Settlement coupled with oceanic processes

Reproductive events include settlement coupled with oceanic processes considered in this section, and
COPULATION & LARVAL DEVELOPMENT
LARVAL BIOLOGY
SETTLEMENT CUES & TIMES
SELECTION OF SUBSTRATUM
ATTACHMENT DURING & AFTER SETTLEMENT
METAMORPHOSIS, and
POST-RECRUITMENT EFFECTS ON COMMUNITY STRUCTURE, considered in other sections.

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 2 sympatric barnacle species Balanus glandula and B. crenatus in Santa Cruz, California indicates that the vertical distribution of adults reflects their vertical distribution as cyprids in the plankton.  As shown in the graph on the Left, adult distributions are segregated, with B. glandula inhabiting high-intertidal positions on the shore, and B. crenatus, subtidal levels.

Settlement patterns of cyprids of each species, determined from weekly examination of vertically arrayed asbestos-cement plates, are shown in the figures on the Right. Note that the data are cumulative over time.  Note also that there is limited overlap at settlement between cyprids of the 2 species. 

Finally, plankton samplings at different depths show a significant non-overlapping vertical segregation of cyprids of the 2 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 2 species are zoned even before settlement. Grosberg 1982 Ecology 63: 894.

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 2 poles are removed weekly and settlers of each species counted.  Thus, there are 2 replicates for each tidal level each week over a period of 6wk

A large body of evidence is accumulating regarding the coupling of oceanic processes with intertidal settlement processes 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, and subsequent community dynamics.  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 3 study sites located only tens of meters apart.  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 from a mass of water through settlement.  A rough estimate of this by the authors suggests that up to 30% of cyprids, equal to 100,000 individuals, could be “drained” off per week as water passes over a single 10-m² area of shore.  The study points out the potential for small-scale oceanic control of benthic populations and communities, thereby emphasising the need for coordinated studies of oceanic and benthic ecology.  Gaines et al. 1985 67: 267.

NOTE  the authors use 2 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 are shown here

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" by cyprids 0.7X

Invertebrate larvae, including those of barnacles, often arrive on the shore in discrete pulses rather than in a continuous trickle. Researchers at Stanford University, California have investigated this problem 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.  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.  Roughgarden et al. 1991 Acta Oecologia 12: 35.

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 drops 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 previously put forward to explain shoreward transport of crab larvae.  Pineda 1991 Science 253: 548.

NOTE:  this idea is considered elsewhere in the ODYSSEY: LEARN ABOUT CRABS & RELATIVES: REPRODUCTION: LARVAL LIFE

Like other species of barnacles, the intertidal barnacle Tetraclita squamosa rubescens 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 5 populations in central and southern California through a survey of genetic variation in 8 enzyme systems. Results show that gene flow is, in fact, high and that the 2 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.  Ford & Mitton 1993 Mol Mar Biol Biotech 2: 147.

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 (see map).  The increase occurs over a large latitudinal scale and results in significantly greater mean monthly recruitment at most of the sites (see histogram on Right). 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.  Connolly & Roughgarden 1999 Limnol Oceanogr 44: 466.

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

Large-scale differences in recrutment of barnacles exist between mid-California and northern Oregon.  A 17-mo 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.  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.  Connolly et al. 2001 Ecology 82: 1799.

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, but the data are not included here

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 map).  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 more going northwards than going southwards (not expected).  The researchers analyse genetic structure in 2 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.  Wares et al. 2001 Evolution 55 (2): 295.

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

Information on the relationship between temperature stratification, upwelling, and settlement of barnacles is provided in a study by researchers from Woods Hole Oceanographic Institution, Maine and Departamento de Oceanografia Fisica, Ensenada at 2 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 (see graphs).  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. Pineda & López 2002 Continental Shelf Res 22: 1183.

NOTE  species used 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

A study by a group of University of California scientists on settlement of acorn barnacles Chthamalus spp.on Santa Cruz Island, California (see map) 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 (1-1.5oC 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.  Broitman et al. 2005 Limnol Oceanogr 50 (5): 1473.

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

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 2 cape areas along the Oregon coast for comparison based on significant differences in long-term chlorophyll a values. The illustration on the Left 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. 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.  Leslie et al. 2005 Proc Natl Acad Sci 102: 10534.

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 above 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

A unique problem 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 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.  Shanks & Eckert 2005 Ecol Monogr 75: 505.

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, see map) 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, but Balanus glandula being 2 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 (see chart for 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, Washington/Oregon, major upwelling occurs seasonally from May-September and is characterised by frequent interruptions or “relaxation” events.  Further 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.  Broitman et al. 2008 Ecol Monogr 78 (3): 403.

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

An investigation by a researcher 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 of and the 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 (see graph 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. Shanks 2009 Mar Ecol Progr Ser 385: 205; Shanks 2009 Mar Ecol Progr Ser 385: 217.

This photograph shows newly settled spat of Balanus glandula, along
with some older recruits. Note the preference for settling in cracks 2X

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 coastal upwelling.  The authors sample adult and newly settled barnacles from 9 sites along the entire Oregon coast (see map) 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 (see graph on Right).  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 (see schematic below).  Barshis et al. 2011 Mar Ecol Progr Ser 439: 139.  

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

NOTE an earlier study on barnacle Balanus glandula recruitment along the central Oregon coast also correlates relaxation of upwelling events with strongest recruitment.  Maximum recruitment is noted under conditions of onshore surface currents and higher seawater temperatures.  Dudas et al. 2009 ICES J Mar Sci 66: 396.