Worms, Pacific Northwest
The Common Earthworm is sometimes called the night crawler because they come to the surface at night. it is a very common worm in gardens and lawns. Common earthworms (Lumbricus Terrestris) are indigenous to European countries but is an invasive species in North America.
During the last ice age, most North American worms had not survived with exception of a few. The forests began to rely on nutrients that would slowly be released from the leaf mulch that would build up at their base. Over thousands of years, the trees evolved to survive this way. But when the Europeans began arriving in their big sailing ships they inadvertently brought the worms with them. They would use dirt as a ballast in the holds of the ships which would be discarded here to make room for goods that would be loaded for the journey back to Europe. The soil was loaded with worms. They also brought potted plants by the thousands that had worms in the soil.
The worms were transported by farmers to all parts of the land thus spreading the worms across North America. The forests back east that relied upon the leaf mulch for their nutrients soon found the mulch being devoured by worms, these trees had no mechanism to utilize nutrients deposited by the worms and now vast forests are dying off due to the inability to gather food.
Earthworms are extremely efficient in both aeration and fertilization of soils, farmers from Europe were very familiar with this and help spread the worms very quickly. The worms can move the leaf mulch down into the soil in a relatively short time span and after eating it, they cast their droppings on the surface. Charles Darwin once estimated that earthworms could move up to 100 tons of soil per hectare, per year. Just think about that, it’s pretty amazing.
Earthworms are hermaphrodites and they all have both male and female reproductive organs, they are unable to self fertilize though and during the summer on rainy days, they will come to the surface to mate. During mating, two worms will join up side by side head to head. They become covered by a mucus tube and inside this tube, they simultaneously begin to secrete sperm. The sperm travels down a groove in the worms body until it enters a small sac in each of the worms. After mating, the worms separate to go their own way and the worms saddle secretes a mucus cylinder that the worm lays eggs along with the sperm into. The worm wiggles its way out of the mucus cylinder which closes up immediately. Within this mucus egg sac fertilization begins and the eggs form into worms.