Category Sea/Ocean

Why don’t sharks just swim up to whales and start eating them?

Sharks do sometimes prey on whales, but adult whales are massive beasts that can and will kill sharks if they come too close. Calves are sometimes attacked, lone mothers with a calf are preyed upon, but whales often live in pods that defend one another. Even peaceful filter-feeder whales still pack a punch with their tails, whales are social and will defend one another and their sonar (okay, okay, echolocation) gives them immense advantage over sharks.

Fish are deaf or nearly deaf. The shark doesn’t even know the whale has a lock on it and is coming down, hard. The shark will smell the whale but by the time it can see the whale coming it will be too late. The whale can accurately locate the shark from kilometers away and act accordingly.

 

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What are the threats to seagrasses?

Like all seat creatures, seagrasses are destroyed by cyclones, over-grazing, fungal and other attacks. Seagrasses some times dry out in the inter-tidal areas. Have you seen them washed ashore? In estuaries, too much of freshwater and suit accumulation destroy seagrass beds.

The biggest seagrass enemy is human activity. We pollute the seawater when we use chemical pesticides and fertilizers and the water runs into seagrass beds on the coast. These chemicals support the growth of algae that block sunlight reaching seagrass. Sediments choke the leaves and sea dredging damages seagrass meadows. Boat anchors and large marine debris can kill sections of seagrass meadows.

Overfishing is bad too. Without fish, the sea-urchin population explodes. Sea urchins eat up seagrass. When large carnivorous fish like sharks are removed predators that eat invertebrates become more abundant. These in turn wipe out small pollinating creatures that the seagrass needs. An increase in herbivorous fish also kills off the seagrasses. You know of the food-chain, right?

Cyclones and tsunamis affect seagrasses. It takes time for left-over seagrasses to regenerate the meadow or for the seed-bank present in the sediment to give rise to new shoots.

Another big threat comes from global warming. When sea temperatures rise, marine heatwaves are created. These occur suddenly, spiking seawater temperature. Examples are the “record-breaking ‘Ningaloo Nino’ (2011) off Western Australia, the long-lasting ‘Blob’ (2013-2016) in the northeast Pacific ad El Nino-related extreme warming in 2016 that affected most of the Indo-Pacific. The number of marine heatwaves has increased around the world, and coral refs, seaweed beds and seagrass meadows are at maximum risk from marine heatwaves.

Biologists tell us that seagrasses bury carbon sediments faster than tropical forests do. When a tree dies, the carbon gets released into the atmosphere. in seagrass meadows, carbon gets stored in seabeds and even if the seagrass dies, carbon stays trapped inside the sediment. Destruction of the ocean food-chain, reducing ocean productivity. Even if protected areas are created for seagrasses, they will survive only if the seawater is clear.

 

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What is the role of seagrass in the marine ecosystem?

Where are they found?

Seagrasses are found is seas surrounding all continents except Antarctica. If you are lucky, you can spot them along the coast in clear, shallow waters. The water has to be clear so that sunlight can get through and help the photosynthesis. Some seagrass species grow in the space between the highest tide line and lowest tide line (inter-tidal zone). This is the area where you can see the seabed when the low tide retreats. Here seagrass grows close to mangrove belts and survives exposure to heat and dryness, thanks to the high humidity. Other species of sea grass (tape grass) are always found submerged in  water in the sub-tidal zone. The eelgrass occurs in estuarine areas. When conditions are suitable, sea grass form dense underwater ‘meadows’-some of which are large enough to be seen from space.

Description

Sea grasses have roots, stems and leaves and produce flowers and fruits. They are closely related to land plants, and probably evolved from land living angiosperms (flowering plants) millions of years ago. The closest relatives to seagrass on land are grasses, palms and lilies.

How many seagrass species are known?

There are 72 different species of seagrasses in the world, belonging to four families. This is not much, but then “It is a huge challenge to evolve the capability to tolerate salt water and live in a submerged environment,” said Frederick Short a marine biologist from the University of New Hampshire and director of Seagrass Net, a global seagrass monitoring network.

The warm waters of the Indo-Pacific region have the highest number (14) of seagrass species in the world. This is because seagrass “evolved first in this part of the world, “Seagrass in this region have had a longer time to diversity and have had suitable growing conditions.

India being in the Indo-Pacific region, has 14 seagrass species. The Gulf of Mannar and Palk Strait house all the 14 species found in India, while the Lakshadweep and Andaman and Nicobar Islands have 8 and 9 species respectively.

How do they ‘breathe?

Land plants have small pores on their leaves called stomata, through which they can take in carbon dioxide from the air and release oxygen. Seagrasses do not have stomata. Instead, they have a thin cuticle layer which allows gases and nutrients to seep directly into the leaves from the surrounding water.

How do they reproduce?

The seagrass produces male and female flowers. The pinkish-green male seagrass flowers produce big pollen, up to 5mm long. Make flowers release pollen covered in a sticky, slimy substance into the surrounding water and the water-currents carry them over to the female flowers. Females have stigmas shaped like tentacles and are supported by coiled stalks. When male flowers release their pollen at night, the female flowers open their petals to ‘catch’ the drifting male flowers.

Their stalk straightens, and then coils and sinks down to the bottom.

Some seagrasses take the help of tiny marine invertebrates like amphipods (tiny shrimp-like creatures) and polychaetes (marine worms) for pollination. The seeds that result float and find suitable seabeds to grow new meadows.

In another method to sustain their species, seagrasses use their massive rhizomes-modified underground stems like ginger and turmeric. These rhizomes spread in the seabed and put out new roots and shoots. This way, a single plant can colonise a large area of seabed, and live on for thousands of years.

How are seagrasses helpful?

Dense seagrass growth traps flowing sediment and nutrients, and creates a world where life can thrive. Their leafy roof supports a wide grow on them: small marine invertebrates, which in turn attract sea anemones and fish; mega herbivores like green sea turtles and dugongs.

 

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What do sea pens do?

True to their names, sea pens resemble old-time quill writing pens. These colonial marine cnidarians (a large group of aquatic invertebrate animals) belong to the order Pennatulacea. Although the group is named for its supposed resemblance to antique quill pens, not all sea pen species live up to the comparison. Colours range from dark orange to yellow to white.

These underwater animals are actually a type of octocoral (soft coral), named for the eight stinging tentacles that they use to capture plankton (tiny floating plants and animals) to feed themselves. In fact, a single sea pen is both an individual and a colony. The basic unit of a sea pen, like all other corals, is a polyp, which consists of a sac-like body cavity enclosed by a mouth and surrounded by a ring of tentacles. The larva usually settles down in sand, mud, rubble or, sometimes, solid rock and this becomes the primary ployp. It buds into daughter polyps, and the sea pen grows. It is supported on a stem-like structure. While the larvae of some species settle close to their parents, others are carried great distances by ocean currents.

Some polyps feed by using nematocysts (a specialized cell in the tentacles of a jellyfish, corals and sea anemones, containing a barbed or venomous coiled thread that can be projected in self-defence or to capture prey) to catch plankton; some force polyps reproduce; and some force water in and out of canals that ventilate the colony.

Sea pens inhabit shallow and deep waters from the polar seas to the tropics. Some sea pens use a bulb inflated with water to anchor them to the sea floor. All have hard, internal skeletons, and at least some of them can glow in the dark.

Although many species live in shallow water, others have been found as deep as 20,013 feet (6,100 metres) below the surface. One species of Umbellula has been discovered living in cold, dark waters near Antarctica. It grows to about 10 feet (3 metres) long with a big, flat head of polyps at its end, the tentacles picking food out of the water as the head is pushed along by the current. Most sea pens, however, grow to between 2 inches and 6 feet, 7 inches (5 cm to 2 metres) in height.

 

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How parrotfish play a vital role in ensuring reef health?

Colourful, tropical creatures with big beaks rather like their feathered namesake – the parrot – parrotfish are known best for two things – eating most of time and pooping. Now, this may seem disgusting, but once you know more about these fish, you will realise that they are saviours of coral reefs. That’s because they spend around 90% of the day eating algae that damage the reefs. This continuous eating cleans the reefs and this ensures the health of the corals.

How does it go about doing this? Each parrotfish has around 1,000 teeth, line up in 15 rows and cemented together to form the beak structure, which they use for biting into the coral. When the teeth wear out, they fall to the ocean floor. But, this does not perturb the parrotfish, because it has so many rows of teeth. These powerful teeth break down bits of coral which are later excreted into nothing but fine white sand that lines the beaches of the South Pacific. This process is known as bioerosion and it helps keep algae under check which provides the right environment for young corals to grow.

You may scoff at the idea that some of the most beautiful beaches in this region are the creation of parrotfish or rather its poop! But, scientists estimate that a single Chlorurus gibbus parrotfish can excrete more than 2,000 pounds of sand each year!

There are about 80-90 species of parrotfish regarded as a family (Scaridae) that live in reefs all around the world, but they all generally live about 5-7 years and grow to 1-4 feet in length. The largest of the family is the bumphead parrotfish found in the Indo-Pacific waters, measuring more than 4 feet and reaching a weight of 100 pounds. Species vary in size from the five-inch Bluelip Parrotfish to the 4 foot Rainbow Parrotfish. Coloring ranges from reds to greens, blues and yellows, as well as grays, browns, and blacks. The parrotfish swims by rowing itself along with its pectoral (side) fins.

They eat and eat all through the day. At night, they sleep in the safety of a cocoon they make out of the mucous they secrete and by finding a suitable hiding place in the coral. It’s nature’s way of keeping them safe from predators.

Like most fish, male parrotfish contains harems of females. This means that in the event of there being no dominant male in a school which may comprise around 30-40 fishes, the largest female changes gender and colour to become the dominant. Once the transformation is complete, they can mate with the other females in the school. Parrotfish release many tiny eggs into the water, which become part of the plankton. The eggs float freely, settling into the coral until hatching.

Unfortunately, humans are their main predators as these fish are considered a delicacy in countries of the South Pacific where they are overfished. And when their numbers are depleted, algae levels shoot up affecting the health of the coral reefs. According to a recent study, reefs where parrotfish were abundant in the 1980s are the ones that are healthy today.

 

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How can a boat sail into the wind?

The wind is the only thing that propels the sailing boat, so how can a boat sail against it? Amazingly, the most important force that drives a boat into the wind is suction.

The boat’s sail is like an aircraft’s wing on its side. On the outwardly curved, leeward side the wind has to flow around the sail, creating a powerful suction effect pulling the sail towards it. The same principle applies to an aircraft, which gets lift from the suction on the top of its wings.

The suction effect is produced by the laws of aerodynamics. The air that is diverted around a curved sail becomes compressed so that it can squeeze past. When a moving stream of air is compressed its speed increases – a draught under a door can be surprisingly strong for this reason. And when the wind’s speed increases, a lot of pressure occurs. This is because the faster the air is moving, the fewer molecules there are in any given space.

The area of low pressure on the leeward side of sucks that sail towards it with the twice the force that the same strength of wind can push into it on the windward side.

So the wind forces the boat sideways. However, the keel – or centreboard – of the boat resists the sideways movement. The wind’s force is then converted partly into a forward movement of the boat and partly into a tilt to leeward which the yachtsman has to counteract by leaning out from the other side of the boat. The boat sailing close to the wind is bound to move substantially sideways – an effect called leeway. But the helmsman can compensate when plotting his course.

Boat can sail directly into the wind, but at 12 m yacht can sail only 12-15 degrees off the wind. To go in the direction of wind is coming from, the boat has to zigzag, or make a series of tacks. The closer a boat sails to the wind, the slower its speed will be. The helmsman can go faster by making wider zigzags at a bigger angle to the wind, but then he has to travel farther.

 

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