Animal life cycles and migration

The leatherback turtle: nesting on the beach where it hatched

A life cycle is the sequence of stages an organism passes through from one generation to the next. For the leatherback turtle (Dermochelys coriacea) almost the whole cycle happens at sea, out of sight of any observer, and the only stage that is seen - and the only stage you are examined on - is the nesting female on a beach at night.

Females do not nest every year. A female reproduces every 2 to 5 years, and when her nesting year comes she does not choose any convenient beach: she returns to the sandy shore where she hatched. This habit of returning to the beach of her own birth is what makes a leatherback colony a colony at all, because each beach is stocked by the turtles that came out of it decades earlier.

At the top of the beach she excavates a nest in the sand, using her hind flippers to scoop out a flask-shaped hole deep enough to keep the eggs at a steady temperature. She lays her eggs into it, buries the eggs and departs, back down the beach to the sea, leaving the nest with no adult in attendance at all.

One visit is not the whole season. She repeats the egg-laying several times at 10-day intervals, so a single female may lay several separate clutches on the same stretch of coast over a few months. Between clutches she is at sea, feeding and building up the yolk for the next set of eggs, and each clutch goes into its own newly dug nest.

The temperature of incubation determines the sex of the offspring. A nest buried in the sand incubates at whatever temperature the sand around it reaches, and in sea turtles warmer nests produce mostly females while cooler, deeper or more shaded nests produce mostly males. This is why the depth of the nest, the shade over it and the time of year all affect the sex ratio of the hatchlings that emerge - the parents contribute chromosomes, but the sand decides the sexes.

• When: females reproduce every 2-5 years, not every year.
• Where: the sandy shore where the female herself hatched.
• The nest: excavated with the hind flippers, eggs laid in it, then buried and the female departs - there is no parental care.
• How often: egg-laying is repeated several times at 10-day intervals within one nesting season.
• Sex: determined by the temperature of incubation, not by chromosomes.
• The exam answer is a sequence: arrive on the natal shore -> excavate -> lay -> repeat at 10-day intervals -> bury -> depart.

From hatchling to adult, and back to the same beach

The eggs hatch after 55 to 60 days. The hatchlings leave the sand together, usually at night when the surface is cooler and predators are fewer, and make for the brightest horizon, which on a natural beach is the sea. From that moment the offspring go to sea, feed and grow - and the beach is not visited again by that turtle for many years.

Males live entirely at sea for the whole of their lives. They never return to a beach, they are never seen nesting, and everything known about adult males comes from animals caught or observed at sea. Only females come ashore, and only to nest, which is why the male half of the life cycle is the part candidates forget.

Growth is slow. Reaching maturity takes 15 to 25 years, so the female digging a nest this season hatched from an egg laid when the beach looked much as it does now, a quarter of a century earlier. During those years the young turtle feeds and grows in the open ocean, and the great majority never reach maturity at all.

Then the loop closes. The mature female returns to the shore where she hatched, mates at sea nearby, and nests, and her hatchlings begin the same cycle. Written as a sequence, the life cycle is: egg, buried in sand -> hatchling after 55-60 days -> juvenile at sea -> adult at sea (males stay there for life) -> mature female returns to her natal beach after 15-25 years -> nests every 2-5 years -> eggs. Everything in that chain is on the syllabus, and a full-mark answer walks along it in order.

• Hatching: 55-60 days after laying, all at once, usually at night.
• Hatchlings go straight to the sea; there is no care from the female, who left before the eggs hatched.
• Juveniles and adults feed and grow at sea; males live entirely at sea and never return to land.
• Maturity is reached after 15-25 years.
• Mature females return to the shore where they hatched, mate at sea nearby and nest.
• The cycle is a loop, not a line: the beach that receives the eggs is the beach that produced the female.

The coral polyp: budding and building a colony

A coral polyp is a small animal with a mouth surrounded by a ring of tentacles, sitting in a cup of calcium carbonate skeleton that it has secreted itself. A coral is not one animal but a colony of genetically identical polyps, all descended from a single original polyp by repeated budding. The colony is what you see as a coral head on a reef, and every polyp in it is a separate mouth feeding for itself.

Polyps reproduce asexually and sexually, and the two methods do different jobs. Asexual reproduction is budding: the parent produces a new polyp next to itself, the new polyp grows its own mouth, tentacles and skeleton, and stays attached. Repeated budding extends the colony sideways and upwards, which is how a coral grows from a single settled larva into a reef-building colony, and it is also how a colony repairs damage.

Sexual reproduction happens through the synchronised release of a huge number of eggs and sperm into the water. The polyps do not pair up and they do not care for eggs: they broadcast their gametes into the open sea, all at once, across many colonies and often across many species. Because eggs and sperm are released together in enormous numbers, the chance of fertilisation in open water is high enough for the species to survive, even though most of the gametes are eaten or simply never meet.

The timing of that release is the key to it working. Spawning is synchronised within a colony, between colonies and often between species, and it is commonly triggered by a rise in water temperature and by the phase of the Moon, and it usually happens at night. A polyp that spawned alone would waste its eggs; a reef that spawns on the same night produces a slick of eggs and sperm at the surface that is far too large for predators to eat all of.

• A coral is a colony of polyps, each with its own mouth, tentacles and skeleton cup.
• Asexual reproduction = budding: the parent produces a new polyp next to itself, genetically identical, which stays attached.
• Budding is how a colony grows and spreads over the reef, and how it repairs damage.
• Sexual reproduction = broadcast spawning: eggs and sperm are released into the water in huge numbers.
• Release is synchronised - within the colony, between colonies and often between species - which is what makes fertilisation likely in open water.
• Spawning is commonly triggered by rising water temperature and by the Moon, and usually takes place at night.

Coral larvae: from surface plankton to a hard surface

A fertilised coral egg is buoyant. Fertilised eggs float to the surface to form planktonic larvae, and the surface is exactly where they need to be: it is the warmest, best-lit water, it carries the most phytoplankton for them to eat, and it is where the currents that will disperse them run fastest.

The larva is a small, elongated, ciliated organism that swims. Larvae swim to the light, which keeps them up in the sunlit surface water instead of sinking out of it. There they feed and grow on the plankton around them, and at the same time they are moved by currents - so their own swimming controls their depth, while the water controls where they end up. A larva that swims well but drifts in the wrong current still ends up on the wrong reef, and most larvae never find a reef at all.

After days or weeks at the surface the larva changes behaviour. It descends and attaches to a hard surface, and once attached it grows into a polyp. Settlement needs a hard, clean surface - bare rock, dead coral skeleton, or rubble - because the polyp must cement itself down and begin secreting its skeleton. Soft sand and mud give it nothing to grip, and a surface covered in algae or sediment offers it no room.

From the settled polyp the cycle restarts, because that first polyp buds to produce the next polyp beside it, and those two bud again, until the colony has spread over the surface it settled on. The life cycle therefore has two completely different halves: a planktonic phase at the surface, which is about dispersal and feeding, and a sessile phase fixed to the bottom, which is about growth, colony building and reproduction. The change from one to the other is settlement on a hard surface, and that is the step most often left out of a written answer.

• Fertilised egg: floats to the surface and becomes a planktonic larva.
• Larva at the surface: swims to the light, feeds, grows, and is carried by currents - dispersal is done by the water, not by the larva.
• Settlement: the larva descends and attaches to a hard surface such as bare rock or dead coral skeleton.
• Growth: the attached larva grows into a polyp, secretes a skeleton, and buds to form a colony.
• Two phases: planktonic (surface, dispersal, feeding) and sessile (bottom, growth, reproduction).
• Settlement on hard substrate is the link between the two phases, and the step candidates most often omit.

Migration: what it is, and the four reasons for it

Migration is the regular, directed movement of organisms from one place to another, usually returning, and undertaken for a reason. The word 'regular' matters: a migrant is not wandering. The movement is repeated, it follows a route, and it happens because the place the organism is going to offers something the place it is leaving does not.

The syllabus gives four reasons for migration, and you should be able to attach an example to each. Finding food: organisms leave water that has been grazed down or that is seasonally unproductive and move to where the food is. Finding mates: adults come together in numbers so that eggs and sperm can meet.

Moving to a different habitat to reproduce: the young need somewhere different from the adults - a spawning ground, a nursery area, a nesting beach, a warm-water calving ground. Avoiding predators: some migrants move out of water where they are heavily hunted, and some move only at night for the same reason.

The last two are easy to confuse. 'Finding mates' is about adults meeting adults. 'Moving to a different habitat to reproduce' is about the eggs or the young needing somewhere the adults do not live - which is why turtles swim to a beach, salmon swim up a river and many fish spawn over seagrass or mangrove roots rather than over the reef they feed on.

• Migration: regular, directed movement between two places, usually with a return, for a reason.
• Reason 1, finding food: moving to water that is more productive or less heavily grazed.
• Reason 2, finding mates: adults gathering so that eggs and sperm can meet.
• Reason 3, a different habitat to reproduce: a spawning ground, nursery area, nesting beach or calving ground that the young need.
• Reason 4, avoiding predators: leaving water where the risk of being eaten is high, often by moving at night.
• The same journey often serves two reasons at once - a turtle that nests on a beach is both finding a mate and using a different habitat to reproduce.

A three-dimensional journey: horizontal, vertical, short and very long

Migration on land is more or less a problem in two dimensions: a herd follows a route across a surface, and rivers and mountains are the obstacles. Migration of marine organisms occurs in a three-dimensional environment, because the ocean has depth as well as a map. An organism can travel north, or it can travel down, and both are migrations.

So marine migration can be horizontal - movement across the surface, from one region or one habitat to another, as when a whale swims from polar feeding water to tropical breeding water. It can also be vertical - movement up and down the water column, from the dark deep water to the sunlit surface and back. Vertical migration needs no coastline, no current and no journey at all in the map sense: an animal can migrate hundreds of metres every day without moving a kilometre across the chart.

Distance varies just as widely, and the syllabus asks only that you recognise the range. Short migrations may cover a few metres, as when reef fish move from the reef flat to the reef slope to spawn, or a few hundred metres vertically in a single night. Very long migrations cross ocean basins and take months, as in tuna, turtles and whales. The same set of reasons - food, mates, a breeding habitat, predators - explains all of them, and the only thing that changes is the scale.

• The ocean is three-dimensional, so a migration can be horizontal or vertical.
• Horizontal: across the surface from one region or habitat to another - whales, tuna, turtles.
• Vertical: up and down the water column - plankton, fish, squid and shrimp moving between the twilight zone and the sunlight zone.
• A vertical migration may cover hundreds of metres without any horizontal distance at all.
• Short migrations: metres to a few kilometres, such as a move from reef flat to reef slope to spawn.
• Very long migrations: across ocean basins, taking months - the transatlantic journeys of tuna and turtles.

Daily vertical migration between the twilight zone and the sunlight zone

The sunlight zone is the upper layer of the ocean, roughly the top 200 m, where enough light penetrates for photosynthesis and where the phytoplankton therefore live. Below it lies the twilight zone, dimly lit but not dark: there is too little light for photosynthesis, yet enough for a predator with good eyes to pick out the silhouette of a small animal from below.

Every day, enormous numbers of plankton, fish, squid and shrimp make the same journey. At dusk they rise from the twilight zone into the sunlight zone, and at dawn they sink back down again, a round trip of tens to hundreds of metres twice in every 24 hours. This is daily (diel) vertical migration, and because so many organisms do it at once it shows up on a ship's sonar as a layer that rises at night and sinks by day.

The reasons are food and predators, and a full-mark answer gives both. The food is at the surface: the phytoplankton and the smaller zooplankton that the migrants eat are concentrated in the sunlit layer, because that is where photosynthesis can happen. The predators hunt by sight, so feeding at the surface is only safe after dark, when the migrant is hidden. By day the migrant retreats into the twilight zone, where it is dim enough for a small animal to be hard to see and where a silhouette is much less obvious than it would be against the bright surface water.

The swim is expensive - hundreds of metres twice a day costs energy - but it is cheaper than the alternatives. An animal that stayed at the surface all day would be eaten, and one that stayed in the deep all the time would be out of reach of the food. Daily vertical migration is the compromise: feed where the food is, at the time of day when it is least dangerous to be seen there.

• Sunlight zone: the upper, lit layer where photosynthesis and phytoplankton are concentrated.
• Twilight zone: below it, too dim for photosynthesis but light enough for predators to see silhouettes.
• Who: plankton, fish, squid and shrimp - a huge proportion of the open-ocean animals.
• What: they rise into the sunlight zone at dusk and sink back into the twilight zone at dawn.
• Why food: phytoplankton and zooplankton are concentrated in the sunlit layer.
• Why predators: at night the surface is dark and hides them; by day the twilight zone is dim enough to make them hard to see.
• The energy cost of the swim is repaid by feeding in the richest water while avoiding being eaten.

Very long horizontal migration, and how migrants find their way

Some organisms cross entire ocean basins. Tuna, turtles and whales all make horizontal migrations of hundreds or thousands of kilometres, and in each case the journey connects two places that do different jobs: a feeding ground where food is abundant, and a breeding ground where eggs, young or courtship are best served. The same individual may make the return journey every year for decades.

Tuna are fast, warm-bodied fish that roam whole ocean basins, feeding in cool, productive water and gathering in warmer water to spawn. Turtles combine the two kinds of migration in one life: a leatherback feeds in temperate and even cold water far from land, then crosses the ocean to nest on a tropical beach, and the female returns to the particular beach she hatched from. Whales feed in cold, highly productive polar water during the summer, where the plankton bloom supports enormous numbers of krill and small fish, and migrate to warm tropical water to give birth and nurse their calves, which could not survive the cold.

None of this is navigation by landmark in the way a walker uses a church spire. The syllabus lists four methods: the Earth's magnetic field, the location of the Sun, Moon and stars, olfaction, and mental maps and landmarks.

The magnetic field works as both a compass and a position-fixing system. An animal able to detect the field's direction knows which way is north; one able to detect its strength and its angle to the surface can also tell roughly where it is, because the field varies predictably across the globe.

The Sun, Moon and stars give direction too, but only if the animal compensates for movement: the Sun crosses the sky, so a sun compass has to be corrected by an internal clock, and the stars turn about a fixed point, which is why the night sky is a usable map. Olfaction carries information over long distances in water, and is the standard explanation for fish returning to a particular river or reef, following a chemical signature they learned as young. Mental maps and landmarks are the local method: a remembered layout of reefs, seamounts, currents and coastal features, used for the last part of a journey when the animal is close to home.

• Tuna cross ocean basins, feeding in cool productive water and spawning in warmer water.
• Turtles feed far from land and cross the ocean to nest on a tropical beach - specifically the one they hatched from.
• Whales feed in cold polar water in summer and migrate to warm water to give birth and nurse calves.
• The journey links a feeding ground to a breeding ground; both are needed and neither can supply both.
• Navigation method 1, magnetic field: gives direction, and its strength and angle also give an approximate position.
• Navigation method 2, Sun, Moon and stars: give direction, but a sun compass must be corrected by an internal clock, and the stars turn about a fixed point.
• Navigation method 3, olfaction: chemical signatures carried in water, used to find a particular river or reef.
• Navigation method 4, mental maps and landmarks: a remembered layout of reefs, seamounts and coastal features for the final approach.