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Phascolosoma nigrescens

Rodrigo Zorrilla Gonzalez (2014)


Fact Sheet



Physical Description


Life History & Behaviour

Reproduction & Development


Musculature and Movement

Anatomy & Physiology


Evolution & Systematics

Taxonomy and Synonyms

Biogeographic Distribution

Conservation & Threats

References & Links

Reproduction & Development

Most sipunculids reproduce dioeciously; their gametes are released from the minute gonad, and grow inside the coelomic cavity (Cutler, 1994). Eggs and sperm are spawned from the nephridia, and fertilisation occurs in open water. Reproductive activity for most temperate water dwelling sipunculids is restricted to a 2-3 month peak during summer or early fall (Cutler, 1994). Phascolosoma nigrescens has ovoid shaped eggs enclosed by a layered envelope (Jamieson, 1999).  Cleavage in all sipunculids is spiral, unequal, holoblastic and the zygote develops the characteristic ‘Molluscan cross’ at the 48-cell stage (Figure 5) (Ruppert et al., 2004). This has lead to phylogenic confusion, which is explained in detail in the Evolution & Systematics section of this website.

Figure 5: 48 Cell stage of Sipunculids. Dotted cells represent Rosette cells, barred cells represent intermediate cells, and cells without shading represent cross cells. Adapted from Cutler, 1999.
Four developmental patterns have been identified for sexually reproducing Sipunculids (Rice, 1981). In depth development stage studies of P. nigrescens have not yet been carried out, but most Phascolosoma species follow the 'IVth pattern of development' (Rice, 1975). Phascolosoma eggs develop into a pelagic lecithotrophic trochophore larvae, then a long lived planktonic pelagosphera, then metamorphose a second time into a vermiform juvenile before reaching adulthood (Rice, 1981). Thus, P. nigrescens has two pelagic stages: the trocophore and planktotrophic pelagosphera. Figure 6 shows the three stages of development leading to the adult body plan. P. nigrescens typically spends 4-10 days as a trocophore larvae and the planktotrophic stage can last from 3 to 6 months (Cutler, 1994). Such extended planktonic stages provide Phascolosoma species with a very wide dispersal range. The dispersal capabilities of P. nigrescens are so effective, that they allow it to cross the East Pacific Barrier (Scheltema, 1968). P. nigrescens does not actively migrate, and thus, must rely on ocean currents to passively transport planktonic larvae to new environments.

Figure 6: Graphical representation of P. nigrescens life history. Arrows show path of development in the following order: ovoid egg, trocophore larvae, late stage trocophore larvae, planktotrophic pelagosphera, vermiform juvenile, adult. Gametes not pictured. Adapted from Cutler, 1999 and Rice, 1976.

The gastrula metamorphoses to a typical trocophore larvae; sipunculan trocophores are usually ovoid shaped, and maintain typical equatorial band of prototroch cells which separate pretrochal and post-trochal hemispheres. These are ciliated, and provide larvae with limited mobility. (Jamieson, 1999). The coelom forms by schizocoely, and occurs in late trocophore development. In addition, the introvert retractor muscles, as well as the terminal organ retractor muscles develop during this stage (Schulze & Rice, 2009) P
. nigrescens then metamorphoses into the pelagosphera larva (Figure 7). This bauplan is easily identifiable, as it develops a definitive head, and expanded metatrochal area with many metatrochal cilia (Jamieson, 1999; Rice, 1986). Shortly after metamorphosis into the pelagosphera, ventral and dorsal retractor muscles become apparent, but the body wall musculature is not yet noticeable (Schulze & Rice, 2009). A mouth is present near the ventral surface of the head, which opens at the base of the ciliated groove. In P. nigrescens, the mouth remains open, but an anus is not present. This said, an oesophagus and complex stomach and intestines are present. At this time, the nervous system consists of  an unpaired ventral nerve cord, and a primitive dorsal brain. Few studies have been done on organogenesis of pelagosphera larvae, and all understanding of Phascolosoma larval internal composition comes from studies done on Phascolosoma agassizii (Rice,1973). From the planktotrophic pelagosphera, development into the juvenile stage occurs very quickly. After a juvenile vermiform stage is reached, individuals adopt a benthic existence throughout adulthood (Jamieson, 1999).

Figure 7: Bilateral view of Phascolosoma nigrescens planktonic pelagosphera larva, adapted from Jackie & Rice, 2002 

Settlement studies of Florida populations of
Apionsoma misakiana describe a heat-labile, species-specific settlement-inducing substance which cues larvae to settle. The response is stronger if sediment is detected by the larvae, and even stronger if adults are present in the sediment (Rice, 1986). Similar responses can be reasonably expected in P. nigrescens, as with most sipunculids, as increased distance from individuals of the same species would limit the probability of gametes meeting. In this way, individuals that fail to perceive members of their own species would be selected against.