Feature: species visibility
In order to keep species number limited but also keep adding new species
As the site's owner
I want every species appears in the sea during only the specified time period
Scenario: species appear
Given dolphin's available time is [4/1, 12/24]
And today is 7/4
When I go to the sea
Then I should see dolphin available
Scenario: species disappear
Given dolphin's available time is [4/1, 12/24]
And today is 12/25
When I go to the sea
Then I should see no dolphin available