mesh, primarily larger diatoms and dinoflagellates. Microzooplankton includes a mixed assemblage of organisms in the 20- to 200-µm-size range. Most organisms in these groups will pass through zooplankton nets or be too small to be noticed, except at high magnification. The predominant microzooplankton are ciliate, flagellate, and amoeboid protozoans and copepod nauplii. Mesozooplankton in the 0.2 to 20 mm size range dominate most samples and are the focus of this book. Copepods alone often represent 50%–90% of the catch, with densities as high as 1 million per cubic meter (m–3). Rotifers, larval barnacles, crab zoeae, and mollusc veligers can also reach impressive densities, but their presence is more sporadic. Macrozooplankton (2–20 cm) shrimps, larval fishes, and other large, mobile animals, though common, tend to be less abundant and less susceptible to capture. The largest zooplankton, primarily jellyfishes and ctenophores, are easily seen from the surface and may be collected with a bucket or dip net. 2 INTRODUCTION TO ZOOPLANKTON ADAPTATIONS TO PLANKTONIC LIFE Life in the plankton poses many challenges. Zooplankton live in a viscous liquid medium in which locomotion is difficult. There is no place to hide from drifting or swimming predators . Survival in the plankton requires specific adaptations to remain in the water column, to secure suitable food, and to avoid predators. Solutions to these challenges have evolved separately in different groups of zooplankton. Feeding While some benthic invertebrates release lecithotrophic larvae with sufficient yolk reserves to complete their planktonic phase without feeding, all of the permanent zooplankton and most of the temporary larval stages feed actively in the water column and are thus planktotrophic. In contrast to terrestrial ecosystems, most of the primary productivity in the water column is in the form of small particles. Phytoplankton and suspended bits of organic matter (and associated microbes) called detritus are usually less than 1 mm long. Most coastal zooplankton graze on these particles. Their task is to collect or to concentrate planula ephyra late trochophore post trochophore trochophore veliger nauplius cyprid zoea megalopa bivalve mollusc polychaete worm barnacle crab scyphozoan jellyfish Fig. 1. Representatives of some common larvae, illustrating only a hint of the great diversity of planktonic larval forms released by benthic invertebrate phyla. Note how several different larval forms may occur in the development of single species and how different these are from the benthic adult. 2 INTRODUCTION TO ZOOPLANKTON ADAPTATIONS TO PLANKTONIC LIFE Life in the plankton poses many challenges. Zooplankton live in a viscous liquid medium in which locomotion is difficult. There is no place to hide from drifting or swimming predators . Survival in the plankton requires specific adaptations to remain in the water column, to secure suitable food, and to avoid predators. Solutions to these challenges...