Toyomitsu Horii

Community and trophic structures of abalone Haliotis diversicolor habitat in Sagami Bay, Japan

The community and trophic structures of the subtidal habitat of the abalone Haliotis diversicolor were examined in Nagai, Sagami Bay, Japan. Conventional community indices showed no significant differences among three sampling stations. The overall continuum of stable isotope ratios was structured into three different trophic linkages: (i) brown algae-dependent benthic food chain; (ii) red algae-dependent benthic food chain; and (iii) planktonic food chain. Brown algae and red algae likely play different roles with respect to carbon sources in the habitat. Conventional fractionation values indicated that the abalone H. diversicolor (δ13C = 12.4 ± 1.0‰, δ15N = 9.3 ± 0.5‰) feeds on the lamina of Undaria pinnatifida during juvenile and adult stages (8.0–65.6 mm shell length). Stable isotope signatures suggested that the juveniles of other abalone species as well as some amphipods and a sea cucumber Holothuria decorata are competitors of H. diversicolor, whereas some Muricidae gastropods such as Thais bronni and Ergalatax contractus are predators. The isotopic differences among macroalgal species and the subsequent transfer to consumers indicate that stable isotopic analysis is an effective means of studying food webs in an open rocky shore community with little influence from external primary production such as terrestrial vegetation.

Dietary value of benthic diatoms for post-larval abalone Haliotis diversicolor associated with feeding transitions

The feeding behavior and growth of post-larval Haliotis diversicolor with initial shell lengths (SL) of approximately 500 μm (Exp. 1–1 and 1–2), 800 μm (Exp. 2), and 1200 μm (Exp. 3) were studied in a laboratory setting while they fed on four species of benthic diatom Achnanthes longipes, Cocconeis sublittoralis, Cylindrotheca closterium, and Navicula ramosissima. Exp. 1–1 and 1–2 revealed no marked differences in post-larval growth rates (mean 24–39 μm SL/day) among the diatom species. However, marked differences in growth rates among the species were revealed in Exp. 2 and 3. Three species, A. longipe, Co. sublittoralis, and Cy. closterium, produced faster growth (Exp. 2 mean 29–51 μm/day, Exp. 3 mean 36–44 μm/day) than N. ramosissima (Exp. 2 mean 18 μm/day, Exp. 3 mean 23 μm/day). Post-larvae fed N. ramosissima had lower digestion efficiency (42.8%) than those fed other diatom species (90.7–100%). Diatom extracellular substances appeared to be principally used from post-settlement to 800 μm SL, and diatom cell contents were required to produce rapid growth of larger post-larvae (>800 μm SL). It is likely that the availability of each diatom for post-larvae was affected by diatom morphology, attachment strength, frustule strength, and post-larval size.

Survival, Growth and Recruitment of Abalone Haliotis diversicolor in Sagami Bay, Japan

Abstract The occurrence of newly settled postlarvae (<500 μm of shell length [SL]) and subsequent growth and survival of the abalone, Haliotis diversicolor, were observed from 2001–2004 at two stations in the rocky shore of Nagai on the coast of Sagami Bay, Japan. Seawater temperature, current velocity, and distance of the weekly stone movement were monitored at both stations to determine factors affecting survival and growth of postlarvae. There were newly settled postlarvae in August and September of 2001, October 2002, August 2003, and September 2004. A total of 6 cohorts were identified between 2001 and 2004. Initial density of each cohort was generally higher at Station 1 than Station 2, but recruits at Station 1 had greater mortality than Station 2. Results from measurements of the physical environmental factors suggested that higher mortality rate of postlarval and juvenile H. diversicolor at Station 1 were produced by greater water turbulence and stone movement caused by storms. Growth rates in the first month after settlement varied between cohorts (35–62 μm SL day−1), and growth rates were slower for cohorts settling later in the spawning season and experiencing lower water temperature. The timing of typhoon-triggered spawning in H. diversicolor population was also suggested to be an important factor affecting growth and survival of postlarvae and early juveniles, along with their subsequent recruitment. The cohorts from 2001–2004 attained 17–40 mm SL in about one year after settlement. To estimate the size and age at the first stage of maturation, the increase in shell length and gonad development of a single cohort from 2001 were measured. This cohort attained 30–55 mm SL and showed high gonad index at 22 mo after settlement, indicating the first stage of maturation of H. diversicolor is achieved at age 2.

Decrease of Abalone Resources with Disappearance of Macroalgal Beds Around the Ojika Islands, Nagasaki, Southwestern Japan

ABSTRACT Because abalone feed mainly on macroalgae, the disappearance of macroalgal beds would have negative effects on abalone. We investigated the relationship between abalone resources and quantitative or qualitative changes of algal beds in coastal waters around the Ojika Islands, Nagasaki Prefecture, southwestern Japan, from 1988 to 2011. During this period, the catch record of 2 abalone species—Haliotis discus discus and Haliotis gigantea—was examined, and the composition and abundance of macroalgae around the Ojika Islands was observed. The annual landing of abalone has decreased steadily from 91 mt in 1987 to 0.4 mt in 2011. The proportion of H. discus discus to the annual abalone landing has also changed; it was stable (30%–60%) between 1988 and 2000, less than 20% from 2004 to 2005, and increased from 2006, reaching more than 90% in 2010. The macroalgal beds have also changed during this period. The perennial Laminariales beds, consisting of Eisenia bicyclis and Ecklonia spp., had disappeared by 2003, shifting mostly to Sargassaceae (Sargassum macrocarpum) beds. After the Sargassaceae beds had disappeared by 2009, there have been no Laminariales or Sargassaceae beds except for spring algal beds (unusual perennial Sargassaceae beds that form dense stands from March to July and lose most thalli after maturing in August) in some fishing ports. The results suggest a differential detrimental effect of the disappearance of macroalgal beds on abalone, with different species having different effects. After 2009, when no perennial Laminariales or Sargassaceae beds were present, there were no mature individuals of H. gigantea, with no recruitment at the juvenile stage of the latter at major fishing grounds. Reformation of the macroalgal beds is indispensable to sustain or restore the H. gigantea population. On the other hand, because most H. discus discus are able to mature, they can sustain a viable population under this condition.

Identification of juvenile abalone Haliotis diversicolor based on number of open and sealed respiratory pores

Changes in the number of respiratory pores (open pores) and their imprints (sealed pores) in post-larvae and juveniles were observed and compared for four abalone species Haliotis diversicolor, H. discus discus, H. madaka, and H. gigantea. The first open pore was evident at a shell length (SL) of 1.5 mm in H. diversicolor, 1.9–2.0 mm Sl in H. discus discus, and 2.3–2.4 mm SL in H. madaka and H. gigantea. The number of open pores in H. diversicolor gradually increased with growth, with four to five pores at 2.5–18.0 mm SL and five to six pores at 18.0–27.0 mm SL. The other three species maintained four to five open pores after they reached 3.4–4.5 mm SL. The total number of open and sealed pores (TNP) was greater in H. diversicolor than in the other species at the same SL. Juvenile H. diversicolor were identified among field-caught abalone by the difference in the relationship between SL and TNP (SL-TNP relationship) and also by the monoclonal antibody reaction method. The results of the two methods were in perfect agreement, indicating that our method using the SL-TNP relationship is reliable for the identification of H. diversicolor.

Tracking Larval, Newly Settled, and Juvenile Red Abalone (Haliotis rufescens) Recruitment in Northern California

ABSTRACT Recruitment is a central question in both ecology and fisheries biology. Little is known however about early life history stages, such as the larval and newly settled stages of marine invertebrates. No one has captured wild larval or newly settled red abalone (Haliotis rufescens) in California even though this species supports a recreational fishery. A sampling program has been developed to capture larval (290 µm), newly settled (290–2,000 µm), and juvenile (2–20 mm) red abalone in northern California from 2007 to 2015. Plankton nets were used to capture larval abalone using depth integrated tows in nearshore rocky habitats. Newly settled abalone were collected on cobbles covered in crustose coralline algae. Larval and newly settled abalone were identified to species using shell morphology confirmed with genetic techniques using polymerase chain reaction restriction fragment length polymorphism with two restriction enzymes. Artificial reefs were constructed of cinder blocks and sampled each year for the presence of juvenile red abalone. Settlement and recruitment were found to vary with year and site from 2007 to 2015. In some years such as 2010 and 2013, there were many larvae and newly settled abalone, whereas in other years there were none. The two exceptionally poor years for larval and newly settled abalone were 2012 and 2015 (warm El Niño years). In 2013, there was spatial concordance between two sites 18 km apart with respect to the sizes of the newly settled individuals suggesting they were spawned and settled on the same day. The methods developed here, quantifying early life history stages, may shed light on the “black box” of recruitment and help addresswhat are the drivers of good and bad recruitment years for red abalone in northern California.

Larval Dispersal of Abalone and Its Three Modes: a Review

ABSTRACT Abalone are planktonic larvae in their early life history stages, and their dispersal predominantly takes place during these stages. Abalone stocks rely on the natural supply of larvae in self-recruiting populations, larval connectivity in metapopulations, and artificial hatchery—produced larvae in regions where larval restoration is needed. Larval dispersal is, therefore, one of the key factors in the management of wild populations as well as in the establishment of resilient and genetically viable populations during restoration. In this review of abalone larval dispersal studies conducted in different regions of the world, evidence is found for three modes of larval dispersal; (1) short distance, (2) long distance, and (3) both short and long distance (dual mode). Four biological factors (spawning, larval duration, vertical behaviors, and presettlement mortality) are proposed to influence larval dispersal. Consideration of larval dispersal mode, influential biological factors, and their interactions may improve estimation of connectivity in metapopulations for establishment of effective marine protected areas, which could help the recovery of declining populations and the conservation of endangered abalone species.