This month I will present to you a very small phylum of animals – both in size and in number of species. The phylum Cycliophora, which means the ring-bearer, comprises exactly one genus and two described species. These species are about 350 µm in body length, which is less than half a millimeter and about the size of the thinnest mechanical pencil leads of 0.2 mm–0.4 mm. The first species to be described was Symbion pandora from material collected in Denmark, Sweden, Norway, and Orkney Islands. These animals occur on the mouth parts of Nephrops norgevicus (the Norway lobster). There they feed on the food the lobster eats. This kind of lifecycle is called commensalism as the species have no benefit or harm for the host and just share their food. Later, the second species Symbion americanus was described from the mouth parts Homarus americanus (the American lobster). Interestingly, this species occurs also in Norway and hence all species of this phylum are part of Norwegian fauna.

Their body organization is relatively simple. The sessile feeding stage, which is the most prominent one, has a mouth opening in middle of the buccal funnel. The funnel is used to collect food by the cilia on the ring of the funnel. The mouth can be closed and only the funnel be replaced by new one, which develops within the trunk. This happens several times during their life. Below the funnel is the trunk with a U-shaped gut, so that the anus open besides the funnel again, and a simple nervous system. Additionally, the trunk contains the developing new buccal funnel or budding stages. The animal is attached by non-cellular stalk with adhesive disc to the mouth parts of the host.

The really interesting aspects of these species is their metagenetic life cycle. Metagenetic means that they reproduce both asexually and sexually. They might be tiny but their life is very complex. It begins quite easy enough. The feeding stages reproduce mostly asexually by a so-called Pandora larva. This is bud developing within the trunk and which then breaks through the skin. The larva settles close to the parent and develops into a new feeding stage. Within a short time, thousands of individuals can populate a host this way. However, when conditions get worse or shortly before the host starts to molt, the sexual reproduction starts and now it gets really complicated.

Some individuals stop feeding and produce asexually male buds, the so-called Prometheus larva, which develops into the primary male. This male attaches to another feeding stage. The primary male produces three secondary males with a penis (see the featured image); again by internal budding. In parallel, the attachment of the primary male triggers the asexual production of female buds. These develop into females with eggs, but they first remain in the trunk of the parent. The secondary male fertilizes egg in a female, which resides still within its parent. Now the female with fertilized eggs hatches and settles on the same host. Except for the attachment disc, the body of this female (the mother) degenerates and the disc develops into a cyst. A cyst is a protective case around the fertilized egg, which develops into the chordoid larva develops. The chordoid larva is released to the open water and searches for a new host to settle on. It can stay several weeks in the open water. However, when the sexual life style was triggered by the molting of the original host, the chordoid larvae usually settles the old host with the new skin after molting.

Source featured image: R. C. Neves & H. Reichert: Microanatomy and Development of the Dwarf Male of Symbion pandora (Phylum Cycliophora): New Insights from Ultrastructural Investigation Based on Serial Section Electron Microscopy. In: PLoS ONE, Vol. 10, No. 4, 2015, e0122364