The hermit crab is a tiny crustaceous species ranging from 2 to 10 centimeters in length, and weighing 7 to 18 ounces. The crab leg shape varies from having sharp, hooked legs, to having blunt legs. Since the sharp feet trait is dominant, a homozygous recessive crab will have blunt legs. Crab legs are only controlled by one gene, they are Single-gene trait. The color of a hermit crab can change from individual to individual. Such as the Elegant Hermit crab which has blue and black limbs to the generic hermit crab which has reddish-brown limbs. Since crab color is controlled by more than one gene, it is classified as a Polygenic trait. The crab legs are also known as homologous structures to claws into objects like eagles to hold onto rocks to feed without being blown away by the water current. The oldest crab fossil ever discovered was from 445 million years ago!
One day, a plane was flying over Island C and suddenly crash landed! This lead to rocks being scattered across the island and causing geographic isolation since there wasn’t any way to get to the other side of the island. By natural selection, the crabs with hooked feet were more fit to survive. The crash-landed plane is an example of Genetic Drift. 2. Since Hermit crabs are not picky eaters, they were not deeply affected by the crash.
The crabs with pointed legs adapted to eating more mosses and lichens growing on rock since they could easily grab onto the rock to eat. Crabs with blunt legs were more adept to swimming in the water fast, and have the ability to hop from rock to rock. They adapted by focusing their diet on fallen fruit, human litter, and other foods they could find on their journey. Since the crash made lots of pieces of the island go stray, they crabs must adapt to make their main source of food since it is the easiest way to attain energy. Maxillopeds (crab mouth) help by engulfing and consuming the vegetation easily. Crabs in the future will have to evolve in a slow process know as gradualism to live comfortably on Island C. On Island C lives the white-bellied mangrove snake. Often referred to as the crab-eating water snake, they engulf the crab whole similar to an anaconda and eat the crab whole. Since the snakes are fast creatures, the crabs with blunt short legs are naturally favored to blitz past the snakes and to survive. Naturally, the crabs with sharp feet will want to reproduce with smaller nimble crabs to eventually birth and evolve into a new species of hermit crab. This is known as directional selection since crabs with smaller nimble feet are more adept than crabs with sharp legs and the hybrid cross. Eventually, the white-bellied mangrove snake will also evolve in response to the evolution of the hermit crab in a process known as co-evolution.
As said earlier, The average size of a hermit crab is 2-10 centimeters which are perfect in order to hide between rocks and be safely avoided. If the hermit crab is too big, it might get stuck in the rocks and will be easily found and eaten by the snakes. On the other hand, if the hermit crab is too small, they might be too weak to keep jumping from rock to rock and might get tired very easily. Therefore, The averagely sized hermit will be more fit to survive in this ecosystem. This is known as stabilizing selection since hermit crabs in the middle size chart will survive at a higher survival rate than small and large crabs.
After 1,000,000, the island that was split into 2 by the plane started to drift. Since the hermit crabs population had been severed in half, the crabs evolved and gained a genetic mutation that helped the hermit crabs react fast and be able to evade snakes. This came in beneficial to how a hermit crab lives and is able to be danger-free even in the most swift of situations. However, the gene that made certain chemicals in crabs nerves, caused a mutation that worked as a disadvantage since it dulled their nerves and slowed their time to react. In the next 50 years, the crabs with a disadvantaged mutation nearly went extinct. The population of crabs with better reactions grew exponentially after the natural disaster known as a Tsunami.
The tsunami caused the already broken islands to split in half again. This process is known as the founder effect since the gene pool and allele frequencies shifted due to another separation. Punctuated equilibrium also occurred because the separation of islands and the mutation caused a roadblock in a long stable pattern of evolution. In the near future, a possibility of adaptive radiation could take place on the isolated island to have better adaptations and live in their brand new habitats. This was reproductive isolation since they were separated and couldn’t reproduce with their fellow hermit crabs Since reproduction was at an all-time low, the crabs had to find a way to reproduce and to repopulate themselves. Female crabs prefer crabs with small tiny legs and huge claws to protect themselves and run. According to the Hardy-Weinberg principle, this type of natural selection is known as sexual selection.
The unbalanced allele frequencies lead to the end of genetic equilibrium since some male crabs didn’t mate. Every 50 years, the rock formations shift causing the crabs with tiny swift feet to move faster and live better. But the rock formations could change in a bad way exposing sharp places for long-legged hermit crabs to hook on and to escape danger better. This is called disruptive selection since organisms at the end of the graph have higher fitness than the middle. 8. Over time the 2 populations of hermit crabs grew to be attracted to different features. For example, the crabs on 1 island like short legs and big claws, while the other island prefers crabs based on their physical color. This is known as behavioral isolation since the 2 species have changed in behavior in response to their geographic isolation.
Over time the hermit crabs developed stronger legs and were able to jump from island to island. After they had arrived on their new island they started to mate with the other island species of crab at night to be undetected by the snakes. This was un-natural to the native crab species since they used to mate in the morning under the sun so they could blend in with the rocks to avoid the then awake snakes. This is known as temporal isolation since the 2 species are active at different times. When these 2 species bred, the process of their breeding is known as speciation.
Snakes were smart to catch on and improve their visual capability to distinguish the crabs from the rocks and eat them. Therefore, crabs with sharper and harder shells were more adept and fit to survive since they could hurt the snake being eaten whole and could even escape in these types of situations. The white-bellied snake and the hermit crab will have a rivalry for generations to come and will keep involving to survive.
