Reproductive Systems Research Task - Science, 2013
^ Examples of Nudibranchs
^ Examples of the habitat and distribution of the Nudibranch
Nudibranchs live in an aquatic environment, found in oceans widespread across the world. Their habitat ranges specifically from shallow costal tide pools, to vibrant coral reefs and the cold depths of the ocean; depending on the genus.
This particular environment places multiple pressures on the Nudibranch – the main issue being
strong waves and currents, which have the ability to upset the species’
environment. Other pressures include predators (especially those who have
evolved to camouflage into the Nudibranch’s environment) and tourists, who
stage a risk to damage both the habitat and Nudibranchs themselves, as they are
drawn to their bright colours and unique markings.
^ Examples of courting and copulation between Nudibranchs
Nudibranchs are hermaphroditic, which means they possess both
female and male sex organs. Despite this leading to a possibility of self
fertilisation, the Nudibranch reproduces by cross-fertilisation. As all
Nudibranchs are both male and female, they have the same tube-structured
genitals. The process of fertilisation for such a primitive animal is rather
intimate, with most species undergoing extensive courting before actually
Nudibranchs have reproductive pores on their necks, and once sexually mature they begin searching for another Nudibranch to mate with. Once two members of the same species have recognised each other, courting begins. This involves both species manoeuvring around in opposite circles to get their reproductive pores aligned. In some cases, one might follow the other around touching on the edge of its tail; others will crawl over one another, nuzzle heads, heads or stroke their mate with their mouth tentacles. Others fight each other, and others devour their mate if he or she doesn’t deem suitable. Mating can only take place when each Nudibranch has moved into a head to tail position with their 'necks' touching.
On contact with the other neck, each penis becomes
erect and seeks out the female genital duct of the other. Sperm sacs are passed
through the penis into the other Nudibranch, fertilising both molluscs;
fertilisation is internal, and the specific amount of gametes is unknown – but
is within the thousands. Both now go their own way and lay egg masses that may
contain up to 100,000 eggs.
An advantage of this strategy would be that by
cross-fertilizing it creates many more variations in the gene pool, thus giving
the species a better chance of survival is a dramatic change to their
environment were to happen. The major disadvantage of cross fertilisation would
be that it takes both more time and energy to complete.
^ Examples of egg ribbons laid by Nudibranchs
Nudibranchs are oviparous animals, and therefore development
is external. Their eggs are laid 1-2 days after fertilisation and have a
gestation period of 8-12 days. These molluscs lay thousands of eggs in a
delicate, ruffled ribbon-like strand, called an egg ribbon; such massive
spawning numbers increase the chance of as many as possible of the Nudibranchs
These ribbons are laid on or near the Nudibranch’s favourite
food (generally sponges or algae), this acting as the food source for the eggs,
and eventually the young. The egg girdles laid by Nudibranchs are rarely attacked and eaten by predators, even though many are laid out in the open and appear extremely vulnerable -the only known predators seem to be other Nudibranchs. This is because the strands are protected by toxic chemicals which cover the ribbon, repelling all potential predators except for its own species. Upon hatching, the young have a small shell. The juvenile Nudibranchs rise into the water column and are distributed by the currents. After drifting around in the currents for days, they cast off their baby shell, and settle to a habitat near their food source.
An advantage of this strategy would be that although out in the open, the eggs are protected by the toxic chemicals covering the ribbon, and the gestation period is relatively quick. A major disadvantage to would be that although protected
within the egg, once hatched and carried away from the ribbon the young have no
means of protection or parental care.
^ Examples of juvenille Nudibranchs
No parental care is provided to the young of the Nudibranch; once the ribbon is laid, the mother Nudibranch abandons the eggs and swims away. This creates a clear link between the number of eggs the Nudibranch produces and the mortality rate of the
offspring. The Nudibranch has a low mortality rate due to the low level of care
provided and the pressures their environment place on them, but this is
compensated by the large masses of eggs that are produced, increasing the
chance of survival for the species. Another factor that increases the chance of
survival for the Nudibranch is the protective capsule surrounding the eggs.
The most probable reason as to why there is no parental care provided for the young
of the Nudibranch is relative to energy expenditure, life span and number of
eggs. To care for thousands of young individually without assistance would be an impossible task in itself, let alone one for a primitive Nudibranch. It would also take up an enormous amount of energy, and a considerable amount of the mother Nudibranchs’ life, considering they live for up to three months. As there are many offspring produced, the gestation period of a Nudibranch is very short as is their body length, the level of parental care they provide is low-nothing and sexual maturity is quick, the Nudibranch is clearly of r-selection.
Advantages of this strategy would be that is uses less energy to produce and care for the young, sexual maturity is fast and a large amount of young is produced. The major disadvantage would be the decreases chance of survival due to minimal care and great exposure.
^ An example of penis fencing (the white spike) between Nudibranchs
To ensure against interruptions or such whilst mating, some species have a modified penis with multiple hooks which act as a stimulator and an anchor, while others have a spike-like penis for deep penetration. Some female vaginas are lined with spines to ensure against premature withdrawal when mating occurs in strong currents, or swell conditions.
One species of Nudibranch takes part in what is referred to
as penis fencing. During mating, they fence with one another using their
penises attempting to stab and inject sperm in their opponent, while avoiding
being fertilized themselves. They are able to inseminate their opponent by
injecting their sperm into any region of the other's body they are able to
penetrate. After successfully injecting the other, the sperm streams through
their partner's body on their way to ovaries, where they will fertilize the
eggs. The victorious Nudibranch swims away, whilst the loser now has to bear
the burden of motherhood.
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