Its a Snail's Life

At first, the reproductive cycle of Thyca crystallina seems typical of a marine gastropod, with separate sexes and external fertilization. After mating, the female lays eggs, which develop into free-swimming larvae. The larvae go through a planktonic phase, drifting in the ocean for a time.

This snail, however, has a life cycle divergence from a typical marine snail; after drifting for a time, it settles on an echinoderm host, the Blue Seastar Linckia laevigata, where it plays the role of a parasite. Evidence of this parasitic relationship goes back to the Late Cretaceous, when dinosaurs, particularly Tyrannosaurus rex, ruled the planet. The Late Cretaceous ended with the K-T extinction event; none of the dinosaurs would survive this.
The planktonic larvae of T. crystallina allow dispersal, and these snails range across the Indian and Pacific Oceans. Reducing its spread is the snail’s tie to the seastar. It appears to be restricted to populations of Linckia laevigata, making it a host-specific parasite. This specificity will limit its distribution. In addition, imperilment may be in the future for the snail and the seastar since the star is over-collected due to its striking blue color, and the snail depends on it.

The snail attaches to the seastar using a proboscis that penetrates the seastar's body and reaches the radial hemal and perihemal systems along the ambulacral groove within the ambulacral ridge. This connection allows the snail to feed on hemal and perihemal fluids, which are rich in nutrients. This is similar in some ways but not the same thing as blood; seastars have no blood, though that does not prevent people from calling this snail the blood-sucking snail. 

Figure 1. The snail, Thyca crystallina, on the Blue Seastar, Linckia laevigata.

Figure 1. The Blue Seastar, Linckia laevigata

Figures 3a and 3b. Two images of Thyca attached to Linkia. These illustrate how the animal blends with the seastar’s color. This is enhanced because the snail shell is partially transparent, allowing the underlying blue to show through.

The attachment of the snail to the seastar is permanent for adult females, and once the proboscis is embedded, it cannot be withdrawn. (Do the males roam around? I couldn't find this information). The proboscis bypasses shallow routes, such as the perivisceral coelom, and instead penetrates deep into the hemal-perihemal complex of the starfish. This complex route ensures that the snail accesses nutrient-dense fluids. This feeding process appears to cause minimal harm to the seastar, although minor structural changes occur, such as the reduction of host ampullae and the displacement of ambulacral ossicles.

 

Seastar fluids and spaces:

In sea stars, the perivisceral coelom is the fluid-filled cavity between the body wall and the digestive system. The organs sit in this space (gonads, for example, as they are one of few organs found in Echinoderms).

 

The term perihemal refers to the anatomical space or structures surrounding the hemal system in certain invertebrates, particularly echinoderms like sea stars. The hemal system in these animals is a network of fluid-filled canals and tissues involved in transporting nutrients and other substances throughout the body. This system is separate to the water vascular system used to run the tube feet, and for respiration.

 

In echinoderms, the perihemal system is a connective tissue structure that surrounds the hemal system and is associated with circulatory functions. This system structurally supports the hemal system. 

 

Components of the hemal systems are named by location, the axial hemal system is along the central axis of the body, while the radial hemal strands extend along the arms following the radial water canals and nervous system.

Figures 4a, b. Two images of seastar insides: In the left image, the coelomic space is gray, while the hemal system is mauve/reddish. The water vascular system, including the entrance point of the madreporite, is in blue. Image stolen from Biocyclopedia.com. In the right image, the hemal system is marked with small circles within, and the water vascular system is with dots. Stolen from: Madreporite Nexus (Jonathan Dale).

The males:

There is sexual dimorphism in this snail. The males are dwarfs residing under the shell of females and do not penetrate the seastar. 

 

These snails are small; large adult females reach 7mm. The shells are nearly transparent and only slightly coiled. The females have a long proboscis, which, as mentioned earlier, is used to penetrate the seastar’s body (Figure 5), seeking the hemal system juices.

Figure 5. Sketch of Thyca parasitizing a host (Thyca sp) mod from Neumann and Wisshak (2009), after Sarasin and Sarasin (1887). Proboscis is shown penetrating the host's coelomic cavity through the test.

Locations and Locations:

This snail’s infestation rates of sea stars vary based on environmental conditions, especially water movement, which influences settlement and attachment patterns. The snail's genetic structure shows moderate variations across its range, indicating gene flow with some limits; regional variations appear to be influenced by larval dispersal ranges.

 

Location on sea stars: Small individuals are generally found on the aboral (upper) side of the seastar arms. In contrast, larger individuals are predominantly located on the oral (under) surface, close to the mouth. In one study, 62% of seastar were infested, with 2-3 parasites per seastar. Larger parasites tended to settle on the right side of the ambulacral groove. They were often accompanied by dwarf males, highlighting the sexual dimorphism and size-based segregation on the host body. I could not find an image of the male.

 

Biology 101 Terminology 

Ectosymbionts:

If they harm the host, they are a parasite. 

No harm to the host, but the ecto-beasty benefits = mutualism. 

If both gain advantages, it's considered commensalism.

 

FYI, I’m trying to make everyone use seastar as one word instead of starfish, which is ugly.

 

Sources and Further Readings:

Complete scientific names:

Crystalline Seastar Snail, Thyca crystallina (A. Gould, 1846)

Blue Seastar, Linckia laevigata (Linnaeus, 1758)

Images: 

Biocyclopedia.com

Madreporite Nexus (Jonathan Dale).

Flicker (Erich Flores), iNaturalist (Buleleng Regency)

 

Crandall, E.D., Jones, M.E., Munoz, M.M., Akinronbi, B., Erdmann, M.V. and Barber, P.H., 2008. Comparative phylogeography of two seastars and their ectosymbionts within the Coral Triangle. Molecular Ecology 17(24): 5276 – 5290.

 

Egloff, D.A., Smouse, D.T., Pembroke, J.E., 1988. Penetration of the radial hemal and

perihemal systems of Linckia laevigata (Asteroida) by the proboscis of Thyca

crystallina, an ectoparasitic gastropod. The Veliger 30, 342–346

 

Elder, H.Y., 1979. Studies on the host–parasite relationship between the parasitic

prosobranch Thyca crystallina and the asteroid starfish Linckia laevigata. J. Zool.

London 187, 369–391.

 

Neumann C, and Wisshak M. 2009.  Gastropod parasitism on Late Cretaceous to Early Paleocene holasteroid echinoids — Evidence from Oichnus halo isp. n. Palaeogeography, Palaeoclimatology, Palaeoecology 284: 115–119