Figure 1. The holotype and allotype of Symbion pandora from Norway lobster Nephrops norvegicus. The holotype is an asexual feeding stage attached to a seta of the lobster. The allotype is a primary male with two secondary males inside. (Modified from Funch and Kristensen 1997, drawn by Stine Elle).

Figure 2. The proposed life cycle of Symbion pandora (numbers in the figure refer to those used in the text). The dominant stages in the life cycle are the feeding stages (4–9, 12–13, 15–16), so named because they are the only stages with a digestive tract. The feeding stage is attached to the mouthparts of the host with an acellular stalk and adhesive disc. The trunk contains a U-shaped gut, and basal to this undifferentiated cells continually produce internal buds. These buds develop into new feeding structures and other organs (5–6). Eventually a feeding stage also produces a Pandora larva asexually (9). Later, feeding stages switch to production of a primary male (13) or a sexually mature female (16). The Pandora larva (9), primary male (13), and female (16) are developed inside the feeding stages in a brood chamber. They are all short-lived and do not possess a gut. The Pandora larva develops a juvenile feeding stage from budding cells, while inside the feeding stage (9). A fully developed Pandora larva escapes from its maternal feeding stage (10), settles somewhere on the mouthparts of the lobster (11), and a new feeding stage develops from buds (4). When the primary male escapes (14), it seeks a feeding stage and attaches permanently (15). Some primary male organs then degenerate, but budding cells inside develop into several new individuals, tiny sexually mature males. Eventually a female develops inside the feeding stage (16). The female develops a single oocyte, while inside the feeding stage. This oocyte is fertilized either by hypodermic insemination, during release of the female or just after the release. Free females have a zygote inside (17) and later a 4-cell embryo may be observed (18). The female then settles on the mouthparts (19), degenerates and an embryo develops to a chordoid larva (19–21). The chordoid larva escapes from the old cuticle of the female (22), seeks an appropriate site on the mouthparts of a lobster, settles (1), degenerates (2), except for budding cells posteriorly, which develop into a new feeding stage (2–4). (Modified from Funch and Kristensen 1995; published in Nature 378: 711–714.)

Figure 3. Free chordoid larva of Symbion pandora. Lateral view. The dorsal side has a uniform cuticle, except for the anterior part on top of the brain forming a rigid hood and the dorsal ciliated organ. The ventral side has cilia and several outlets of glands. The chordoid organ, composed of 50 stacked muscle cells, runs through the whole length of the larva. (Modified from Funch 1996, drawn by Stine Elle.)