Tomio Itow

Horseshoe crabs (Limulus polyphemus) in an urban estuary (Jamaica Bay, New York) and the potential for ecological restoration

We assessed the suitability of intertidal habitats for spawning by horseshoe crabs (Limulus polyphemus) at 12 proposed restoration sites identified by the United States Army Corps of Engineers along the shore of Jamaica Bay, a highly developed estuary in New York City. Based on beach geomorphology, we chose to quantify horseshoe crab activity at five of the sites during the May–July 2000 breeding season. Horseshoe crabs spawned intensively on small patches of suitable sand within larger areas of eroding shoreline with bulkheads and rubble fill. Small areas of sand behind grounded barges at Brant Point and Dubos Point had densities of over 100,000 eggs m−2, which was equal to or greater than the egg densities on longer, more natural appearing beaches at Spring Creek and Dead Horse Bay, or at a sand spit at Bayswater State Park. There were no significant differences in the percentage of Jamaica Bay horseshoe crab eggs that completed development when cultured using water from Jamaica Bay or lower Delaware Bay, a less polluted location. Only 1% of the embryos from Jamaica Bay exhibited developmental anomalies, a frequency comparable to a previously studied population from Delaware Bay. We suggest that the distribution and abundance of horseshoe crabs at our study areas in Jamaica Bay is presently limited by the availability of suitable shoreline for breeding, rather than by water quality. Restoration efforts that increase the amount of sandy beach in this urban estuary have a good likelihood of benefiting horseshoe crabs and providing additional value to migrating shorebirds that use horseshoe crab eggs as food.

Induction of Secondary Embryos by Intra‐ and Interspecific Grafts of Center Cells under the blastopore in Horseshoe Crabs

When the cell mass (center cells) of the early gastrulae in both American and Asian horseshoe crabs was grafted into the embryo of the homologous species, secondary embryos were formed as a result of these grafts. Secondary embryos were also formed in a similar way when the center cells of heterologous embryos were grafted between the American and Asian horseshoe crab embryos. The characteristics of the secondary embryos were similar to the host embryos in both cases, indicating that the center cells played the roles same as those by the amphibian organizer. The homogenate of center cells also induced the formation of secondary embryos. This is the first published study in which secondary embryos of horseshoe crabs have been induced by grafting. The fact may mean that this type of embryonic induction is widespread in the animal kingdom.

The Effects of Water Quality on Horseshoe Crab Embryos and Larvae

It is well established that horseshoe crab eggs can develop successfully across a wide range of temperatures and salinities. However, many estuaries in which horseshoe crabs spawn have been heavily impacted by pollutants, and degraded water quality may be affecting the survival of horseshoe crab eggs laid in such areas. Laboratory bioassays using Limulus polyphemus embryos and larvae have shown a very high tolerance to a variety of contaminants, including heavy metals, oil, and organic compounds, in comparison to similar stages in other marine arthropods. Of the metals tested, acute toxicity was highest for mercury and tributyltin (TBT), but much lower for cadmium, zinc, and copper. Possible mechanisms of pollution tolerance, including stress proteins (Hsps), are discussed. Sublethal levels of heavy metals (especially mercury and TBT) induced a variety of embryonic malformations and impaired regeneration of walking legs. The frequency of malformed L. polyphemus embryos was low (ca. 1%) both in relatively pristine habitats (lower Delaware Bay, NJ) and in urban estuaries (Sandy Hook Bay, NJ, and Jamaica Bay, NY). In contrast, a much higher percentage of malformed Tachypleus tridentatus embryos have been found from several locations in Japan, and severely polluted water may be hampering Japanese efforts to re-establish horseshoe crab populations. Pollutants accumulated by adult females may become incorporated into their eggs and could conceivably be passed up the food chain to egg predators.

MORPHOGENIC MOVEMENT AND EXPERIMENTALLY INDUCED DECREASE IN NUMBER OF EMBRYONIC SEGMENTS IN THE JAPANESE HORSESHOE CRAB, TACHYPLEUS TRIDENTATUS 1

Active morphogenic movement in the horseshoe crab, Tachypleus tridentatus, was observed during the 2 days following completion of the germ disc.Main migration of cells is as follows: Peripheral cells of the germ disc migrate to the anterior margin of the germ disc, and the cells of the anterior margin of the germ disc migrate posteriorly along the median body axis. During such migrations, the transverse furrow appears at the anterior margin of the germ disc.The 3rd prosomal segment is formed from the region just behind the furrow. By treatment of embryos with chemical reagents such as cytochalasin B, dithiothreitol and α-lipoic acid, morphogenic movement was disturbed and segment-defective embryos were formed.Defect or fusion of appendages and segments in the segment-defective embryos occurred mainly in the 3rd prosomal segment. A possible explanation is that the prospective region forming the 3rd prosomal segment cannot take part in the formation of segments as a result of the inhibition of morphogenic m...

Induction of multiple embryos with NaHCO3 or calcium free sea water in the horseshoe crab

SummaryWhen horseshoe crab embryos were treated with NaHCO3 at the developmental stage when the germ disc appears, multiple embryos were formed. NaHCO3 may effect the formation of multiple embryos by binding Ca2+ ions of the embryo since multiple embryos were also formed by treatment with Ca2+ free sea water.The treatment caused the blastoderm layer to tear. When the embryos were returned to normal sea water after the treatment, the blastoderm recovered. Some cell masses, probably derived from the germ disc or its prospective cells, formed during the process of the recovery. Each cell mass developed into an embryo.

Inhibitors of DNA synthesis change the differentiation of body segments and increase the segment number in horseshoe crab embryos (Chelicerata, Arthropoda)

SummaryThe inhibitors of DNA synthesis change the differentiation of abdominal segments of the horseshoe crab and increase the number of segments in 90%–100% of the surviving embryos. The data suggest the following:1.The primordia of segments are formed one by one from the cells at the posterior end of the embryonic area, and they are determined soon after formation.2.When DNA synthesis inhibitors are applied, the forming primordium acquires a character intermediate between the anterior segment already determined and the segment next to be determined.3.The abnormal differentiation is possibly caused by a time lag between DNA synthesis and the rest of metabolism.4.The specific character of each segment of normal embryos and malformations successively determines that of the next segment.5.By this route the whole number of segments is established.

Effect of the Glutamine-Analogue “Azaserine” on Embryonic Development of the Horseshoe Crab

Embryos of the horseshoe crab were treated with the glutamine‐analogue “azaserine”. This compound induced malformed embryos at the stage of enlargement of the germ disc. The embryonic areas of these monsters were separated into two pieces. This is the first report of monsters induced with azaserine.