Research study 3

Further studies on the influence of GABA on the development of Haliotis rufescens veligers show that in the presence of 10^-3 M concentrations of the molecule, metamorphic competency is reached within 160h from fertilisation (graph on Left, 15^oC). Control larvae not treated with GABA are still not competent to settle even as long as 240h after fertilisation.

Tests of other amino-acid neurotransmitters, some homologues of GABA and others differing by a little as one carbon atom in chain length from GABA, show variable results on 8d-old larvae (table on Right: asterisked substances are homologues of GABA). The authors note that the closely related GABA homologues and other amino acids are less effective, requiring higher concentrations and longer exposures to induce metamorphosis (data not shown here).  GABA is essentially 100% effective after 1h at concentrations of 10^-3-10^-4 M.  Morse et al. 1980 Fed Proc 39: 3237.

Another chemical inducer for settlement and metamorphosis of larval abalone Haliotis rufescens is increased concentration of K⁺ in the surrounding seawater.  Studies at the Marine Sciences Institute, Santa Barbara, California show that addition of K⁺ in concentrations in excess of those already present in seawater of 6-12mM KCl or K₂SO₄ significantly increases the percentage attachment of larvae (see graph). Note the dose-dependency of the settlement response in the larvae.  The researchers also show that a decrease in external potassium ion concentration will inhibit the larval settling response to GABA.  The authors propose that increased K+ may act in a manner similar to GABA, by directly depolarising excitable cells involved in perception of inductive stimuli by the larva.  Baloun & Morse 1984 Biol Bull 167: 124. Photograph courtesy Kevin Lee, Fullerton, CA diverKevin.

NOTE  the authors choose not to have a control treatment in the experiment, perhaps because they know that without addition of potassium the larvae will not settle

Haliotis rufescens 1X

Like most species of Haliotis, larvae of green abalone H. fulgens are free-living for just a few days.  Consequently, their potential for dissemination over any but short distances is severely limited.  This is tested by researchers from the Scripps Institution of Oceanography, La Jolla by releasing drift tubes from several abalone-populated sites in the Channel Islands and recording time taken to arrive at the mainland.  At the time of publication of this work, mainland stocks had been largely depleted, and it was thought that recruitment might potentially come from Channel-Islands stocks.  Of 1225 tubes released in June, only 4% are recovered on the mainland (see map).  More importantly, only a handful (about 5 in total) is recovered within the normal competency period of the larvae (average crossing time is about 16d, while larvae of H. fulgens would be expected to settle within about 10d at 21^oC, the seawater temperature at the time).  The authors remark that while some transport occurs between isolated mainland populations within a realistic time period, the potential for this to have any significant impact is likely slight.  In comparison, a high proportion of tubes released from both island and mainland sites travel just a few km to suitable sites within biologically realistic times.  These observations suggest that the present fishery closure (dating from 1977) is unlikely to promote recovery of mainland populations any time soon.  The study is an interesting one, and is sure to have been pursued in later papers.  Tegner & Butler 1985 Mar Ecol Progr Ser 26: 73. Photograph courtesy Kevin Lee, Fullerton, California.

NOTE  the “larval mimics” are clear plastic test-tubes of 15 x 1.6cm size, sealed and weighted to float upright just at the sea surface.  A stamped and addressed card inside instructs the finder to provide date, time, and precise location of recovery.  Tubes are released from both island and mainland sites in June and October, but only data for the island releases are reported here.

NOTE  a partial assumption made by the authors with regard to these data is that recovery is simultaneous with landing on the shore, which is unlikely.  Also, of the 5 tubes crossing to the mainland in a realistic time, only “a portion” is found near suitable rocky habitat.  The authors do not speculate on the actual number of larvae that might be represented by this “portion”, based on numbers of individuals spawning on the islands, nor on what size population of upstream spawners would be required under any circumstances to provide “adequate” numbers of larvae for an “x-sized” estimated recruitment. Perhaps 5 out of 1225 drift tubes would actually represent multi-billions of larvae being transported

Research study 7

As the larvae move along in currents at the time of settlement, are they active or passive on a small scale?  This is tested in laboratory flow tanks in Santa Barbara, California where settlement of Haliotis rufescens larvae is compared with settlement of larval-mimicking particles under different current intensities flowing over plastic casts modeled after natural rock substrata.  The current carries the larvae and mimic-particles over high-velocity regions of the model (on the tops of projections, indicated in yellow on the model at Right) and past low-velocity regions (in and around depressions, indicated in pink).  In each location chips of coralline algae are placed as cues to induce settlement.  In low-velocity regions of the model where deposition of mimic-particles is great (left pink bar in histogram), the author favours a prediction that larval settlement will be low because of poor contact with the settlement-inducing algal chips.  An alternative prediction is that it would be high because particles would settle out. In fact, larval settlement is actually high, indicating that the larvae behave more like passively deposited particles.  In comparison, in high-velocity areas of the model where deposition of mimic particles is small and where we predict that larval settlement will be great because of good contact with the settlement-inducing algal substrata (right yellow bar in histogram), larval settlement is much lower than prediction but significantly more than predicted for a passive particle.  The author sums up this part of a much larger study by noting that the pattern of larval settlement in the flow tanks closely resembles that of a passively deposited particle, but one that still responds to a cue when able.  Boxshall 2000 J Exper Mar Biol Ecol 254: 143.

NOTE the mimicking-"particles" are actually dead or inactive larvae, and empty shells, most of a size of about 320µm

NOTE  veliger larvae are able swimmers, but would be swept along in even the slowest current velocities used in the flow tanks (3.5 ^. sec^-1) were it not for boundary-layer flow effects.  Shear forces may be high away from the substrate surface, but they diminish as the substrate surface is approached until, just at the substrate-water interface, they ultimately decrease to zero.  This means that even on the tops of projections on the model where the current is fast, near-zero velocities are accessible close to the substratum, just a short distance away.  For the tiny veliger larvae, the algal surfaces represent an oasis of calm within the storm