Arrowhead

Splitting from its ancestor, the arrowhead represents a unique evolution from the mainline rorms, differing from them in several ways. While they retain a chitinous shell that covers the majority of their bodies, much like it was with the armororm, the exoskeleton now has grown both denser and stronger with the incorporation of aragonite, a calcium carbonate mineral, into it. This is not unlike the shells of earth crustaceans, and like them, they routinely molt this shell in order to grow larger, a trait not seen in other rorm species, which instead tend to shed plates one at a time as they grow.

Reproductive-wise, arrowheads have adapted their ancestral budding system. Instead of producing a few, smaller copies of themselves, they spawn a mass of tiny budded “spores” that, supplied with little more than a stomach and mouth, must gorge on other planktonic species so that they may mature. While many of these offspring will be consumed by larger planktivores, a good portion will be lucky enough to survive, reach maturity, and reproduce themselves. These spores are released from in-between any body segment, but primarily through the base of the tail. This differs from most other rorms, which typically reproduce offspring around the head region, though this adaptation came about so as to reduce the risk of adults consuming their offspring as well as to compensate for the presence of armor there.

Lacking true jaws, Arrowheads have compensated for this by extending their first armor segment to create a pair of “tusks” that serve to help funnel plankton-laden water directly into the gaping maw. In conjunction with this, tiny tentacle-like extensions extend from around the maw. Capable of limited movement, they each possess a tiny stinging cell within them that is used to incapacitate larger planktonic species before they are swallowed whole.

The presence of an exoskeleton reduces the amount of open surface area through which oxygen and carbon dioxide can be diffused through the skin. As such, Arrowheads had to compensate for this. Slight grooves in the shell of the armororm’s head, which were already thinner in an attempt to solve this issue, have now evolved into complete openings, and function akin to spiracles in earth insects in the sense that gas exchange occurs here. Oxygenated water is sucked into these opening, passed through the body, then expelled through the spaces in-between the tail segments. This, however, would not be sufficient to transport oxygen throughout the body, which would require yet another evolutionary adaptation.

Unlike any other rorm species, the Arrowheads have evolved a sort of blood. Akin to the haemolymph of earth insects, this substance lacks blood cells and is primarily water, alongside a variety of ions, lipids, amino acids, etc. This substance is mostly translucent, with a faint yellowish to greenish tinge in color, stemming from the presence of trace amount of pigments in the liquid acquired from the food they eat. Gas exchange in this liquid is passive, though the small size of the Arrowhead makes it sufficient. The blood also functions in the role of the lymphatic system, as well as transporting nutrients to cells and cellular waste away from them. A pair of hearts, located behind the head and extending towards the base of the tail, help to make this entire open circulatory system function, though they are little more than a few rings of muscles surrounding a pair of tubes that effectively squeeze the blood throughout the body.

Perhaps the most prominent feature on the Arrowhead is the presence of a pair of yellowish lumps on either side of their bodies. Filled with jelly and a small bundle of several electrical sensors, they are used in lieu of eyes in order to sense the world around them. Capable of picking up electrical signals, they grant the Arrowheads a limited worldview beyond what their chemoreceptors can detect. While not capable of granting long-range detection of predators - and prey items are numerous enough to not require it - it does allow Arrowheads to keep in contact with one another and know their positions, allowing for shoaling to occur.

Exploding in regards to population, this tiny planktivores have become prey for a huge variety of larger predators, and thus have become important in many food chains.