Half A World, The Earthly Width

Red Prowfish (Pataecus fronto), family Pataecidae, order Perciformes, found in the coastal waters of western and southern Australia

and they have a specific relationship with crested caracaras, in which the black vultures assist them by not only preening them after meals but also leading them to food in the first place— due to their superior sense of smell— while the caracaras assist the black vultures by acting as a warning signal in case of danger

and while this is more typical of black vultures, this is not common at all for any member of the falconidae family— it’s a special bond!

Red-legged Seriema (Cariama cristata), family Cariamidae, order Cariamiformes, Brazil

Seriemas were once placed in the Gruiformes, but in 2014 were elevated to their own order. They are most closely related to Falconidae, Psittaciformes and Passeriformes.

They are territorial, and some people in South America use them as guard animals.

Seriemas eat a wide variety of small prey, and are known to pick up snakes with their beaks and slam them hard onto the ground repeatedly, to subdue & kill them.

Seriemas are the closest living relatives of the extinct Terror Birds, and are even similar in size and lifestyle to the smallest members of that group!

Seriemas are limited to two species now, but in addition to Terror Birds, had an EXTENSIVE fossil record of relatives with many species spread around the world!

Seriemas are the earliest derived members of the Australaves and helped researchers develop the hypothesis that Telluraves, the group that includes the vast majority of tree-dwelling birds, evolved from a predatory ancestor!

Lesser mouse-deer Tragulus kanchil

Perched on the tendril of a Passiflora plant, the egg of the Julia heliconian butterfly may be safe from hungry ants. This species lays its eggs almost exclusively on this plant’s twisted vines.

These photos show the embryonic form of Limulus polyphemus, commonly known as the Atlantic horseshoe crab. The eggs of this species are initially opaque, with a grayish-blue, green, or pink coloration, but they become increasingly translucent as the embryos mature, providing a glimpse of the tiny horseshoe crabs developing within.

Above: several embryos twirling around in their eggs

The legs become visible roughly five days after fertilization, and the embryos begin to move shortly thereafter, eventually flexing their legs and twirling their bodies. They molt for the very first time just a few days later. Each embryo will shed its skin and grow a new one four times in total before it even hatches from the egg.

Above: Limulus polyphemus embryos

The hatchlings finally emerge after 2-4 weeks. The freshly-hatched larvae measure less than 1cm long, and they look just like miniature versions of their adult form, except that they do not yet have tails (which are actually known as telsons) and their exoskeletons are still soft and translucent. These young horseshoe crabs are often described as "trilobite larvae."

Above: a young horseshoe crab discarding its egg

Atlantic horseshoe crabs generally spawn in May and June, with hundreds of thousands of individuals gathering along the coast on the night of the full moon and new moon. Each female lays up to 100,000 eggs per season, but very few of those offspring actually survive to adulthood. Most of the eggs are eaten or destroyed before they can even hatch, and many of the remaining larvae perish at some point during the 10 years that it takes for them to reach full maturity (i.e. the age at which they begin to reproduce).

Above: the freshly-hatched larvae

Wild horseshoe crabs can live to be more than 20 years old, and they can measure up to 60 centimeters (2 feet) long. They have 10 eyes in total, including two compound eyes that are specifically adapted for the purpose of finding a mate:

The most obvious eyes are the two lateral compound eyes. These are used for finding mates during the spawning season. Each compound eye has about 1,000 receptors or ommatidia. The cones and rods of the lateral eyes have a similar structure to those found in human eyes, but are around 100 times larger in size. At night, the lateral eyes are chemically stimulated to greatly increase the sensitivity of each receptor to light. This allows the horseshoe crab to identify other horseshoe crabs in the darkness.

Above: a close-up of a horseshoe crab's compound eye, which is covered in tiny hatchlings for some reason

Horseshoe crabs have been around for at least 445 million years, which means that these creatures are about 200 million years older than the dinosaurs and at least 50 million years older than trees, and yet their morphology has changed very little in that time. In fact, modern horseshoe crabs are frequently described as "living fossils," because they still look strikingly similar to their fossilized ancestors.

Above: the juvenile form of Tachypleus tridentatus, commonly known as the Chinese horseshoe crab

It's important to note that horseshoe crabs are not true crabs. In fact, they're not even crustaceans -- they belong to a completely different group of arthropods known as chelicerates, and they are more closely related to spiders and scorpions than they are to crabs.

Above: Tachypleus tridentatus and Limulus polyphemus

This is a revised/updated version of a post that I published about two years ago, with much more information, photos, and sources.

Sources & More Info:

iNaturalist: Horseshoe Crab Eggs

Maryland Department of Natural Resources: Horseshoe Crab Life History

Current Zoology: Developmental Ecology of the American Horseshoe Crab

PBS: Once a Spawn a Time: Horseshoe Crabs Mob the Beach

Smithsonian National Zoo and Conservation Biology Institute: Limulus polyphemus

National Wildlife Federation: Horseshoe Crabs

The boundary of Fog