The Effect of Size-limited Brood Capacity on Brood Size in a Freshwater Bivalve

Size limited brood capacity is common among species with hard exoskeletons or shells. In these species, brood size is limited by the physical capacity to hold offspring. Here we present evidence that brood size is limited by physical constraints in Sphaerium striatinum , a small brooding bivalve. Sphaerium striatinum is a sequential brooder and produces offspring throughout the year. Offspring are brooded in marsupial sacs located on the inner demibranch. In an unconstrained brooder one would predict that brood size would increase as a function of adult length cubed, a volumetric relationship. In S. striatinum, brood size increases as less than a function of adult length squared. We demonstrate that brood size is limited by two general constraints: marsupial sacs and the retention of extra-marsupial offspring. The number of marsupial sacs increases as less than a function of adult length squared. This relationship may be a result of physiological process such as feeding and respiration. Offspring size at independence is a crucial factor in determining offspring survivorship. The retention of extra-marsupial offspring promotes growth inside a safe environment and increases survivorship upon independence. However, the exponent relating brood size to adult length is significantly less for adults that contain extra-marsupial offspring than compared to adults that do not contain extra-marsupial offspring. Although the evolution of brooding in S. striatinum has resulted in severe constraints on brood size, the benefits of brooding outweigh the cost of limited brood capacity. We discuss our results in relation to brooding strategies and size limited brood capacity in other brooding bivalves

The Mismanagement of Limulus polyphemus in Long Island Sound, U.S.A.: What Are the Characteristics of a Population in Decline?

Over the past 15 years, horseshoe crabs in Connecticut have gone from being considered a nuisance species to a species of Greatest Conservation Need in 2015. This has happened through first, its discovery as an economically important species, second through research of its ecological role in coastal estuaries, and third, through education of the public concerning its role in the environment and their own health. To manage horseshoe crab populations successfully requires long term monitoring, research and education. The use of annual or biannual trawl data trends to assess the success of management decisions is limited due to the high variance of counts by location, season, and year, and is prone to misinterpretation. Trawling also damages the very habitat that the target species needs to survive. What is required to reach the goal of a sustainably managed population is the determination of local population dynamics and then modifying management practices to maximize reproductive success of the species. In Long Island Sound (LIS), these basic requirements for sustainable management have not been available until recently. The two states that manage horseshoe crabs in Long Island Sound (Connecticut and New York) have drastically different harvest regulations and management plans that are clearly not working. The LIS horseshoe crab population has declined and remains low as shown by both the long term trawl data and our long term mark-recapture data. After 13 years of a classic ecological study we found the Long Island Sound horseshoe crab population to be one discrete management unit where at least 3 % of the recaptures were found to cross the Sound. The LIS population is aging with low recruitment of newly molted adults and is reproducing well below its maximum rate. We observed very low spawning densities, increasing numbers of single females on the beach, and less than 6 % of polyandrous mating behavior. These basic population trends over the length of this study indicate the current harvest quotas and management techniques are not sustainable. We recommend at the very least the implementation of a unified management plan for Long Island Sound with one shared harvest quota for the entire LIS population. Second, we suggest that the number of no-harvest zones should be increased on both sides of the Sound and a ban of the harvest of spawning females should be added to existing harvest regulations. Ultimately, we would recommend the establishment of multiple Marine Protected Areas (MPAs) within LIS where the entire MPA is off limits to commercial or recreational fishing of any type

Project Limulus: What Long-Term Mark/Recapture Studies Reveal About Horseshow Crab Population Dynamics in Long Island Sound

Project Limulus is a long-term study of the population dynamics of the horseshoe crab population in Long Island Sound (LIS). We have tagged over 20,000 spawning adults from \u3e20 beaches ranging from Greenwich to Stonington, CT since 1997. Cumulative recapture rates have reached 9%. On average 90% of the crabs are recaptured within a few miles of their original tag site within the first season. Between seasons, on average, 45% of crabs are recaptured within the same locality of where they were tagged. Of all recaptures, 99% of recaptured individuals are found within LIS. This past year we expanded the study into RI, NY, and MA collaborating with many groups for a regional horseshoe crab census. Preliminary findings reveal low spawning numbers compared to Delaware Bay across the region

Effect of Food Availability on Reproduction and Brood Size in a Freshwater Brooding Bivalve

Brood size is often constrained by the amount of energy available to produce offspring. Yet, energetic constraints may not be as important if the physical capacity to brood offspring is limited. Investigating the relative importance of energetic and physical constraints on brood size is necessary to understand how reproductive strategies are molded by natural selection. We investigated how food availability affects brood size in Sphaerium striatinum, a freshwater bivalve. We reared juveniles to adulthood under three food levels in a common garden experiment. The number of reproductive attempts, brood size, and stage of offspring development were measured. Clams reared with the most food reproduced more often, produced more offspring per reproductive attempt, and contained larger broods than clams reared with less food. These data support the notion that food availability is an important factor in the production of offspring and overall brood size. However, the number of offspring surviving to later stages of development was not different among treatments. In fact, clams reared with the most food lost proportionately more offspring than clams reared with less food. We conclude that physical constraints are more important in determining overall brood size than energetic constraints in S. striatinum

Horseshoe Crab Eggs: A Rare Resource for Predators in Long Island Sound

In Delaware Bay, the spawning of several million horseshoe crabs (Limulus polyphemus) coincides with the arrival of migratory shorebirds that feed on their eggs. High horseshoe crab spawning densities and resulting high egg densities drive egg availability and predation rates. At high spawning densities, female horseshoe crabs perturb previously deposited clutches causing eggs to rise to the sediment surface (surface egg densities average 100,000 eggs m− 2). At the surface (0–5 cm), the eggs are quickly depleted by shorebirds and other predators. This interaction between egg density and egg predation has not been explicitly explored on beaches with low spawning densities such as on the Connecticut (CT) shore of Long Island Sound (LIS). Spawning indices in LIS (range: 0.002 to 0.02 females m− 2) are two to three orders of magnitude less than in Delaware Bay (0.7–1.0 females m− 2). Given the low spawning density and correspondingly low subsurface egg density (x = 1.5 eggs cm− 2), we predicted that the frequency of egg predation would be rare. A series of exclosures was constructed on two known horseshoe crab spawning beaches to test the frequency and rate of predation on horseshoe crab eggs. There was no significant difference between egg masses initially placed in artificial nests and recovered at the conclusion of each experiment. While shorebirds were observed probing sediments within and outside of experimental plots, eggs were never observed at the surface during this study. Foraging by shorebirds for buried eggs may be energetically prohibitive given the lack of eggs on the surface and the low subsurface egg densities on CT beaches in LIS (range = 0.6–2.4 egg cm− 2). Limited egg predation by fish was observed but tidal scouring of eggs buried below mean tide primarily drove egg availability for these predators. We conclude that horseshoe crab egg predation is a rare occurrence in Connecticut due to low egg density as a direct result of low spawning densities

Parental Care in Sphaerium Striatinum Lamarck: Evidence for Retention of Competent Offspring

The timing of offspring release is a fundamental turning point in the life history of any organism. It represents the end to many of the most costly forms of parental care (e.g., provisioning of nutrients for developing eggs and zygotes) and the beginning of an independent life for the offspring. Generally temporal variation in this event is attributed to a variety of physiological and evolutionary trade-offs. Here we examine the retention of offspring in the freshwater clam Sphaerium striatinum Lamarck. Brooded offspring are typically not released into the environment until they are 4.0 mm shell length (SL). We provide evidence that offspring as small as 2.0 mm SL are competent to process particles and produce both fecal and pseudofecal material. Furthermore, such small clams are shown to experience higher mortality than larger individuals once they are removed from brood pouches. Hence, the retention of competent offspring within brood pouches represents a form of extended parental care

Reproductive Behavior in Horseshoe Crabs: Does Density Matter?

While the four species of horseshoe crabs share many common reproductive traits with respect to their reproductive systems, they do differ with respect to their mating behavior (monogamy vs. polygynandry). Past research has attributed these differences to a number of factors including: spawning densities, operational sex ratios (OSR’s), male condition (or age), environmental and/or genetic factors, or a combination thereof. Mating behaviors in the three Asian horseshoe crab species (Tachypleus gigas , T. tridentatus, and Carcinoscorpius rotundicauda ) with low spawning densities and 1:1 operational sex ratios are typically monogamous. In Limulus polyphemus , mating behavior is more variable ranging from monogamy to polygynandry. Here we provide evidence, through a long term behavioral study, that variation in mating behavior is influenced by population density in L. polyphemus . Our study population on two beaches in Connecticut (Long Island Sound) have a spawning density 400 times less than that found in Delaware Bay (0.002 females/m2 vs. 0.8 females/m2) but similar operational sex ratios. Between 90%–95% of all spawning females in CT were paired with only one male, thus exhibiting monogamous behavior. In contrast, between 30 and 60% of spawning females in Delaware Bay have more than one mate and produce clutches of eggs with multiple paternities. Male condition played no role in mating behavior in CT populations. We also observed that on average 18% of the females on the spawning beaches are single. These results suggest that population density is an important condition that determines mating behavior. Also, low population density may lead to decreased mate finding ability and lost opportunities for spawning

Zebra Mussels Affect Benthic Predator Foraging Success

The introduction of zebra mussels (Dreissena spp.) to North America has resulted in dramatic changes to the complexity of benthic habitats. Changes in habitat complexity may have profound effects on predator-prey interactions in aquatic communities. Increased habitat complexity may affect prey and predator dynamics by reducing encounter rates and foraging success. Zebra mussels form thick contiguous colonies on both hard and soft substrates. While the colonization of substrata by zebra mussels has generally resulted in an increase in both the abundance and diversity of benthic invertebrate communities, it is not well known how these changes affect the foraging efficiencies of predators that prey on benthic invertebrates. We examined the effect of zebra mussels on the foraging success of four benthic predators with diverse prey-detection modalities that commonly forage in soft substrates: slimy sculpin (Cottus cognatus), brown bullhead ( Ameirus nebulosus), log perch (Percina caprodes), and crayfish (Orconectes propinquus). We conducted laboratory experiments to assess the impact of zebra mussels on the foraging success of predators using a variety of prey species. We also examined habitat use by each predator over different time periods. Zebra mussel colonization of soft sediments significantly reduced the foraging efficiencies of all predators. However, the effect was dependent upon prey type. All four predators spent more time in zebra mussel habitat than in either gravel or bare sand. The overall effect of zebra mussels on benthic-feeding fishes is likely to involve a trade-off between the advantages of increased density of some prey types balanced against the reduction in foraging success resulting from potential refugia offered in the complex habitat created by zebra mussels

Negative Effect of Zebra Mussels on Foraging and Habitat Use by Lake Sturgeon (Acipenser fulvescens)

1. Lake sturgeon (Acipenser fulvescens) are threatened or endangered throughout much of their range. Juvenile sturgeon utilize sandy and silty habitats extensively during their growth. Invasive zebra mussels change the nature of sandy and silty habitats because they settle on and coat the habitat with the shells of living and dead individuals. The potential impacts of this increased habitat complexity on lake sturgeon is unknown. 2. Juvenile lake sturgeon habitat choice was assessed in laboratory experiments, and zebra mussel impact on the foraging success of juvenile lake sturgeon on three different prey species was measured. 3. Sturgeon foraging on chironomids was virtually eliminated by 95% zebra mussel cover of the sand floor of the foraging arena, and 50% cover reduced foraging significantly. Foraging on more mobile prey items (amphipods and isopods) was essentially eliminated by either 95% or 50% zebra mussel cover of the arena floor. In habitat choice experiments, sturgeon avoided the zebra-mussel- covered habitat more than 90% of the time. 4. This combination of zebra mussel avoidance and reduced foraging in the presence of zebra mussels may be detrimental to sturgeon restocking programmes utilizing smaller sturgeon in zebra-mussel-infested waterways

Horseshoe Crab Research in Urban Estuaries: Challenges and Opportunities

Horseshoe crabs rely on estuaries for food resources, places to spawn and for larvae and juveniles to develop and grow. Many of these estuaries are becoming increasingly urbanized and dominated by human activity. An urban estuary is characterized by armored shorelines, high nutrient loads, large fluctuations in algal and bacteria populations, increased levels of pollutants like heavy metals and pesticides, and seasonally low oxygen levels and pH. While urban estuaries are challenging for horseshoe crab survival and to researchers trying to study them, there are also opportunities for involving the public in research and increasing public awareness of the importance of the conservation and survival of horseshoe crabs. Two recent studies in New York and Connecticut have involved citizen scientists to tag and gather valuable data on horseshoe crab population dynamics. It has been discovered that there is very low recruitment of new adults into the two urban estuaries studied and that spawning populations of horseshoe crabs are relatively low when compared to less disturbed more rural estuaries