By Ella Baldwin
Argonauts are a type of cephalopod that are closely related to the octopus. The scientific name is Paper Nautilus, because it has a paper-thin shell around it that protects it. However, the males do not have this shell, which is why the species is sometimes incorrectly referred to as a species of octopus. Females tend to grow to between 20 and 40 centimetres, whereas males are significantly smallerat only 1 to 2 centimetres in length. The argonaut has eight arms, and is capable of swimming, but they tend to drift in the open water. The female argonaut has an interesting way of making sure she has the perfect buoyancy in the water, using her shell. She will swim up to the surface and retrieve the air she needs into the top of her shell, and then use her weight to drop down so that she is not on the surface but not quite on the bottom of the ocean. The female will wrap herself around the shell to keep her eggs safe, which are located inside the shell, and also to keep herself in the shell. The shell is made of calcium carbonate, and the female secretes it through two of her first pair of arms when preparing for fertilisation. The female is not attached to the shell, and can leave it if she needs to, but they tend to stay in the shell for protection. Click here to see a female argonaut out of her shell. The female's shell is designed to protect her eggs, which are in long threads.
Female Argonaut using her shell for buoyancy
Female Secreting her shell
Argonauts are an aquatic species, and are epipelagic, which means they live in waters 100 metres deep or less from the surface. They live in open tropic and subtropic waters of mild temperatures, and are faced with minimal pressures, but have some predators that live also in the epipelagic zone, including yellowfins, bigeye tunas, seabirds and lancelets. The environment that they live in is fairly unstable, as it is constantly changing, and sometimes there will be boats and other floating objects passing by.
Argonauts have an interesting process
and method of fertilisation, involving the male detaching his third arm, which
is then attached to the female. Before fertilisation occurs, the female will
secrete her shell from her two front arms to protect her eggs. Until the conditions are right for fertilisation to occur, the elongated arm is stored under the eye of the
male in a pouch, and bursts from the pouch when needed. The arm is energised
and can swim, but has minimal or no sense of direction, meaning the male
detaches it when he is next to the female. The arm has suckers on the inside of
it, used to attach itself to the female’s mantle, which is her shell, and then
burrows into her mantle cavity, where it waits to be fertilised. Once the arm
of the male has been detached, the male can no longer survive, and sinks to the
bottom of the ocean. The style of fertilisation is cross fertilisation, as
there are specific male and female species, and in the process, gametes are
released to be fertilised, although it is uncertain about the exact amount.
Unlike most cephalopods, the female argonaut can reproduce multiple times, and
will not die after reproduction.The advantages of internal fertilisation include - eggs will be fertilised in a safe environment, and they are less likely to be lost, therefore the sperm will not die as easily, and the majority of the eggs will be fertilised.
Diagram of Fertilisation - after the penis has been detached
type of development that the argonaut uses is oviparity, or external, but she
keeps the developing eggs in her shell to protect them. This means she uses part
oviparous and part viviparous development. The gestation period tends to refer
to the amount of time a mammal has it’s baby in her womb, and as the argonaut
lays eggs, it is difficult to calculate this figure. It is also difficult to
calculate the length of time the baby argonauts are in their eggs, so this
figure was hard to research and come up with an answer. An advantage of
viviparous development is that the eggs are safe and can be looked after
closely by the mother. Some disadvantages of external development include the eggs can be broken or eaten by other animals as they are a rich
source of nutrients.
Argonauts primarily use the r-selection method of parental care, although research has shown that in the Aegean Sea, the eggs have become increasingly larger in size and fewer in number, and the mother has started to show a more K-selection approach to the care of her young, partly because the environment in the Aegean Sea is more stable than the regular environment of the argonaut. As the main method of parental care is the r-selection, the amount of eggs produced and fertilised is very large. The mortality rate of the argonaut offspring is difficult to estimate, as the species is hard to observe in the wild, however, from the statistics gathered, it seems that it cannot be insignificant, as there are so many eggs fertilised. There is minimal parental care in the argonaut species, and once the eggs have developed and have been removed from the female’s shell, she will look after them until they are a little larger and more developed, then she will abandon them in search of another mate. A disadvantage of r-selection parenting is that the babies have no knowledge of what the outside world is like, and are easy prey for other larger fish and predators. An advantage of K-selection is the mother will attempt to teach her young everything they need to know before she goes away or dies, preparing them for the outside world of the ocean.
Baby Argonaut caught
EOL. (n.d.). Discover Life: Point Map of Argonaut. Retrieved 09 16, 2013, from EOL: http://eol.org/data_objects/21112091
Finn, D. J. (n.d.). Argonaut buoyancy video. Retrieved 09 14, 2013, from Museum Victoria: http://museumvictoria.com.au/about/mv-news/2010/argonaut-buoyancy/argonaut-video/
Orenstein, M. (n.d.). Marine Invertebrates of Bermuda | Paper Nautilus (Argonauta argo). Retrieved 09 13, 2013, from BBSR: http://www.thecephalopodpage.org/MarineInvertebrateZoology/Argonautaargo.html
Planet, A. (2013). MOLLUSKS | Argonaut. Retrieved 09 13, 2013, from Animal Planet: http://animal.discovery.com/marine-life/argonaut-info.htm
The Regents of the University of Michigan and its licensors. (n.d.). Argonauta argo. Retrieved 09 15, 2013, from EOL: http://eol.org/pages/492812/details#reproduction
University of Bristol. (n.d.). Argonautidae. Retrieved 09 14, 2013, from University of Bristol: http://palaeo.gly.bris.ac.uk/palaeofiles/fossilgroups/cephalopoda/argonautidae.html