Research Study 1

Fig. 1.  Vertical separation of barnacle species Balanus glandula and B. crenatus on intertidal shores around Santa Cruz, California.  Although the axis designations are perfectly correct, wouldn't the presentation be clearer if tidal height were on the Y-axis?

Fig. 2. Vertical stratification of larvae of Balanus glandula and B. crenatus in the plankton, with those of the latter species being deeper 

The vertical segregation of barnacle species in the intertidal region is usually thought to result from differential effects of post-settlement mortality.  However, a study on two sympatric barnacle species Balanus glandula and B. crenatus in Santa Cruz, California suggests that the vertical distribution of adults may largely reflect their vertical distribution as cyprids in the plankton.  As shown in Fig. 1, adult distributions are segregated, with B. glandula inhabiting high-intertidal positions on the shore, and B. crenatus, subtidal levels.  Fig. 2 shows settlement patterns of cyprids of each species, determined from weekly examination of vertically arrayed asbestos-cement plates.  Note that the data are cumulative over time.  Note also that there is limited overlap at settlement between cyprids of the two species.  Finally, plankton samplings at different depths show a significant non-overlapping vertical segregation of cyprids of the two species, with larvae of B. glandula consistently being at the surface and those of B. crenatus being deeper.  It is not known how the cyprids position themselves at different levels in the plankton.  However, it is clear from these data that the larvae of the two species are zoned even before settlement. 

NOTE 12 plates are attached to each of 12 vertical poles set in the intertidal zone, each plate separated vertically by 0.3m (from -1.2m to +2.1m).  Plates from two poles are removed weekly and settlers of each species counted.  Thus, there are two replicates for each tidal level for each week over a period of 6wk

Research Study 2

Fig. 1. Whatever free space might have been available on this Balanus glandula-dominated upper shore area is now occupied by a younger generation of barnacles, mostly B. glandula (lighter-coloured) but intermixed with  Chthamalus dalli (darker-coloured), settled after a mass "swamping" of the area by cyprids 

Fig. 2. Variability in settlement of Balanus glandula larvae at three neighbouring sites in Pacific Grove, California

A large body of evidence is accumulating regarding the coupling of oceanic processes with intertidal settlement of marine invertebrates. The study presented here concerns how water-column conditions can directly alter settlement rates of barnacle cyprids and thus affect distributions and abundances of adult populations, as well as subsequent community dynamics (Fig. 1).  For example, in Pacific Grove, California monthly settlement of Balanus glandula is directly coupled with nearshore abundance of cyprids, but there is large heterogeneity in settlement density in three study sites located only tens of meters apart (Fig. 2).  Given that similar numbers of cyprids may be reaching each of the sites, the reasons for the heterogeneity could relate to differences in water movement, density of previously settled adults, availability of free space in the sites, and loss of cyprids through settlement.  A rough estimate of this by the authors suggests that up to 30% of cyprids, equal to about 100,000 individuals, could be “drained” off per week by settlement as water passes over a single 10m² area of shore. The study points out the potential for small-scale oceanic control of benthic populations and communities, and emphasises the need for studies of benthic ecology to be coordinated with with matching studies on oceanic processes.

NOTE the authors use two different methods for estimating cyprid densities: 1) in April 1983 counts are made from timed plankton-net tows, and 2) in 1984 counts are made directly from seawater pumped from the study sites through fine-mesh filters. Only the first set of data is shown here

Research Study 3

Fig. 1.  Settlement pulses of barnacle larvae Balanus glandula, Chthamalus dalli, and C. fissus off the California coast are associated with periodic cessations of upwelling

Invertebrate larvae, including those of barnacles such as Balanus glandula, Chthamalus dalli, and C. fissus, often arrive on the shore in discrete pulses rather than in a continuous trickle.  Researchers at Stanford University, California have investigated this and suggest that on the shores of central California the larvae accumulate in an offshore front that represents the boundary between the warm southward-flowing California Current and the cold, saline water upwelling close to shore.  When the alongshore winds causing the upwelling lessen and upwelling ceases, the front is thought to move onto the shore carrying the larvae with it (Fig. 1).  During strong alongshore winds and strong upwelling, the front may be positioned up to 130km offshore; during weak upwelling it may move in to a few km from shore.

Research Study 4

Another idea relating to transport of larvae of acorn barnacles Chthamalus spp. and goose barnacles Pollicipes polymerus shoreward involves internal tidal bores. The author correlates decreases in surface temperature in waters around La Jolla, California with the advection of subsurface cold water to the shore at predictable intervals within the lunar cycle. This upwelling brings deep water to the surface in a direction perpendicular to the coastline. Thus, instead of the traditional idea of upwelling along this part of the California coast being associated with wind-driven events, this new idea invokes internal tidal bores. Further, the author states that with respect to barnacle cyprids and, to a lesser extent, other planktonic organisms, the internal tidal-bore hypothesis better explains larval transport shorewards than does the surface-slick hypothesis put forward to explain shoreward transport of crab larvae. 

Research Study 5

Fig. 2. Collecting sites for gene-flow study of volcano barnacles Tetraclita rubescens along the California coast

Fig. 1.  Tetraclita squamosa is a similar-looking volcano barnacle found in south-east Asia

Like other species of barnacles, the intertidal barnacle Tetraclita rubescens (Fig. 1) produces planktonic larvae and has the potential for long-distance dispersal.  The species occurs from Baja California to San Francisco Bay. Two geographical locations, Point Conception and Monterey Bay, are possible “interrupting” features along the species’ range, and the question arises as to how much of a barrier they may present to movement of the planktonic larvae and thus to gene flow between populations. This is investigated in five populations in central and southern California (Fig. 2) through a survey of genetic variation in 8 enzyme systems.  Results show that gene flow is, in fact, high and that the two geographical features noted do not appear to impede this flow.  An exception to this is a population at Moss Landing, individuals of which appear to be more closely related to one another than would be expected by chance alone.  Thus, although gene flow is high between the populations sampled, some isolated groups may have retained larvae for several generations.

NOTE   the author uses the species name Tetraclita squamosa rubescens  (the species Tetraclita squamosa is a similar-looking barnacle found in south-east Asia), but this compounded name is no longer recognised, and the proper species name for the California barnacle is Tetraclita rubescens

Research Study 6

Fig. 1.  Sites along the California coast used in study of shoreward transport of larval Balanus glandula and Chthalmalus spp. during El Niño events.  Sites studied are major headlands

Fig. 2. Comparison of Balanus glandula larval recruitment to nine major headland sites in California during the 1997 El Niño year (red bars) versus the 1996 non-El Niño year (dark bars)

By its large-scale effect on onshore transport of water, an El Niño event in 1997 is thought to have increased markedly the settlement and recruitment of barnacles Balanus glandula along the coast of California (Fig. 1).  The increase occurred over a large latitudinal scale and resulted in significantly greater mean monthly recruitment at most of the sites studied (Fig. 2).  This could be interpreted as a “one-off” event were it not for its accord with earlier, smaller-scale studies showing similar transport anomalies associated with El Niño events. The authors consider alternative explanations for their data, but return to the one most likely, that the high recruitment is caused by increased onshore transport of larvae associated with El Niño. 

NOTE the authors present data on Chthamalus spp. as well, but the effects are not so clear as with B. glandula and are not included here

Research Study 7

Fig. 1. Sites selected for study of upwelling effects on recruitment of barnacles Balanus glandula and Chthamalus dalli

Large-scale differences in recrutment of barnacles exist between mid-California and northern Oregon (Fig. 1).  A 17mo study of settlement and recruitment of barnaces Chthamalus dalli and Balanus glandula shows a 1 - 2 order of magnitude greater number of cyprid larvae in the plankton and the same-magnitude greater recruitment to test plates attached to the substratum, between locations in mid-California and ones in northern Oregon (Fig. 2). The break point is at Cape Blanco, Oregon.  Shores south of the Cape are characterised by strong upwelling, which tends to transport larvae offshore. North of the Cape upwelling is less prevalent and larvae stay closer to shore. 

NOTE another species Chthamalus fissus also settled onto the plates but could not be distinguished from C. dalli at such a young age, so the data are lumped together by the authors. The same effect is shown for recruitment of mussels, also included in the study, but the data are not included here

Fig. 2.  Latitudinal effects on recruitment of Balanus glandula and Chthamalus dalli, likely associated with upwelling

Research Study 8

Fig. 1.  Collection sites for genetics study of barnacles include several in California, one in northern Washington and three in Baja California

Point Conception, California has long been recognised as a boundary between the Californian and Oregonian biogeographic “provinces” of intertidal marine invertebrates.  The reason for this is two-fold.  First, the predominantly year-long southward-flowing California Current is partially interrupted at the Point by upwelling and, second, the circulation pattern south of the Point forms a slow-moving eddy of warmer water (see Fig. 1). An interesting finding in a review of distributions of 750 intertidal invertebrates by researchers from Duke University and University of California Santa Barbara is that the boundary mostly affects species with pelagic larval development (as expected), but much greater going northwards than going southwards (not expected). The researchers analyse genetic structure in two species of acorn barnacles Balanus glandula and Chthamalus fissus whose ranges span Point Conception and find patterns consistent with greater southward migration across Point Conception than northward. They conclude that the Point is really more of a one-way biogeographic boundary, affected not so much by water temperature than by action of ocean currents. 

NOTE mitochondrial DNA sequence data. The authors also re-analyse similar data sets published previously by other authors for purple urchins Strongylocentrotus purpuratus and whelks Nucella emarginata, results not included here

Research Study 9

Fig. 1.  Upwelling effects on settlement of barnacles Chthamalus fissus, Balanus glandula, and Pollicipes polymerus

Information on the relationship between temperature stratification, upwelling, and settlement of barnacles is provided in a study by researchers from Woods Hole Oceanographic Institution (Massachusetts) and Departamento de Oceanografia Fisica (Ensenada) at two sites, La Jolla, California and La Salina, Baja California.  Records over a 5yr period indicate that settlement of Chthamalus spp., the most abundant settler, is significantly greater at the northern La Jolla site than at the southern La Salina site (Fig. 1). The investigators credit this with a more shallow thermocline combined with higher and more energetic water motions at the La Jolla site, leading to greater onshore transport of larve. The shallower thermocline at La Jolla is correlated with wind-driven upwelling. The water motions manifest as internal waves or bores that, in their first phase, bring larvae close to shore along the sea bottom and, in their second phase, transport larvae close to the surface and then onto the shore. 

NOTE species studied include acorn barnacles Balanus glandula and Chthamalus spp., and goose barnacles Pollicipes polymerus

NOTE data are obtained from grooved PVC settlement plates set out on the shore

Research Study 10

Fig. 1.  Study sites on Santa Cruz Island, California

A study by a group of University of California scientists on settlement of acorn barnacles Chthamalus spp. on Santa Cruz Island, California (Fig. 1) reveals significant differences in recruitment on either side of the island based on differences in water temperature and upwelling patterns.  Thus, low recruitment at western sites in comparison with eastern sites is thought to be associated with lower and more variable sea-surface temperatures on the western side of the island. The colder water (1 - 1.5^oC lower) bathing the western side is apparently recently upwelled and devoid of larvae, while the eastern side is bathed in warmer water containing abundant larvae, thought by the authors to originate outside the Santa Barbara Channel. 

NOTE mussel Mytilus spp. recruitment is also included in the study, but not dealt with here

Research Study 11

Fig. 1. Map of study sites and productivity levels in Oregon

Fig. 2. Larval densities of Balanus glandula at two sites in Oregon

It should be axiomatic that barnacle populations associated with high primary productivity in adjacent nearshore ocean areas would produce more larvae than ones associated with less productive areas but, surprisingly, only a few such “bottom-up” studies have been done.  In one investigation on larval production in barnacles Balanus glandula in Oregon, researchers select two cape areas along the Oregon coast for comparison based on significant differences in long-term chlorophyll a values.  Fig. 1 indicates that waters around Cape Perpetua have significantly greater chlorphyll a contents than waters around Cape Foulweather.  Furthermore, field assessments using settlement plates with standardised densities of same-aged barnacles Balanus glandula show that the mean larval production per 100cm² shore area is more than 120-fold greater at the Cape Perpetua sites with higher primary productivity than at the less productive Cape Fouweather sites (Fig. 2).  The authors discuss other variables that may have influenced their results that they could not control for, such as differences in temperature, current flow-rates, potential zooplankton food, but conclude that their findings are most consistent with an initial hypothesis of bottom-up “forcing”. The researchers note the importance of their findings in the context of designing objective means for evaluation of sites for marine reserves and other protected areas. 

NOTE the northernmost site is at Cape Foulweather and the southernmost at Cape Perpetua (80km apart). Two study sites are nested within each cape area as shown on the map

NOTE the plates are 10 x 10cm polyvinylchloride plastic attached to the mid-intertidal substratum at each site with stainless-steel lag screws.  After 8mo in the field the plates are collected and barnacles later analysed for brood numbers. Similar analyses are done on natural populations at each site

Research Study 12

A unique situation associated with unidirectional currents, such as the California Current, is the potential for species to go extinct from upstream to downstream locations.  An analogous situation is the Drift Paradox in freshwater streams.  An extensive review of life-history traits of nearshore and shelf/slope fishes and benthic crustaceans in the Southern California Bight and Baja California done by researchers at the Oregon Institute of Marine Biology shows several parallel adaptations that may minimise this evolutionary tendency.  Thus, such traits as long life, high fecundity, and production of numerous broods per year all tend to improve the chances of larvae recruiting via eddies and countercurrents to “upstream” parental populations.  The authors conclude that rather than being selected for broad dispersal, the pelagic stages of these organisms may have evolved to achieve a migration between larval pelagic and adult benthic habitats.

NOTE  simply put, how do organisms such as insects faced with downstream drift  persist in upper reaches of a stream?

Research Study 13

A far-ranging and ambitious project to determine spatial and temporal patterns of recruitment of marine invertebrates on the U.S. west coast involves monitoring of barnacle recruitment at 26 sites from northern Oregon to southern California (1750km, Fig. 1) over an 8yr period from 1997 - 2004. Recruitment is monitored monthly and correlated with satellite-derived records of monthly sea-surface temperatures (SSTs).  Results show a large-scale pattern with areas of similar recruitment dynamics separated from other areas by abrupt transitions. For example, peak recruitment in late summer/early autumn in Oregon switches abruptly to early spring in northern California and then back to summer in southern California.  Recruitment rates vary greatly with species, with Chthamalus spp. exhibiting variable patterns across the region examined, and Balanus glandula being two orders of magnitude less at sites south of Oregon.  As expected, correlation with SSTs is mostly positive, indicating recruitment generally in warmer months.  Despite considerable spatial and temporal variation within region and among the barnacle species monitored, major biogeographical breaks are identified not just at Point Conception, California, but also at Cape Blanco, Oregon (Fig. 2 shows data for Chthamalus spp.).  Note in the chart that larval recruitment rates for Chthamalus spp. are high in central Oregon (sites 2 - 7), decline abruptly near Cape Blanco (sites 8 - 10), are negligible south of Monterey Bay (sites 19 - 22), and increase again dramatically south of Point Conception (site 23). The breaks appear to owe mainly to differences in upwelling duration in the different areas.  For example, in the northern-most region, Oregon, major upwelling occurs seasonally from May - September and is characterised by frequent interruptions or “relaxation” events.  Farther south, in the area between Cape Blanco and Point Conception, the upwelling season is longer (April - October) and relaxation events occur less frequently.  Finally, in the topographically complex area south of Point Conception, upwelling/relaxation patterns are more seasonally mixed. 

NOTE both barnacles and mussels are included in the study, but only the former are considered here

Fig. 1.  Chthamalus dalli study sites and major recruitment breaks

Fig. 2.  Recruitment of Chthamalus dalli at 26 sites along the west coast from northern Oregon to southern California over an 8yr period

Research Study 14

Fig. 1.  Effect of temperature on survival of recruits of Balanus glandula

An investigation at the Oregon Institute of Marine Biology on barnacles Balanus glandula shows that both settlement and recruitment, although commonly used to infer mechanisms of larval delivery to shore areas, may be providing different answers depending upon the timing and methodologies used for the assessments.  For example, settlement assessments tend to suggest that larval delivery varies mainly with wind direction and tidal cycles, while recruitment assessments tend to suggest that frequencies of upwelling and downwelling may be primarily responsible. The commonly held assumption that recruitment varies directly with settlement will hold only if settlement and post-settlement mortality are density-independent, and this has been assumed to be the case over several decades of barnacle research.  In the present study, however, simultaneous monitoring of settlement and recruitment on standardised plates set out at different locations and aspects on a rocky shore, reveal frequent catastrophic, density-independent mortalities that disrupt the presumed cause-and-effect relationship between the two events. The principal mortality events during the study are caused by excessive and rapid heating by the sun (Fig. 1, for one example). These events are exacerbated by the types and positioning of settlement plates employed, and the author provides some suggestions as to which methodologies may work best. The study is valuable in that it takes a fresh, objective look at an old problem in the study of larval settlement.

Research Study 15

Fig. 1.  Collecting sites for study of genetic variability in relation to upwelling in populations of Balanus glandula

Fig. 2.  Settlement of larval Balanus glandula in relation to strength of upwelling

A study by researchers from Hopkins Marine Station, California and Oregon State University discloses an interesting correlation between genetic variability in acorn barnacles Balanus glandula and incidences of coastal upwelling. The authors sample adult and newly settled barnacles from 9 sites along the Oregon coast (Fig. 1) over a 9yr period and, consistent with results of other studies, show little or no spatial genetic variation among the sites.  However, what does appear in the data are small but significant temporal changes in genetic composition correlative with seasonal upwelling strength (estimated from levels of wind-stress).  Weaker levels of upwelling are associated with higher levels of larval settlement and with greater genetic structuring in the population (Fig. 2). The authors hypothesise that during periods of weak upwelling the larvae are kept closer to shore nearer to parents of origin, mixing is less, and differences in genetic structure are enhanced (Fig. 3). 

NOTE analysis of mitochondrial DNA at the cytochrome-oxidase I locus

NOTE an earlier study on Balanus glandula recruitment along the central Oregon coast also correlates relaxation of upwelling events with stronger recruitment.  Maximum recruitment is noted under conditions of onshore surface currents and higher seawater temperatures (Dudas et al., 2009)

Fig. 3.  How weak upwelling conditions favour genetic heterogeneity in populations of Balanus glandula

Research Study 16

El Niño events in the south Pacific Ocean can have serious disruptive  effects on larval settlement of barnacles on Californian coasts.  To assess these effects in the nearshore waters off La Jolla, California, researchers from the University of San Diego, CA and Woods Hole Oceanographic Institution, MA collect barnacle larvae from stations located within 1km of the shore before, during, and after the 2015 - 16 El Niño event.  Barnacle cyprids are sampled from the entire water column at water depths of 4, 6, 8, 10, and 12m.  Overall, some 33 collection cruises are made over the 28mo period of study.  Results show that cyprid density is highest for most species before and after the El Niño event and lower during it, but with no apparent change in species diversity.  This last is unexpected based on results of other studies, and is thought by the researchers to owe to lack of anomalous northward-current flow during the event. 

NOTE collections contain 6 species of known identity Pollicipes polymerus, Chthamalus fissus, Balanus glandula, Tetraclita rubescens, Balanus trigonus, and Megabalanus rosa, and four species of unknow identity, with C. fissus by far being the most common.  Identifications are made using morphological and molecular (mitchondrial COI gene) means

NOTE  a shallow-water warm-temperature anomaly known as the "blob" was also present along with El Niño, and may have contributed to larval density variations