Caulerpa: Sea Grapes, Feather Algae and Other Variations on a Tube

Invasive Caulerpa taxifolia in the Mediterranean, from here.

There is an observation (it has been dubbed 'Gorton's Law') that any discussion of any marine organism will invariably lead to someone asking whether it can be served with chips. In most cases, the answer will be some variation on 'yes', 'no', 'eww' or 'that's just stupid'. Sometimes, however, the answer will be 'it depends'.

Caulerpa is a genus of marine green algae found in tropical and subtropical waters around the world. Over seventy species of Caulerpa are currently recognised, with many further divided into bewildering arrays of subspecies, varieties and formae. At least two of these species, C. taxifolia and C. racemosa, have become notorious as invasives in the Mediterranean and nearby parts of the Atlantic. Apart from the closely related genus Caulerpella, it cannot really be mistaken for anything else (at least if you have a microscope). The thallus of Caulerpa takes the form of a long branching, creeping tube (the stolon) from which arise numerous tubular, flattened or globular fronds. A Caulerpa thallus can grow reasonably large: the stolon may be over three metres in length, with fronds several centimetres high. Despite this size, the thallus is not divided into individual cells: the multinucleate cytoplasm is freely connected throughout. Instead of cell divisions, branching ingrowths of the cell wall called trabeculae provide strengthening for the thallus. Large Caulerpa individuals are therefore one of the leading contenders for the title of 'world's largest single-celled organism', though I've noted many times that the concept of 'largest' doesn't mean much when talking about multinucleate structures that are indeterminate in size.

Caulerpa racemosa, photographed by Guillermo Diaz-Pulido.

The two genera Caulerpa and Caulerpella that share this unique cell structure (there are other tubular, non-cellular algae, but they lack the trabeculae) are distinguished by their reproductive morphology. Caulerpa usually reproduces vegetatively: older pieces of a thallus die, separating the growing tips, or pieces of the thallus break off and settle separately. When sexual reproduction does occur, Caulerpa fronds do not produce dedicated reproductive structures. Instead, the cytoplasm within an entire frond becomes divided into gametes that are released through slender papillae that grow on the frond surface. In the single species of Caulerpella, C. ambigua, fronds may bear specialised reproductive structures formed from a compound whorl of tightly-branched lobes lacking trabeculae (Price 2011).

Caulerpa nummularia, photographed by D. S. Littler.

Species and varieties are distinguished by the morphology of the thallus, using features such as the shape of the fronds (which may be simple tubes, or branched and feathery, or disc-shaped, or broad and flat, or globular and looking like clusters of grapes, or any number of further variations) and their manner of branching. However, many species may vary considerably in habit, and in some species forms described as separate varieties may later be found growing as separate sections of a single thallus. In a detailed study of variation in four species of Caulerpa found in the Philippines, involving comparisons of specimens collected in the wild, thalli cultured in the laboratory, and molecular data, de Senerpont Domis et al. (2003) found that three of the four species showed morphological variation consistent with their previous classification, but the fourth species (C. racemosa) varied to the extent that previously recognised 'varieties' could not be reliably distinguished.

Caulerpa prolifera, photographed by Elizabeth Lacey.

But to get back to the important question: can it be served with chips? It depends. Many Caulerpa are widely eaten, particularly in eastern Asia, and particularly those with globular fronds (which are referred to as 'sea grapes'). Nevertheless, Caulerpa species have also been referred to as toxic, particularly in relation to their noxious weed status in the Mediterranean (where their invasiveness has been attributed to the absence of suitable grazers). Toxicity of Caulerpa, it appears, can vary between taxa and possibly between seasons (the taste of edible varieties in the Philippines becomes more peppery during the rainy season). The identity of the toxic compound(s) in Caulerpa has been subject to debate: a number of candidates have been identified, but studies have disagreed on their effects and severity, and it remains uncertain whether Caulerpa poisoning poses a serious risk to humans (Higa & Kuniyoshi 2000). One of these candidates, caulerpicin, was tested by Doty & Aguilar-Santos (1966) using the straightforward method of feeding it to 'volunteers'. They recorded the results as follows:

Different people respond differently to caulerpicin. Some merely obtain a mild anaesthetizing sensation which is not immediate but is delayed for a minute or two. Others also obtain a numbness of the tongue or lips. In one subject exposed to the substance at various times truly toxic symptoms have become stronger and stronger following each contact. Almost immediately on chewing the raw dried Caulerpa material, the subject felt a numbness at the tip of the tongue. This has developed to a point at which the reaction is one of numbness of the extremities coupled with a cold sensation in the feet and fingers, rapid and difficult breathing, slight depression and, finally, loss of balance requiring the subject to lie down. The symptoms wear off, depending on the dosage, in a few hours to a day. Coupled with these reactions to the impure and pure substance, the same subject has developed a sensitivity to oysters and crabs and eating them produces the same symptoms.

It's the 'following each contact' bit that worries me. "Feeling better? Good. Have some more!"

REFERENCES

Doty, M. S., & G. Aguilar-Santos. 1966. Caulerpicin, a toxic constituent of Caulerpa. Nature 211: 990.

Higa, H., & M. Kuniyoshi. 2000. Toxins associated with medicinal and edible seaweeds. Toxin Reviews 19 (2): 119-137.

Price, I. R. 2011. A taxonomic revision of the marine green algal genera Caulerpa and Caulerpella (Chlorophyta, Caulerpaceae) in northern (tropical and subtropical) Australia. Australian Systematic Botany 24: 137-213.

de Senerpont Domis, L. N., P. Famà, A. J. Bartlett, W. F. Prud’homme van Reine, C. A. Espinosa & G. C. Trono Jr. 2003. Defining taxon boundaries in members of the morphologically and genetically plastic genus Caulerpa (Caulerpales, Chlorophyta). Journal of Phycology 39: 1019-1037.