Detailed Study
Sea Urchins (Echinoidea)
With nearly 200 different species, sea urchins are small, slow moving globular animals that mainly feed on algae. They typically come in black and dull green, brown, purple, blue red, and are hunted by sea otters, starfish and wolf eels, amongst many others animals. A sea urchin's round and spiky shell usually measures between 3 and 10 cm across. Sea urchins are found in warm or cold waters in rock pools, in kelp forests in coral reefs and sea grass beds. |
Gender and mating systemsIt is difficult to distinguish a male from a female sea urchin simply by sight, but there are a few key differences which can help differentiate genders. Firstly, male and female sea urchins occupy different areas of the ocean: males are usually found in more elevated and exposed locations, to ensure their sperm, when released, can be readily broadcast by ocean currents. Females on the other hand are found in lower, more protected locations, which means that their eggs will be more protected from predators when laid. The mating system of seas urchin is mainly polygamous.
FertilisaitonLike fish, female and male sea urchins spawn directly into the open ocean simultaneously in a process called broadcast spawning. Broadcast spawning typically occurs in the regions of the reef that are affected by strong currents. The eggs, termed pelagic eggs, are quickly carried away from the reef, and the many predators that threaten them.
In a study carried out at the Bamfield Marine Sciences Centre in British Columbia, multiple parenting was detected in 98% of females and 83% of males. Although, what researches found the most intriguing was the proportion of individuals that mate with one another. Eg. in a spawning event, 68% of males will produce at least some offspring with every female. It is important to take note that when only a few males spawn, most males produce offspring with females, but this number proportion falls as the density of males increases (seen in the graph above). Development and nutrition of embryo Hatching is to a 4-arm pluteus larva, called an echinopluteus, that floats around the water as a part of the ocean plankton until is is ready to settle on the sea floor. In the successive 4, 6 and 8-arm stages, the sea urchin feeds on phytoplankton for 5 to 8 weeks. After that, the 8-arm pluteus larva starts to show internal adult features, and a few days later, spines and tubefeet develop externally. Metamorphosis completes the process within a week, and shortly after, the juvenile sea urchin can be seen crawling on the sea floor. Some sea urchins have very little nourishment in the egg; in purple urchins (strongylocentrotus purpuratus), they start feeding as larvae within days or less. Their larvae can float around for weeks or more without having to feed. Paternal care and numbers of offspringSea urchins are classified as being 'spawners', which means that they take little to no care of their young, produce many offspring, fertilise and develop externally and can repopulate a niche quickly. This is also referred to as "r-selection" or the 'quick and many' strategy. Sea urchins can have up to a million eggs, but never tend to them, which makes it harder for their young to survive.
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