11th August 2011 - 1 First Pencils, Now Pens - Pen Urchins

Back in April, I wrote about the beautiful pencil urchins. Today, I came across this lovely collection of photographs of sea pens, so I thought I'd carry on with the stationary theme.

Name

The sea pens are types of soft coral of the order Pennatulacea, sub order Subselliflorae. The name 'sea pen' arises from their feather-like appearance which resembles a quill pen.

Photo by fiveinchpixie

The visible part of the sea pen can be up to 2 m in some species, such as the tall sea pen which can be found off the west coast of Scotland.

Life Cycle

A sea pen is a colonial animal, which means it is made up of individual polyps which together function as a whole animal. It begins life as a larva which roots itself and then develops into a stalk. This stalk is called a rachis. It has a root-like structure at its base which anchors it to the sea floor. Both this and this stalk are strengthened by calcium carbonate. The feather-like protrusions are made up of two new types of polyps formed through asexual reproduction. These polyps are responsible for feeding (gastrozoids) and respiration (siphonozoids). The gastrozoids capture food while the siphonozoids move water around to allow for gas exchange.

Some species reproduce by releasing eggs and sperm into the water, whereas in others the female retains the eggs and fertilisation is internal. The developing embryos are brooded until they have reached an advanced larval stage when they are released into the water, where they root to form new pens.

Habitat

Sea pens prefer deeper water, from 10 m to 2 km deep, where they are less likely to be uprooted. They tend to stay in one place but can re-establish themselves if necessary. They position themselves so currents ensure a steady flow of plankton, their main source of food.

Threats and Defences

Unfortunately, sea pens are often destroyed by prawn trawlers and dredgers as well as being prey for sea stars and nudibranchs. In defence, a sea pen may luminesce or deflate and retreat completely underground.

Sources

1. http://en.wikipedia.org/wiki/Sea_pen
2. http://www.bbc.co.uk/nature/blueplanet/factfiles/jellies/seapen_bg.shtml
3. http://www.naturalengland.org.uk/ourwork/marine/protectandmanage/mpa/mcz/features/habitats/seapen.aspx
4. http://www.marlin.ac.uk/speciesfullreview.php?speciesID=3353#
5. http://www.advancedaquarist.com/2003/10/inverts
6. http://www.ehow.com/about_6418563_sea-pens.html