Aquatic and Marine Biology

Benthic animals are those that live on or in the bottom substrate of aquatic environments, such as the deep sea or lake beds.

Sharks are cartilaginous fish, meaning their skeletons are made of cartilage rather than bone. Cartilage is lighter and more flexible than bone.

Seals are marine mammals that are capable of sleeping in the water, often by bobbing vertically or resting on the surface. They are carnivores, not vegetarians, and are found in various oceans globally rather than being limited to the Gulf of Mexico.

Ammonotelic animals are those that excrete nitrogenous waste primarily as ammonia. This typically includes aquatic insects, bony fish, and some amphibians (like tadpoles). In the list, aquatic insects, bony fish, and marine bony fish are definitely ammonotelic; the classification of others varies, but four is the standard answer in this context.

• Sea snakes are very poisonous. The beaked sea snake is the most poisonous sea snake. They have flattened or compressed paddle-like or oar-like tails.
•  This is the distinguishing feature of sea snakes. They have laterally compressed bodies.  

Animals which give birth to young ones are known as mammals which are vertebrates.

In viviparous animals, females give birth to young ones. The development of zygote takes place inside the female's body. Females deliver young ones and the chances of survival of offsprings are more. Mammals (except egg-laying mammals), like whale, shark, mouse, humans are examples of viviparous animals.

So, the correct answer is 'Sharks'.

Fresh water fisheries are found in ponds, tanks, rivers and various water bodies catla, rohu, mullets, and bhetki are some fresh water fisheries. They have important contribution to the growth of the economy of the country. There has been a very impressive growth in fish production during 1990's in the country.

The western mosquitofish (Gambusia affinis) is a species of freshwater fish, also known commonly as simply mosquitofish. Mosquitofish were introduced directly into ecosystems in many parts of the world as a biocontrol to lower mosquito populations, which in turn negatively affected many other species in each distinct bioregion. Based on diet, mosquitofish are classified as larvivorous fish. Their diet consists of zooplankton, small insects and insect larvae, and detritus material. Mosquitofish feed on mosquito larvae at all stages of life. Adult females can consume hundreds of mosquito larvae in one day.

Pontobdella is the type of leech. They are also known as skate sucker. It is the genus of leeches which includes bilaterally symmetrical animals. They belong to the class of Clitellata. It is marine leech. They are parasitic worms and attack skates and rays. They attach to the rock and wait to attack the skate. They also attack certain plaice. 

Vertebrates, and a few invertebrates, have a closed circulatory system. Closed circulatory systems have the blood closed at all times within vessels of different size and wall thickness. In this type of system, blood is pumped by a heart through vessels, and does not normally fill body cavities. In fish, blood flows from the heart to the gills for gas exchange, then to the rest of the body, and finally back to the heart. This is called a single circulation since the blood flows through the heart only once during each complete trip around the body. Amphibians evolved a double circulation; blood flows from the heart to the gills or lungs for gas exchange, then back to the heart to be repressurized before flowing to the rest of the body. The vessels that serve the respiratory organs are called the branchial circuit (for gills) or pulmonary circuit (for lungs). Vessels that serve the rest of the body are called the systemic circuit. The amphibian heart and most reptilian hearts have only three chambers, two atria and one ventricle and there is some mixing of oxygen-rich and oxygen-poor blood in the single ventricle. Endothermic vertebrates, the birds and mammals, have higher metabolic rates and require stricter separation of the pulmonary and systemic blood. Thus, they have four-chambered hearts. Oxygen-rich blood flows through the other ventricle to the systemic circuit.

The amphibians have blood directed in two circuits with two atria and one ventricle where mixing of blood takes place.

• Planktons are passively floating in upper water, nektons are actively swimming while benthos lead sedentary life upon the sea bottom. 
  
• Plankton (singular plankter) are a diverse group of organisms that live in the water column of large bodies of water and that cannot swim against a current.
  
• They provide a crucial source of food to many large aquatic organisms, such as fish and whales.
  

Aquatic mammals, such as whales and dolphins, often have larger hearts relative to their body size to support efficient circulation and oxygen delivery during long periods of diving.