***Cakile maritima***

**Sea rocket**

**Brassicaceae**

*Cakile maritima* is a prostrate or mound-forming to erect fleshy annual
with obovate to spoon-shaped, petioled leaves. Fruits are 1.5 - 3 cm,
the upper segments conic to cylindrical, with acute to blunt tips.
Flowers are lavender or white. This species is found in beach dunes
below 50m and is native to Europe. It is gradually replacing *C.
edentula*, which is native to eastern North America (Rollins 1996).

Much of the ecological information available for *Cakile* in north
America has resulted from interest in the observed ecological
replacement of *Cakile edentula* with *Cakile maritima* on the western
coasts of North America and Australia. In both cases *C. edentula* var.
*edentula* was naturalized first, followed by *C. maritima* from Europe.
Currently *C. maritima* is found within large areas of habitat formerly
supporting both species (Boyd and Barbour 1993). Replacement of *Cakile
edentula* with *Cakile maritima* has attracted attention for a number of
reasons. First, it has been extremely well-documented (Barbour and
Rodman 1970, Rodman 1974, Heyligers 1985), and there is no evidence that
hybridization has occurred that might mask a true replacement event
(Rodman 1974, 1986). Secondly, the fact that replacement occurs
simultaneously on both continents argues against coincidence in favor of
some underlying ecological mechanism (Rodman 1986). Finally, beach
communities appear to be relatively open to invasion due to low plant
density and high levels of disturbance (Breckon and Barbour 1974).

Several explanations for the observed replacement have been offered.
Interference for pollinators (Barbour and Rodman 1970) is a possible
mechanism - *Cakile maritima* has larger flowers - but *Cakile edentula*
is capable of extensive self-fertilization. Direct interference is
unlikely due to low plant densities but was investigated by Barbour
(1970) and Barbour and Rodman (1970). A limited growth chamber study by
Barbour (1970) showed that *Cakile edentula* outperformed *Cakile
maritima* in mixture, so competition or allelopathy seem unlikely
explanations. Rodman (1986) suggested that *Cakile edentula* is more
r-selected and reproduces more rapidly than *Cakile maritima*, but that
the latter outcompetes *Cakile edentula* by preempting limiting safe
sites.

Indirect effects have also been considered. Baker (1986) suggested that
a virus might be acting differentially on the two species, but little
supporting data is available (Rodman 1986). Differential herbivory was
considered by Barbour and Rodman (1970), and Boyd (1988) examined the
preferences of three *Cakile* herbivores at Point Reyes, using growth
chambers or artificial field tests. One herbivore, larvae of the moth
*Platyprepia guttata*, preferred *Cakile maritima* but another, the deer
mouse, *Peromyscus maniculatus* preferred *Cakile edentula* Boyd
concluded that *P. maniculatus* might have accelerated the replacement
but was not a major factor because *P. maniculatus* is found both within
and outside the extinction zone of *Cakile edentula.*

Boyd and Barbour (1993) conducted a field experiment to determine why
*Cakile maritima* has replaced *Cakile edentula* in most of coastal
California but not Oregon or Washington. They found that both species
have seed banks persisting for at least 2 years in foredune sandune. No
significant interference between the two species was detected in either
the foredune or open beach habitat. In the foredune, it was
experimentally determined that insect herbivory had no influence on
survivorship, though rodent herbivory had a significant and similar
effect on both species. Both *Cakile* species grew best in open beach
habitat, where reproductive output per plant was similar for the two.
*Cakile maritima* was found to have higher survivorship than *Cakile
edentula* in the foredune, with enough more individuals surviving into a
second reproductive year to produce an 18-fold difference in seed output
into the surrounding habitat. The authors conclude that replacement of
*Cakile edentula* by *Cakile maritima* is due to the latter\'s ability
to better tolerate foredune conditions and survive into 2nd or 3d
reproductive seasons. At more northerly latitudes (e.g. Oregon and
Washington) plants may be forced into strictly annual habits by
environmental conditions, facilitating coexistence (Boyd and Barbour
1993).

The effects of habitat and predation on the population biology of
*Cakile maritima* on the west coast of North America have been
investigated by Boyd (1991). Survival patterns of seedlings in adjacent
open-beach and foredune habitats differed; open-beach plants were
usually killed by winter storms. Approximately 5% of foredune plants
survived into a second reproductive season, however, and these plants
produced 85% of the seed in the foredune. *Peromyscus maniculatus*
removed 95% of the seeds produced by foredune plants.San burial, which
is much higher for open-beach plants tht foredune plants, enhanced the
probability of escaping predation. Seeds cached by *P. maniculatus* had
low survival rates relative to those not cached, so seed dispersal by
*P. maniculatus* is not beneficial. While foredune plants produce
sufficient seeds for replacement, predation by *P. maniculatus* reduced
seed survival to less than 20% of the replacement rate (Boyd 1991).

Maun and Payne (1989) investigated the adaptive significance of fruit
and seed dimorphism in *Cakile maritima* and two subspecies of *Cakile
edentula*, *Cakile edentula* var. *edentula* and *Cakile edentula* var.
*lacustris*. Dimorphic fruit segments were tested for differences in
sizes of propagules, dispersal ability, germination behavior, and growth
rate. Fruit dimorphism in *Cakile* consists of two morphologically
distinct types of fruit segments, upper and lower. The upper fruit
segments are deciduous, an adaptation for dispersal, while the lower
segments remain attached to the parent plant. The presence of two morphs
may contribute differentially to plant fitness because of differences in
fruit and seed size, dispersability, germination, growth, and
reproduction (Maun and Payne 1989). The mean mass per seed, fruit, and
shell to seed mass ration of upper fruit segments of all three taxa were
found to be significantly higher than the lower segments. The upper
seeds of the two maritime species, *Cakile edentula* var. *edentula* and
*Cakile maritima* were similar in mass, but the lower seeds of *Cakile
edentula* var. *edentula* were significantly lighter than those of
*Cakile maritima* The fruit mass was significantly higher in upper and
lower segments of *Cakile maritima*than in either group of *Cakile
edentula* With respect to dispersal, the upper fruit segments of all
three taxa did not differ significanly in floating ability from lower
fruit segments. Compared with the two groups of *Cakile edentula*,
fruits of *Cakile maritima* remained afloat for a long time; 50% of the
fruit segments of *Cakile maritima* remained floating after 100 days,
compared with 21 days for *Cakile edentula* var. *lacustris* and 15 days
for *Cakile edentula* var. *edentula*. Larger upper seeds of *Cakile
edentula* var. lacustris emerged from significantly deeper depths than
smaller lower seeds; the authors suggest that sand accretion may be a
strong selective force in the evolution of *Cakile* seeds. Morphological
dimorphism in the fruits of *Cakile edentula* var. *lacustris* is linked
with physiological dimorphism in that lower fruit segments germinate
better over a wider range of temperatures than do upper segments. Light
did not alter the final germinateion percentage of upper or lower fruit
segments, but inhibited the rate of germination. The relative growth
rate decreased in all three taxa with an increase in the age of a
seedling, regardless of it\'s seed weight (Maun and Payne 1989).

Pakeman and Lee (1991a and b) evaluated environmental factors
contributing to differences in the performance of *Cakile maritima* in
strandline and foredune habitats. These authors found a marked
difference in growth dependent upon their position of establishment.
Large, rapidly growing plants are associated with sand containing
macroalgal litter, whereas small stunted plants are found on foredunes
above the limit of tidal innundation. Edaphic factors and plant nutrient
concentrations were measured throughout the 1988 growing season at a
site in Wales to determine possible causes for these differences.
Pakeman and Lee found that the observed differences in performance were
not associated with water or potassium availability, but that nitrogen
availability was much higher in the strand habitat as compared with the
foredunes, and the concentration of N in the leaves of plants growing in
the strand environment was higher than that measured for foredune
plants. The authors conclude that nitrogen differences account for the
observed growth differences, probably by controlling photosynthetic
rates. Phosphorus may also be important (Pakeman and Lee 1991 a and b).

***References cited:***

Barbour, M.G. 1970. Seedling ecology of *Cakile maritima* along the
California coast. Bull. Torrey Bot. Club 97:280-289.

Barbour, M.G., and J.E. Rodman. 1970. Saga of the westcoast sea rockets:
*Cakile edentula* ssp. *californica* and *C. maritima*. Rhodora
70:370-386.

Boyd, R.S. 1988. Herbivory and species replacement in the West Coast
searockets (*Cakile*: Brassicaceae). Am. J. Bot. 75:1540-1548.

Boyd, R.S. 1991. Population biology of west coast *Cakile maritima*:
effects of habitat and predation by *Peromyscus maniculatus*.

Boyd, R.S., and M.G. Barbour. 1993. Replacement of *Cakile edentula* by

*C. maritima* in the strand habitat of California. Am. Midl. Nat.
130:209-228.

Breckon, G.J., and Barbour, M.G. 1974. Review of North American Pacific
Coast beach vegetation. Madrono 22: 333-360.

Heyligers, P.C. 1985. The impact of introduced plants on foredune
formation in southeastern Australia. Proc. Ecol. Soc. Aust. 14: 23-41.

Maun, M.A., and A.M. Payne. 1989. Fruit and seed polymorphism and its
relation to seedling growth in the genus Cakile. Can. J. Bot. 67:
2743-2750.

Pakeman, R.J., and J.A. Lee. 1991a. The ecology of the strandline
annuals *Cakile maritima* and *Salsola kali,* 1. Environmental factors
affecting plant performance. J. Ecol. 79: 143-153.

\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_. 1991b. The ecology of the
strandline annuals *Cakile maritima* and *Salsola kali,* II. The role of
nitrogen in contrilling plant performances. J. Ecol 79:155-165.

Rodman, J.E. 1974. Systematics and evolution of the genus Cakile
(Cruciferae). Contrib. Gray Herb. Harv. Cambridge, Massachusetts. 146
pp.

Rodman, J.E. 1986. Introduction, establishment, and replacement of sea
rockets (Cakile: Cruciferae) in Australia. J. biogeog. 13:159-171.

Rollins, R.C. 1996. Brassicaceae. In The Jepson Manual of the Higher
Plants of California. Hickman, J.C., ed. p 406. Universtiy of California
Press, Berkeley.