Plankton Diversity Zooplankton
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Plankton Diversity Zooplankton Zooplankton in the context of marine life Zooplankton is considered the most important link between planktonic primary producers and large carnivores, amongst them fish species subject to human exploitation. Zooplankton can also contribute the role of marine systems as sinks of CO2, the main greenhouse gas. When feeding, zooplankton compacts small, slowly settling particles into larger ones (fecal pellets) that can reach the bottom before being recycled, so that biogenic carbon can be sequestered in the sediment, thus enlarging the time for CO2 to return to the atmosphere. Nutritional modes in zooplankton: All zooplankton -indeed all animals and some micro-organisms- are heterotrophic. That is, they require organic substrates as sources of chemical energy in order to synthesize body materials. Animal species differ in how their energy is obtained: as follow: Herbivores: feed primarily on phytoplankton Carnivores: feed primarily on other zooplankton (animals) Omnivores: feed on mixed diet of plants and animals and detritus Detrivores: feed primarily on dead organic matter (detritus) Crustacean Zooplankton Phylum Arthropoda Crustaceans comprise a class of the phylum Arthropoda, whose distinctive features include (1) an external skeleton of chitin, a flexible but stiff material, that is relatively impermeable to the external environment, and (2) some degree of segmentation, with paired, jointed appendages (e.g., legs, antennae). The crustaceans possess antennae, mandibles, and maxillae as head appendages, and usually have compound eyes. They are notable for their mobility, armored exoskeleton, and generally good vision. ( ﻣﺠﺪاﻓﻴﺎت ا ﻷَرْﺟُﻞ Copepods (oar-footed • The word Copepoda derives from Greek, from koipe, that means oar, and podos, which means feet Hence Copepod = oar-footed, referring to the pair of swimming legs that are moved together, like the oars. Copepods are covered by a chitinous carapace. Generally, the body is elongated and has two distinct parts, the anterior, the cephalothorax or prosome and the posterior, the abdomen or urosome. The last somite which is called the anal somite is ending in a two branched structure called furca or caudal rami (singular ramus). In the cephalothorax two filiform appendices are laterally widespread, the antennae. These are sometimes modified into a sexually modified grasping organ in the males. Following the antennae, there are several feeding appendages, plus four to five double branched swimming feet. ٢١ Fig. Adult calanoid copepod. Lateral view with swimming legs in the anterior position. Enlarged image of one of the filtering mouthparts shown at left. Copepods represent the largest group of crustaceans in the zooplankton and the most abundant zooplankton in all the oceans and seas, they are called “insects of the sea“ and can be found in all seasons and marine environments in the plankton, generally forming the bulk of holoplankton communities both in biomass and in number of individuals. They range in length from less than 1 mm to a few millimeters. They are either Herbivorous, carnivorous or omnivorous species; in fact there is no clear separation between feeding habits, the size of food items ٌ .being usually more important than their possible animal or vegetal origin Development involves twelve different stages (after egg), each separated by moulting, or casting off of the exoskeleton, and marked by the appearance of new segments and additional appendages. The first six stages are nauplius (plural, naupli) larvae (designated NI to NVI); the last six are copepodite stages (CI to CVI), with CVI being the sexually mature adult. Females can lay eggs free in the water, or carry them in egg sacs. Copepods are classified into three major groups (or orders, with three other minor orders) – calanoid, cyclopoid and harpacticoid copepods The calanoid (Suborder Calanoida) are the most of marine planktonic species. Calanoids are usually larger, barrel shaped and the body is com- posed of head, thorax, and abdomen o They have long first antennae that almost reach the length of the animal and a thinner abdomen. o Copepods have two swimming speeds. The first is slow, steady, and accomplished using their mouthparts. The second looks like a succession of jumps separated by stillness. This jumpy form of swimming is accomplished by the appendages on the thorax. ٢٢ o Calanoids have a median naupliar eye but lack the compound eyes found in many other crustaceans. o Females often retain eggs in one egg sac until they hatch or they scatter their eggs into the water. In the genus Calanus, the female lays eggs in clutches of about 50, every 10-14 days. o The first stage in identifying them is the number of segments behind the head (three, four or five). o Calanoids feed mainly on phytoplankton, some organic particles, and smaller zooplankton. Detection of food occurs first on the first antennae, which are festooned with sensory hairs. They trap particles on hairlike maxillary setules, located on setae, which, in turn, are located on the maxillipeds. The animal flaps four pairs of appendages (Figure ) to propel water past itself. As a diatom comes near the copepod, the maxilliped reaches out and grabs it, rather than sieving it through the setules (as was originally thought). Actually, because of the viscosity of its environment, the animal grabs the diatom with a surrounding envelope of water, which may have to be strained off the particle before it is passed to the mouth parts. In the larger copepods, sieving by the setules may occur. The animal must detect the presence of a diatom by means of chemosensors on the feeding appendages before reaching out for the food. Feeding current (green) generated by thoracic appendages Maxilliped reaches out and grabs particles entrained in current Another copepod group, the Order Cyclopoida, differs in that members are often smaller with relatively shortened antennae and more segments in the posterior third of the body. Females often retain eggs in two ovisac. The order contains over 1000 species, but the majority live among benthic algae or in bottom sediments; only about 250 species are planktonic. The third group, the harpacticoids are smaller still, elongate and with no difference in width between the thorax and abdomen. They have short antennae, two egg sacs and are typically benthic – although they may be found in the plankton at night or on drift algae. The harpacticoids usually have benthic adults, but the larvae of some species may dominate the estuarine zooplankton. Members of the Lernacopodoida are wormlike copepod that parasitize marine mammals and fish. ٢٣ Figure Planktonic Crustacea. Calanoid copepods: (a), Pseudocalanus elongatus; (b), Centropages typicus; (c), Calanus finmarchicus. (d), Microsetella norvegica, a harpacticoid copepod. (e), Oithona simi/is, a cyclopoid copepod. • Krill Family Euphausiidae, Next to copepods in importance are the Euphausiacea. They are shrimp-like creatures from one to a few centimeters long (from 15 mm up to 5 cm long) that form enormous swarms in high latitude seas. They are fast swimmers, often undersampled by nets because of their visual perception and avoidance capabilities. They dominate the zooplankton of much of the Antarctic Ocean. Euphausiids are generally omnivorous with food consisting of detritus, phytoplankton, and a variety of smaller zooplankton. They are a crucial link between primary producers and carnivores of high latitude (both arctic and antarctic) food webs, and the main food source of food for baleen whales, penguins, seals, etc. They are important as prey for commercial fish (e.g. herring, mackerel, salmon, sardines, and tuna). They harvested commercially Adult krill have bioluminescent dots along their abdomen, stalked eyes, a loosely fitting carapace around the abdomen and their long and setose feeding limbs. Euphausia superba, Antarctic Krill Gnathophausia zoea, a mysid. ٢٤ • Mysids (Figure ) they seldom are important components of the plankton community. Many of these shrimp-like animals spend part of the time on the seafloor, but rise into the overlying water at night or when forming breeding swarms. They have been introduced in some lakes for increasing large particle food availability for fish. Instead, they sometimes became competitors for young fish and even predators for newly hatched fish! are more important in fresh water than in ( ﺑﺮاﻏﻴﺚ اﻟﻤﺎء Cladocera (water fleas • seawater, but they are sometimes abundant in estuaries. Eight marine species worldwide. The genus Podon preys on other zooplankton. Because they are capable of producing by parthenogenesis (i.e. reproduction without males and without fertilization), Cladocera are able to rapidly increase their numbers when environmental conditions are favorable. Cladocera are often found on the surface of samples, which may be due to trapped air inside the carapace. Cladocera: Podon leuckarti (left), Podon leuckarti; (right) Evadne nordmann Figure ) are usually minor components of the) ( اﻟﻘﺸﺮﻳﺎت اﻟﺼﺪﻓﻴﺔ ) Ostracods • zooplankton community. These crustaceans have a unique, hinged, bivalved exoskeleton into which the animal can withdraw. Most species are rather small. Little work has been done on feeding habits in this group, but some species are regarded as scavengers. Ostracods cumacean amphipods • Cumaceans are truly planktonic but rarely dominate the zooplankton.. They look superficially like a large calanoid copepod, but with a bulbous head and thorax, and a slender abdomen