Life Cycles Of Zooplankton

In general, the smallest plankton have the shortest life cycles: bacteria and flagellates generally multiply within a few hours to one day. Most mesozoo-plankton have life cycles of a few weeks, while the macro- and megaplankton usually have life cycles spanning many months and longer.

Many zooplankton spend their entire life cycle as part of the plankton (for example, copepods, salps and some jellyfish) and are called holo-plankton. The meroplankton, which are seasonally abundant, especially in coastal waters, are only planktonic for part of their lives (usually at the larval stage). Most bear little, if any, resemblance to the adult form and drift for days to weeks before they metamorphose and assume benthic

Polyp Oceanography Life Cycle
pianula polyp strobilating polyp ephyra adult mudusa

nauplius cyprid CYP"d settled adult

Figure 2.6 Life stages (larval to adult form) of a typical copepod, barnacle and jellyfish. Names in italics refer to those life stages that are not planktonic, when the animal becomes attached to hard surfaces.

nauplius cyprid CYP"d settled adult

Figure 2.6 Life stages (larval to adult form) of a typical copepod, barnacle and jellyfish. Names in italics refer to those life stages that are not planktonic, when the animal becomes attached to hard surfaces.

or nekton lifestyles. Examples of meroplankton include the larvae of sea urchins, starfish, crustaceans, marine worms and most fish. Planktonic and sessile life stages of some common zooplankton types are shown in Figure 2.6 and are described below.

The general copepod life cycle includes six nauplius stages (larvae) and five copepodid stages (juveniles) prior to becoming an adult. Each stage is separated by a moult and, as the stages progress, the trunk of the copepod develops segmentation. Sexes are separate, sperm is transferred in a sper-matophore from the male to the female, and eggs are either enclosed in a sac until ready to hatch or released as they are produced. Development times from egg to adult are typically in the order of 2 to 6 weeks, and are significantly affected by temperature and food availability. The life-span of adults may be from one to several months.

Barnacles also have free-swimming nauplius stages, followed by a carapace-covered cyprid stage after the final naupliar moult. Cyprid larvae are attracted to settle on hard substrates by the presence of other barnacles, ensuring settlement in areas suitable for barnacle survival and for obtaining future mates. After settling, the cyprid releases a substance to permanently

BOX 2.1 PLANKTON DIVERSITY

In 1961, the great biologist GE Hutchinson wrote a speculative essay entitled 'The paradox of the plankton', expressing surprise at the high diversity of plankton in an otherwise fairly uniform environment (Hutchinson 1961). Classical competition theory would suggest that, without disturbance, there should be very low diversity - particularly for holoplankton. The key is that the ocean environment is not uniform, but is divided into characteristic water masses, and is not without disturbance caused by seasonal changes and storms. Modelling also suggests high diversity is possible when there are hundreds of species (rather than tens of species), each with their own life cycles, sizes and physiology.

cement itself to the substrate. Calcareous plates then grow and surround the body. The appendages face upwards to form cirri which sweep food particles into the organism. The adults are hermaphroditic (each with both male and female parts) and reproduce sexually by cross fertilisation. The adult broods the fertilised eggs within the shell until they develop into nauplius larvae. Over 10 000 larvae may be released by a single adult.

Life cycles ofjellyfish are complex, with generally two adult morphologies: polyp and medusa (typical jellyfish form). The sexes are separate and mature adult medusae release eggs and sperm, which, upon fertilisation, form free-swimming, hair-covered larvae known as planulae. After a few days to weeks, the planulae settle on hard substrates and metamorphose into tiny sessile polyps (which look like upside-down jellyfish), which clone themselves and bud (strobilate). Juvenile jellyfish (ephyrae) peel off from the stack, float into the plankton as young jellies and grow into adult medusae. This transformation can take a few weeks up to a few years, depending on the species ofjellyfish.

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