Nekton and Benthos. Nekton DISCUSSION:

Size: px

Start display at page:

Download "Nekton and Benthos. Nekton DISCUSSION:"

May Bridges
6 years ago
Views:

1 Equipment for general class use: Nekton and Benthos a set of videos from selected sources a set of well labeled specimens in phylogenetic order DISCUSSION: Nekton Nekton is a term that applies to those animals that are active swimmers. Unlike the plankton we learned about recently, nektonic animals are capable of undergoing significant horizontal as well as vertical motion. Fish make up the largest fraction of the nekton, but this group also contains large crustaceans, cephalopods, reptiles and mammals. Sometimes oceanic birds are also included in this category, such as albatrosses, petrels, gannets and tropic birds. These animals can have a profound effect on marine communities through predation. Humans have harvested many nektonic species throughout history and still do today. Invertebrates - Cephalopods include squid, cuttlefish and octopuses. Squid can swim at speeds up to 10m/sec (22 miles/hour) and some species of squid show extensive migrations. One such squid species is Todarodes pacificus, which migrates almost 2,000 miles from the northern East China Sea to the Kuriles. They vary in size from a few cm, or the size of a pea, to 20 meters or 6 stories in a building. Squid have a short life span of 1 to several years, so many larger species grow very quickly, eating 15-20% of their body weight every day. Cephalopods fisheries have grown around the world in the past few decades with many nations using large-scale drift nets. One problem with this type of fishing is that many incidental species, such as sharks, turtles, and porpoise's, are also caught and killed as by-catch. Giant squid (Fig. 1) live on the continental slope. We know very little about them, but occasionally they wash up on shore either dead or moribund. A few years ago, Japanese scientist caught one alive for the first time. In the summer of 2009, in a rare case, one was caught in a sampling net in the Gulf of Mexico. The squid measured 5.93 meters or about 20 feet long and weighed nearly 100 pounds. The largest giant squid on record was about 17 meters long. In February of 2007, a colossal squid was caught off the New Zealand coast. These are close relatives to the giant squid. Colossal squid grow to about 13 meters in length and can weigh more than 1000 pounds. There is a family of squid, the Ommastrephidae, that are common in all areas of the world s oceans except the Polar Regions. Some species of this family grow to 3-4m. A specimen may be on display in the lab. An individual of this size was fitted with a transmitter off Japan and was tracked for a day and a half. It hunted from 0-50m at night and descended to about 800m during the day. In one of the videos you will see another species of this family, the Humbolt squid. They are veracious predators and will attack each other (as you will see in the video). Currently, American scientists are investigating why more of these types of squid are being found regularly off the west coast of the United States. 143 of 5

2 FIGURE 1 Vertebrates - Marine reptiles include sea turtles, sea snakes and the marine iguana. The largest sea turtle is the leatherback, which grows to 9 feet and one ton and migrates long distances to bear their young on beaches. Sea snakes are very poisonous, though they are only found in the Indian and Pacific Oceans. They bear their young at sea and are not aggressive. The only species of marine iguanas are found on the Galapagos Islands, Ecuador. During the day, these cold-blooded reptiles sun themselves on warm rocks at the equator. They feed on red algae growing on rocks in the ocean. In order to graze, they must dive into the cold waters of this up-welling region and hold their breaths for several minutes. It is critical that their body temperatures not drop too low, so they must return to sun bathe frequently. The most commonly thought of marine vertebrates are fish. There are many families and we will only discuss a few here. The Class Chondrichthyes, or cartilaginous fishes, lack scales and includes sharks, skates and rays. This group appeared in the fossil record about 450 million years ago and presently includes about 300 species. The largest sharks, the basking and whale sharks attain lengths of 14m and 20m respectively, but are filter feeders. Sharks have low fecundity (low number of eggs) and bear their young live. They are slow growing and do not recover easily from heavy mortality. Since the villianization of sharks in 1978 by the movie Jaws, shark populations have declined worldwide due to recreational as well as commercial fishing. In many cases, the shark itself is either killed for its jaws only, or the dorsal fin is sliced off and the animal returned to the ocean to die. Class Osteichthyes, or bony fishes, evolved about 300 million years ago and presently include over 20,000 species. Nektonic species in the open ocean include those that live in the upper waters both day and night including tunas, dolphinfish, wahoo, etc. They are countershaded by their pigmentation, being darker on the dorsal surface and lighter on the ventral surface. They are voracious predators and can reach speeds of up to 40mph or faster over short distances. Another group is those species which live deeper during the day and migrate up into surface waters at night. The most common representatives are myctophids or lanternfishes. They countershade using the bioluminescence from their photophores. There are also active, deeper living groups which are mostly small such as hatchet fish, anglerfish, gulpers and stargazers. Most have photophores for use in attracting prey, mating or as an alarm. Marine Mammals are a group that includes three orders that have evolved from different terrestrial ancestors and independently adapted to life in the sea. Order Cetacea - whales, porpoises and dolphins. The ancestors were large land animals that entered the sea about 55 million years ago. The largest are the baleen whales (suborder Mysticeti) which are filter feeders, using baleen plates which hang from the roof of the mouth. There will be a large piece of a baleen plate on display in the lab. Some baleen whales use the technique of bubble netting 144 of 5

3 to feed, as you may see in the videos. Many have extensive migrations, such as humpback whales, which winter in the tropics and summer in the Polar Regions. The other suborder, Odonticeti, includes another 66 species; all equipped with teeth and a single blowhole instead of the two that baleen whales have. They do not rely only on surface living prey and use echolocation for prey targeting. They consume fish and squid. Before extreme pressure on whale populations, it is estimated that they ate 190 million tons of krill annually in the Antarctic. Another order of marine mammals is the pinnipeds which include seals, sea lions and walruses. They are different because of their four swimming flippers. They spend part of their time in the water and part on ice or land. The majority are found in the Arctic or Antarctic. The third order includes manatees and dugongs. They are herbivorous and feed on larger plants so they are restricted to shallow coastal waters. Although not officially endangered species, they are seriously threatened by recreational boats whose propellers kill numerous individuals every year. EXERCISE 1: Nekton Video Clips You will see several video segments from a variety of sources. Most of these are from programs such as NOVA, NATURE or DISCOVERY. They are intended to give you an appreciation of the various nektonic groups, their size, their diversity and their different modes of feeding. You should see a good variety of the different nektonic organisms as well as learn more about their behaviors and interactions. DISCUSSION: Benthic Species Having considered the organisms which live in the water column, actively swimming or floating, we will now turn our attention to the organism which live associated to or permanently anchored to the sea floor. These are known as benthic organisms and they exhibit an incredible diversity both taxonomically and morphologically. We will look at this diversity from the intertidal zone all the way to the deep floor of the oceans. The intertidal zone is the region many of us have had contact with. The kind of organisms that we see there depends mainly on the substrate, or type of bottom material, which is related to the intensity of wave action. Rocky intertidal zones have greater visible diversity than sandy or mud flats. The rocky intertidal is characterized by intense wave activity and a hard surface, which supports many species that anchor to rocks. Barnacles, mussels, starfish, macroalgae and sea anemones are all commonly found in these environments. Both the diversity and the biomass of benthic organisms is much lower in sandy intertidal areas because, in many ways, it is like living in liquid sandpaper - the substrate moves every time waves break and wash up the beach. Mud flats have more burrowing organisms, like clams and worms, than sand beaches because wave energy is at a minimum; therefore the substrate is stable. In the subtidal zone, wave energy is much less of a factor (remember the characteristics of shallow water waves). Many species of fishes live here. Fish that live on or in association with the bottom are called demersal fishes. There are many other free-living species of invertebrates such as sea urchin, sand dollars, starfish, lobster, snail's etc. Although these appear to move slowly and do little, you would see how active this group can be with time-lapse photography. They perform important tasks as detritus feeders, by cleaning and mixing the surface sediments. 145 of 5

4 DISCUSSION: The Food Web Revisited The main source of energy for almost all life on this earth is the sun. Photosynthesis fuels all life forms as primary producers convert the sun s energy into sugars. Primary consumers then graze upon these autotrophs. Energy moves through the food webs in this manner (Fig 2). FIGURE 2 The top of a pyramid is called the apex and the animals found there are known as apex predators. In a terrestrial food web, such apex predators might be lions or bears. Typically, apex 146 of 5

5 predators have no natural predators and are viewed as the top of the food chain. Humans are also considered an apex predator. Bioaccumulation- As you may have discussed in the plankton lab, bioaccumulation can be a problem in predators. Dinoflagellates sometimes produce a neurotoxin. Heterotrophic animals, such as clams, consume dinoflagellates eventually leading to a build-up of these toxins in their tissues. If humans ingest contaminated clams, they can become severely ill, suffer permanent nerve damage, paralysis and may even die. Bioaccumulation of anthropogenic materials can affect other organisms as well. You may have already heard warnings of mercury contamination in fish. More recently, scientists have been examining the effects of chemical contaminants such as PCBs and plastic compounds in the environment. Studies are being conducted to examine the affects these contaminants have upon apex predators. It is suspected that some contaminants are already at lethal levels for animals such as orcas. Large quantities of plastic wastes enter the ocean every year. Some of these break down into smaller pieces and can be found in the stomachs of small fish and birds, causing death. The Deep-Sea Food Web- We know from our previous labs that the sun s light energy does not penetrate very far into the ocean. Because of this deep-sea organisms depend on sources of food from above. The rain of organic matter from the water column is known as marine snow and organic matter can be washed out to sea by rivers or carried by turbidity currents. Occasionally, large food parcels, such as dead carcasses of whales, squid, and sharks, aren t consume before they reach the deep sea floor and become a flurry of scavenger feeding activity. The biomass of benthic organisms decreases as one goes from shallow water to 4000m by two orders of magnitude because of this lack of energy input to the environment. However, if an alternate form of energy is provided, in the form of a deep-sea hydrothermal vent or methane seep, a vast variety of organism can be found thriving in high quantities. EXERCISE 2: Benthos Video Clips In order to introduce you to the wide range of marine benthic communities, you will see videos depicting some of these and their inhabitants. Additionally, there will be specimens from shallow water (fishes, molluscs, coral skeletons) and deeper water (mostly crustacea). They will be labeled by depth of collection. 147 of 5

ZOOPLANKTON. Zooplankton: 2. Crustaceans Copepods. Diverse -- protozoans and others

ZOOPLANKTON. Zooplankton: 2. Crustaceans Copepods. Diverse -- protozoans and others ZOOPLANKTON Diverse -- protozoans and others Nutrition modes: Herbivores (graze on phytoplankton) Carnivores (predators) Radiolaria Foraminifera Zooplankton: 1. Protozoans: Foraminifera and Radiolarians