Meroplankton vs. Holoplankton! In the diverse and unseen world of plankton, scientists have found that all of the zooplankton fall into one of two categories. The first group is called holoplankton. Combining the Greek words of “holo” meaning whole or entire and “plankt” meaning drifter, these zooplankton spend their entire lives drifting through the epi- and meso- pelagic zones. These organisms can range in size from tiny but abundant copepods to the extremely large gelatinous cnidarians such as sea jellies and siphonophores. These animals are incredibly important food source for both small fish such as mackerel and sardines as well as some of the largest baleen whales.
The second group is called meroplankton. This name comes from the Greek terms “mero” meaning part and “plankt” meaning drifter. This group of organisms begins life drifting throughout the sea until they grow and mature enough to settle in another area. This adaptation allows many of our favorite invertebrates to colonize vast areas of sea floor and prevents competition between parents and offspring.
Bioluminescent organisms can create their own light! There are many weird and wonderful bioluminescent creatures in the ocean. Some emit light as a predatory tactic, like the anglerfish, which has a light-emitting photophore that protrudes from the top of its head. The anglerfish has a symbiotic relationship with bioluminescent bacteria that collect on the photophore and help lure prey towards the fish’s mouth. This is helpful in the darkness of the deep sea where food is scarce and hard to find.
Other organisms use bioluminescence to defend themselves. Dinoflagellates are a type of phytoplankton that flash a blue-green light when they get agitated by waves or predators at nighttime. This light can startle and distract the phytoplankton’s predators, or it can act as a burglar alarm that attracts bigger predators to come to the feeding site. Sperm whales are known to linger around places with lots of these bioluminescent organisms because their glowing alerts the whale that there is prey in the area.
Next time you are by the ocean at nighttime, try splashing around in the water and see if these dinoflagellates will light up for you!