Reproductive events include larval biology considered in this section, and
COPULATION & LARVAL DEVELOPMENT,
SETTLEMENT CUES,
SELECTION OF SUBSTRATUM,
ATTACHMENT,
SETTLEMENT COUPLED WITH OCEANIC PROCESSES, and
POST-RECRUITMENT EFFECTS ON COMMUNITY STRUCTURE, considered in other sections.

The naupliar phase of barnacle development includes 6 instars, and it is mainly during the last of these that energy in the form of lipid and protein is sequestered.  This energy store must last the larva through the approximately 2-wk cypris stage, as well as through settlement and metamorphosis to the juvenile.  Does the moult to the cypris occur only after some minimum size is attained by the last instar nauplius?  Or does it occur only after some minimum amount of lipid that ensures metamorphosis to the juvenile is sequestered?  And how flexible is the timing?  These questions are addressed in a study at the Oregon Institute of Oceanography, Coos Bay on metamorphic plasticity in Balanus glandula in which size and lipid contents of nauplii are measured under conditions of varying food concentrations. As expected, higher rations lead to larger size and an earlier moult to the cyprid stage (see histogram on Left).

Early (1st-3rd stages) and late (6th stage) variations in food supply do not affect the timing of this moult.  During the middle stages of naupliar development, however, enhanced food decreases the time to cypris moult, while reduced food lengthens it.  This is the period of greatest plasticity in timing of development.  About 60% of naupliar development is complete at the beginning of the 6th stage, when the timing of the moult to the cypris stage becomes fixed. 

Constant high rations do not necessarily lead to higher relative lipid concentrations. In fact, relative lipid concentrations only respond to shifts in rations that occur during the 6th naupliar stage.  While not affecting age at moult to the cypris, such changes do affect the cypris’ size and relative lipid concentration. These have fitness implications, as more relative energy will allow the cypris to search more prospective settlement sites for a longer time, and metamorphosis to a larger-sized juvenile will reduce early mortality to predators and bulldozing limpets. Hentschel & Emlet 2000 Ecology 81: 3495.

NOTE larvae are cultured at different concentrations of food diatoms Skeletonema costatum and under conditions of varying rations (high-to-low and low-to-high, in different patterns).  Only some of the results of this comprehensive study are considered here, and then only in condensed form

There has been much research attention over the past decade to “upstream” and offshore events that influence or regulate settlement of marine invertebrates. For example, settlement of barnacles Balanus glandula and Chthamalus spp. in the Monterey Bay area of California is strongly dependent upon the extent of onshore/offshore transport of surface water associated with upwelling events.  In each of 4 “pulses” of settlement monitored in spring/summer by scientists in the Montery Bay area, most settlement occurs during periods of relaxation of alongshore winds and cessation of upwelling. 

During summer the net surface transport is usually offshore as a result of upwelling. This is shown in a 17 July photograph of Monterey Bay as a predominance of cold upwelling water that tends to carry the larvae offshore (see photo on Left). Records of barnacle settlement on plates at 2 sites around Monterey Bay confirm that settlement of barnacles does not occur at this time.  However, 10d later the alongshore winds abate, upwelling ceases or is markedly reduced, and warmer surface waters move close to shore and stay that way for about 2wk (see photo on Right). Coincidentally, the settling plates now show large settlement pulses on 27-29 July, with smaller pulses continuing over the next 2wk.  The authors consider 2 alternate hypotheses that could explain their data.  The first one, that the pulses of settlement are caused by temporal variation in release of larve from the adults, is dismissed because regular collections of adults from around the Bay show no evidence of synchronised pulses in release of larvae.  The second, that periodic onshore waves such as related to fortnightly tidal cycles causes the pulses in recruitment, is also dismissed.  Such waves are present, but they occur during both high and low settlement periods. Farrell et al. 1991 Limnol & Oceanogr 36: 279. Photos courtesy Scripps Satellite Oceanographic Facility, La Jolla, California

NOTE  breeding of these barnacle species in central California lasts for 6mo

NOTE photos are “advanced very high resolution radiometers” (AVHRR) images from NOAA Weather Satellites.  Colours: black=land, white=clouds, orange=warm, dark blue=coldest

Inshore transport of barnacle larvae may also occur in the convergent zones (or slicks) generated by internal tidal waves.  Such slicks are visible on incoming tides as lines of flotsam.  Not only is surface matter transported, but also sub-surface particles (<20cm depth) such as larvae are moved along.  Comparison of abundances of larval barnacles Balanus glandula and Semibalanus cariosus in the San Juan Islands, Washington show almost 13-fold greater numbers in surface waters of convergent zones (slicks) as compared with between-slick areas.  The authors remark that not only do these internal waves have the potential to carry larvae shorewards but, by the nature of tidal movements, likely deposit them in differing amounts along the shore.  Shanks & Wright 1987 J Exp Mar Biol Ecol 114: 1; for a review of this subject see Le Fèvre & Bourget 1992 Trends Ecol Evol 7: 288.

NOTE convergent-zone transport of larvae is considered elsewhere in the ODYSSEY: LEARN ABOUT CRABS & RELATIVES: REPRODUCTION: LARVAL LIFE

With larval life-spans of several weeks, barnacles have broad dispersal potentials and potentially unrestricted gene flow.  A study of genetic differentiation measured in the mitochondrial¹ (cytochrome oxidase I=COl) locus in 433 barnacles Balanus glandula from 12 populations over a 1500km spread from California to Vancouver Island, British Columbia, however, reveal a strong disjunction beginning at about 40^oN latitude extending south (see map on Left). These patterns indicate that gene flow within southern California populations is restricted spatially from that in the northern populations. 

By monitoring movement of surface drifters² (mimicking larval dispersal) released at 2 locations, one in the north in Oregon (44^oN), the other, in the south near Santa Barbara, California (34^oN), the researchers show essentially no long-term (up to 90d) intermixing (see schematic on Right).  They propose that the lack of communication between waters originating in Oregon and southern California helps to maintain the strong genetic differentiation³ between the regions.  Sotka et al. 2004 Molec Ecol 13: 2143.

NOTE¹ the authors also measured haplotype frequencies for nuclear (elongation factor 1-alpha) loci but, as both sets of results are similar, only the mitochondrial data are presented here

NOTE² 75 of these drifters are released in Oregon (within 120km of the shore) and 541 in California.  The drifters float at 15m depth

NOTE³  in a follow-up study, researchers from Duke University, North Carolina suggest that the genetic diversification in B. glandula may have occurred as much as 100,000yr ago, long before the last major glaciation event.  Since the ensuing time period is clearly enough for the separation to resolve itself by genetic drift and/or migration, the authors reiterate the liklihood of strong oceanographic mechanisms maintaining the split.  Wares & Cunningham 2005 Biol Bull 208: 60

A study by researchers at Scripps Institution of Oceanography, La Jolla provides information on distribution and mortality of larvae of barnacles Balanus glandula and Chthamalus spp. in inshore waters.  First, nauplii and cyprids tend to be spatially segregated, with early-stage nauplii and cyprids being more abundant inshore, and later-stage nauplii being more abundant offshore (see sample graphs for early- and late-stage nauplii and cyprids of Chthamalus spp.).  Note in the graphs the considerable temporal variability in abundances of larvae over 7 consecutive days of sampling.  Second, there is a tendency for barnacle larvae to be closer to the sea surface and, because of prevailing onshore winds and local shoreline topography in the study area, this favours their retention in shallow, inner-shelf waters.  Finally, the authors report relatively high in situ mortality rates for larvae of both species, of up to 20-40% per day, much greater than previously supposed.  In fact, at the high end these rates are almost an order of magnitude greater than the 5% daily mortality of earlier estimates, which would have predictable consequences on settlement/recruitment. Tapia & Pineda 2007 Mar Ecol Progr Ser 342: 177.

NOTE  the larvae are likely to have been a mix of C. fissus and C. dalli but, as their larvae are hard to distinguish, the authors lump them together

NOTE  statistically significant only for Chthamalus spp.  Offshore stations are about 1km off the shoreline

A final hurdle in the transition from ocean to settlement site for the larva of an intertidal rock-inhabiting species like a barnacle is the surf zone.  A group of researchers primarily from Oregon and California hypothesises that differences in water exchange across the surf zone will cause significant temporal and spatial variation in larval delivery to settlement sites on the shore.  There is nothing new in this idea, but the researchers tackle it head on, so to speak.  They measure larval¹ settlement in relation to wave heights and wave periods at 2 sandy beaches², and compare larval settlement on 4 dissipative³  and 6 reflective sandy beaches (see photographs).  Results show, as predicted, that settlement of barnacles Balanus glandula is significantly correlated with the average ratio of wave height to wave period (H/T; data not shown) and that settlement is significantly greater, again as predicted, on boulders on dissipative beaches. Size-frequency distributions on dissipative beaches are dominated by small, recently recruited individuals, with only a few larger individuals (see Left histogram).  On reflective beaches, the reverse is true (see Right histogram). Shanks et al. 2010 J Exp Mar Biol Ecol 392: 140.

NOTE¹  the authors record settlement of barnacles in the wave-height part of the study, and of barnacles, limpets, and algae in the beach-topography part of the study.  Only data on barnacles are presented here

NOTE²   the 2 beaches are at Dike Rock near La Jolla and Bastendorff Beach in southern Oregon.  Barnacle settlement is recorded on plates attached to 3 boulders at each beach

NOTE³   a dissipative beach is a flatter one where wave forces are dissipated over a wide surf zone; in contrast, a reflective beach is a steeper one with a narrower surf zone.  Dissipative beaches tend to have finer-grain sands than reflective beaches.  Actual conditions, of course, grade between these two extremes