Entoprocts are small marine or aquatic creatures that superficially resemble little sea anenomes or hydroids, with a crown of tentacles, and a short vase-shaped body with a stalk that holds the animal to its substrate. They are usually found in colonies. Closer examination reveals that they are not related to the sea anenome and hydroid phylum, Cnidaria, at all, but have a closer relationship with pseudocoelomate worms such as the nematodes. There are less than two hundred described species of Entoprocts but they are common on reefs and intertidal and subtidal communities. Their former name was bryozoan or moss animal. Buchsbaum (1968) describes them thus:
"Some of the small and more delicate 'seaweeds' admired by visitors to the seacoast are not seaweeds at all but the branching colonies of members of the (old) phylum Bryozoa, a name that means 'moss animals' and refers to the plantlike appearance of many bryozoans. Because of the colonial habit of its members, some prefer to call the phylum Polyzoa ('many animals'). Some colonies are shrublike and hang from blades of kelp or grow out from crevices of rocks; others form flat incrusting growths on seaweeds and rocks; and some freshwater bryozoans growas gelatinous masses around stems and twigs that have fallen into the water."
When I read this paragraph again, after many years since my Invertebrate Zoology course, and in preparation for writing about this group for Helium, I suddenly realised that I had been seeing entoprocts for years on the Barrier Reef and had made many mistaken identities, thinking they were soft corals or anenomes. A pretty serious oversight by someone who imagines themself as at least an amateur invertebrate zoologist, but also exciting. Next time I go north to the Reef, I have some different animals to look at, animals that were right under my nose all the time, but not recognised.
Buchsbaum called them Bryozoa but the Phylum has since been broken apart into two Phyla, the Entoprocts and the Ectoprocts. The relationships between these two superficially similar groups have certainly been difficult to work out. Ectoprocts and Entoprocts both have vase-like bodies and rings of 'lophophorate' tentacles but internally they are quite different. Their names come from the location of the anus: External to the tentacle ring in Ectoprocts and inside the tentacle ring in Endoprocts. Their embryological development is radically different, which shows they cannot be closely related. Ectoprocts have radial cleavage while Entoprocts have spiral cleavage. Entoprocts also have a pseudocoelom and while the ectoprocts have a true coelom.
Entoprocts live their lives upside down. The ciliated free living larvae settled on its head and glues itself to the substrate. The body becomes the stem and the vase with tentacles growing at the upper end which are used to capture food. The gut becomes U-shaped with the mouth migrating upwards so that it and are both within the ring of tentacles. The entoproct tentacle ring can be U-shaped or circular. They feed on plankton, detritus and algae and in turn are fed upon by small fish crustaceans and sea urchins.
Entoprocts can reproduce asexually by budding or sexually. Budding is used to increase the size of the colony while sexual reproduction provides genetic diversity and a new generation of larvae that can swim away and disperse to new habitats. There are both hermaphroditic species and species with separate sexes. Fertilisation is internal, taking place in the ovaries or gonoducts.
Adults can live on their own or in colonies. Mostly they don't move around but they can crawl or swim if necessary. Colonial species always have separate sexes. Solitary species are usually sequentially hermaphroditic, beginning as males and then turning female later in life. The larvae are planktonic and resemble annelid trochophore larvae. These little free living predators have a front end sense organ and mouth and cilia at the back end. The larvae may spend a long or short time in the plankton but when they settle, they go through some metamorphic changes because they settle on their heads, a bit like barnacle larvae. The old 'brain' disappears and a new ganglion then develops at the top end which used to be the back end and where the adult tentacles now grow.
I am looking forward to going back to the Great Barrier Reef this year with the year 11 Biology students and looking for moss animals and seeing them with new eyes.
1. Buchsbaum, R. 1968. Animals Without Backbones. Penguin Press Edition.
2. Meglitsch, P. 1972. Invertebrate Zoology. 2nd Ed. Oxford University Press