Re: Caulerpa prolifera

March 14, 2003
From: Cynthia Trowbridge

Dear Bill and Rob,
Concerning Rob's message:
There are at least 3 invasive species of Caulerpa: C. taxifolia (aquarium strain), C. prolifera, and C. brachypus. For those of you unfamiliar with the last species, it is a NW Pacific species that recently appeared on Florida shores; it is overgrowing and killing the coral reefs.

All 3 species of Caulerpa are eaten by sacoglossan slugs and possibility of marine biological control keeps being suggested. I find it particularly disturbing that the suggestions for biological control are made extensively in the popular media (internet, newspapers, conference proceedings, books) but rarely in the scientific venue where peer review can put the issue in a more balanced light.

There are 2 major issues about biological control:
(1) does the potential consumer reduce the abundance or survival of the intended pest species?
(2) is the potential consumer highly specific?

Bill, you are entirely correct about the issue that sacoglossans rarely cause damage of host plants. I can think of only a few cases and they are primarily SPECIAL cases:
Placida dendritica controlling the distribution of the encrusting Codium setchellii on Oregon shores (Trowbridge 1992).
Placida dendritica causing extensive branch loss of the native Codium fragile on Oregon shores (Trowbridge 1993)
Placida cf. dendritica on New Zealand shores destroying the introduced C. fragile ssp. tomentosoides in high intertidal rockpools (Trowbridge 1996).
Placida cf. dendritica on Australian shores causing serious damage to the introduced C. fragile ssp. tomentosoides at Sisters in Port Phillip Bay, VIC (Trowbridge 1999, 2003/4).
Placida dendritica on Scottish shores causing serious damage to the introduced C. fragile ssp. tomentosoides at Oban, Argyll (Trowbridge 2002).

All of these studies demonstrate the potential role of sacoglossan herbivory. But in all of these cases, the slugs, at best, eliminate their host from a specific microhabitat (e.g., high pools or wave-protected shores). My recent JMBA note (2002) is the strongest case of herbivore control BUT I would emphasize that all of these cases are due to unusual circumstances. For example, Oregon shores have unusually high larval recruitment of invertebrates (due to oceanographic conditions) so large populations of sacoglossans develop. At Oban in Scotland, the currents between islands result in unusual hydrodynamic and larval settlement patterns. Thus, to extrapolate from these special cases to a general statement that sacoglossan herbivory will be important is presumptuous.

Now to the second point about the host-specificity paradigm (HSP). I would refer people to a recent invited review by Secord (2002). At the Marine Bioinvasions conference a few years ago, the proponents of marine biological control met and discussed the issues in a public forum. Secord was invited to review the issue. One of the 4 or so cases was the sacoglossan sea slug - Caulerpa proposal. Scientific consensus is that the proposal has too many unexplored risks.

A final brief word about risks, I have a paper accepted (pending minor revisions) about specialized sacoglossans that change their diets on ecological time-scales in Europe and Australia. Sacoglossans are specialized but their associations are dynamic. The view of static associations is not supported by field-based experimental data.

• Secord, D. (2002) Biological control of marine invasive species: cautionary tales and land-based lessons. Biological Invasions (in press).
• Secord, D. & Kareiva, P. (1996) Perils and pitfalls in the host specificity paradigm. BioScience, 46: 448-453.
• Trowbridge, C.D. 1992. Mesoherbivory: the ascoglossan sea slug Placida dendritica may contribute to the restricted distribution of its algal host. Mar. Ecol. Prog. Ser., 83: 207-220.
• Trowbridge, C.D. 1993. Interactions between an ascoglossan sea slug and its green algal host: branch loss and role of epiphytes. Mar. Ecol. Prog. Ser., 101: 263-272.
• Trowbridge, C.D. 1996. Introduced versus native subspecies of Codium fragile: How distinctive is the invasive ssp. tomentosoides? Mar. Biol., 126: 193-204.
• Trowbridge, C.D. 1999. An assessment of the potential spread and options for control of the introduced green macroalga Codium fragile ssp. tomentosoides on Australian shores. Consultancy report, CSIRO/Centre for Research on Introduced Marine Pests: Hobart. 43 pp.
• Trowbridge, C.D. 2002. Local elimination of Codium fragile ssp tomentosoides: indirect
evidence of sacoglossan herbivory? J. Mar. Biol. Assoc., U.K., 82(6): 1029-1030 .
• Trowbridge, C.D. in press. Emerging associations on marine rocky shores: specialist herbivores on introduced macroalgae. J. Anim. Ecol. (accepted pending minor revisions).

Thank you for considering my comments.

Trowbridge, C., 2003 (Mar 14) Re: Caulerpa prolifera. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

Thanks Cynthia,
Bill Rudman

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