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Phylum Mollusca > Class Gastropoda > sea slugs > Order Sacoglossa
Sap-sucking slugs
Order Sacoglossa
updated Sep 12
Where seen? These slugs come in a wide range of shapes from minute to large, and some are commonly seen on all our shores. They often closely resemble the seaweed that they eat! Some are seasonal, being abundant when their seaweed food is plentiful. And disappearing when their seaweed food dwindles away.

What are leaf slugs? Sap-sucking slugs belong to Phylum Mollusca and Class Gastropoda like other snails. Like many other sea slugs (Subclass Opistobranchia), sap-sucking slugs lack external shells as adults. Sacoglossans are also called sap-sucking slugs because this is what they do (see below).

Features: Sap-sucking slugs come in a wide variety of shapes and sizes. Many are tiny (1cm or less). They have one pair of 'rolled up' tentacles, not solid tentacles like other slugs. Some have small external shells, others have internal shells, while yet others have no shells at all. Those without shells have a pair of 'wings' or flaps (called parapodia) that surround the body. In some, the parapodia can be large and leafy, in others short and tucked around the long body. Yet others have other structures on their bodies. They often also take on the same colouration and even appearance of their seaweed food. Thus they are often overlooked.

Sometimes confused with other sea slugs that appear similar but belong to different orders. Here's more on how to tell apart sap-sucking slugs from other sea slugs.

What do they eat? These slugs suck the sap of seaweeds. They have a distinctive radula with a single row of sharp, knife-like teeth, used one at a time. A single tooth is used to pierce the seaweed cell. The 'sap' or contents of the seaweed cell are then sucked out with a tube that acts as a powerful pump. Most feed on green seaweeds, each sacoglossan species usually specialising in a restricted range of seaweed species. Some sacoglossans, however, eat the eggs of other slugs. The worn out teeth are stored in a sac, hence the name of the Order.

Stolen food factories: Some of these slugs retain the seaweed's chloroplasts (the part that contains chlorophyll). Some sacoglossans that eat red or brown seaweed also retain alive, the parts of the seaweed that photosynthesise. It was believed that these chloroplasts continue to carry out photosynthesis inside the slug and provide the slug with extra nutrients. But recent studies suggest something more complicated is going on. Some sacoglossans also recycle the defensive poisons of seaweed into their own secretions to repel potential predators.

Baby Sacoglossa: These slugs are simultaneous hermaphrodites, that is, each animal has both male and female reproductive organs at the same time. They practice internal fertilisation. When two slugs mate, they may both act as males, extending the penis (usually a white tube that emerges from the side of the neck). Some may insert the penis into the female genital pore, others may simply pierce the partner anywhere in the body. They lay eggs in ribbons.

Status and threats: None of our sacoglossans are listed among the endangered animals of Singapore. However, like other creatures of the intertidal zone, they are affected by human activities such as reclamation and pollution. Trampling by careless visitors can also have an impact on local populations.

Sap-sucking slugs often resemble
the seaweed that they eat!
Labrador Mar 05


Internal shell of the Bushy slug.
Changi, May 11



Mating slugs

St. John's Island, May 05

Sap-sucking slugs on Singapore shores



Order Sacoglossa recorded for Singapore
from Tan Siong Kiat and Henrietta P. M. Woo, 2010 Preliminary Checklist of The Molluscs of Singapore.
+added from our observation
^from WORMS

  Family Caliphyllidae
  Lobifera sp.=^Polybranchia sp.

Polybranchia orientalis
(Bushy slug)

  +Family Juliidae
  +Berthelinia sp. (Bivalve slug)

  Family Limapontiidae
  Costasiella sp. (Strawberry slug)
Costasiella kuroshimae
Costasiella paweli
Costasiella usagi

Placida daguilarensis
(Bryopsis slug)
Placida dendritica

Stiliger smaragdinus
(Emerald slug)

  +Family Oxynoidae
  +Lobiger viridis (Tendril slug)

  ^Family Plakobranchidae (Elysia slugs) with list of species recorded for Singapore. Previously Family Elysiidae

  Family Volvatellidae
  Volvatella vigourouxi (Volvatella slug)
Volvatella sp.

Links References
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