Balanus glandula Darwin, 1854
Balanus glandula, the acorn barnacle, is probably the most common intertidal barnacle along the shores of Monterey Bay. Although the acorn barnacle is found in bays and estuaries, the main population occurs along the open coast where, throughout its latitudinal range, the barnacle forms a band a third of a meter or more in width, in association with the red alga Endocladia muricata. Along the shores of the Hopkins Marine Life Refuge, this band centers at about 1.4 m above mean lower low water (Morris, Abbott, & Haderlie 1980). Balanus glandula produces two to six broods during the winter and spring in central and southern California. It stores yolk in the summer and remains ripe in the fall until cold temperatures induce brooding. Depending on the size of the parent, 1,000-30,000 nauplius larvae are produced per brood. Settlement occurs in all but the highest part of the intertidal zone in the spring and summer. Peak settlement usually occurs in the spring (Morris, Abbott, & Haderlie 1980).
Variation in barnacle larval recruitment and the effect this variability has on community structure have been well studied for populations of acorn barnacles that occupy the high intertidal zone of the Hopkins Marine Life Refuge. For example, research by Gaines et al. 1985 found that barnacle settlement could vary among sites monitored for settlement by almost two orders of magnitude and that this variability was correlated with the concentrations (number of, density) of larvae carried by passing water masses. In addition, larval concentration may depend not only on input to the water mass by a source population, but also on subsequent history. For instance, water masses carrying barnacle larvae onto the shore at Hopkins Marine Station, pass through a kelp forest that shelters large populations of planktivores, particularly juvenile rockfish. Research conducted by Gaines & Roughgarden (1987) found that by feeding upon the barnacle larvae these predators can reduce recruitment to 1/50 of the level in comparison to adjacent areas where offshore kelp is absent. Further research by Farrell (et al. 1991) that included oceanographic measurements determined that alternating periods of onshore and offshore transport of surface water, which are responsible for carrying barnacle larvae onto shore, were associated with the waxing and waning of equatorial winds that force coastal upwelling events. Therefore recruitment of barnacle larvae onto shore is enhanced by the onshore advection and severely limited by offshore transport during upwelling events.
A program directed toward monitoring the arrival of larvae of the acorn barnacles (Balanus glandula) on a weekly basis has been established as part of the Marine Life Observatory. As presented in the figure below, barnacles arrived steadily from March through July 2008, after which recruitment declined. Month-to-month barnacle recruitment did not resume comparable levels until March 2009, when a significantly high level of recruitment occurred during July 2009 (and a huge recruitment in 2010). The purpose of this project is to link oceanographic data with genotypic data in an effort to determine how and what factors into gene flow along the west coast, specifically focusing on the Monterey Bay.
Each data point represents a total of 7 days of settlement of acorn barnacles on
six 10 cm x 10 cm plates.
A new model of ocean currents, produced in cooperation with the Partnership for Interdisciplinary Studies of Coastal Oceans (PISCO), suggests that larvae settling at these times come mostly from the north, carried in the prevailing southward flowing California Current. In contrast, genetic analysis of these larvae suggests that a substantial number come from the south. Further monitoring, being developed in association with PISCO, will let us keep track of the way marine larvae are delivered to the local area.
Farrell, T. M., D. Bracher, and J. Roughgarden. 1991. Crossshelf transport causes recruitment to intertidal populations in central California. Limnol. Oceanogr. 36: 279288.
Gaines, S., S. Brown, and J. Roughgarden. 1985. Spatial variation in larval concentrations as a cause of spatial variation in settlement for the barnacle, Balanus glandula. Oecologia 67: 267272.
Gaines, S. D., and J. Roughgarden. 1987. Fish in offshore kelp forests affect recruitment to intertidal barnacle populations. Science 235: 479481.
Morris, Robert H., Donald P. Abbott, and Eugene C. Haderlie, 1980. Intertidal Invertebrates of California. Stanford University Press, Stanford, CA. 690 pp