Per 
Item #: SCP-XXXX
Juvenile SCP-XXXX B instance in containment
Object Class: Euclid Thaumiel
Special Containment Procedures: A controlled population of 500 instances of SCP-5405 A and 500 of SCP-5405 B are to be maintained at Site ███ in two separate 10m x 40m x 10m terrarium habitats and fed daily with 1,500 milliliters of nutrient mixture A30-B. 20 SCP-5405 A must be bred per month for █████████ ██████.
Wild instances of SCP-5405 and their hosts are to be contained or terminated. MTF-Theta 5 (“Brain Bugs”) is currently responsible for the elimination and capture of wild SCP-5405 instances and their hosts.
Amendment to Special Containment Procedures:
Due to the Foundation's efforts at containing SCP-5405, wild populations are estimated to be below 500. To more effectively contain remaining instances, Agents are now instructed to track unbonded instances until they locate a mate, at which point both may be captured.
Description: SCP-5405 is a parasitic Eurypterid1 species, which shares some visual similarities with the tongue-eating louse Cymothoa exigua, and by extension SCP-150. Juvenile instances average 2 millimeters in length and vary between yellow and orange in coloration. Adult male instances, designated SCP-5405 A, are dark red in color and between 1.5 and 2 centimeters long, while adult female instances, designated SCP-5405 B, are pinkish-red and between 2.5 and 3 centimeters long.
All SCP-5405 instances possess a specialized telson2, which allows them to interface neurochemically with larger mammals and some avians. To do this, adult SCP-5405 instances use their mandibles to cut openings slightly below the shoulder of the host and settle under the skin. Their saliva contains analgesics3 and blood clotting proteins which make minimize sensation during the process. From this position, the instance's telson punctures the host's back muscle to access their spinal nerve, to which they connect. This connection allows control over the host's emotional state, as well as limited control of motor functions.
SCP-5405 instances can also communicate with each other by a poorly understood anomalous means. These transmissions are primarily used by instances as a "mating call", as well as for communication between bonded SCP-5405-A and -B instances. Currently, no known method of signal interception exists, nor is there any known anomaly capable of receiving SCP-5405's signal besides SCP-5405.
Removing SCP-5405 instances from their hosts is possible, but has lead to severe mental instability in all known cases. Without heavy amnestitization, former hosts enter a state of acute apathy similar to that found in severe Alzheimer's disease, punctuated by extreme psychotic episodes. In the absence of constant care or medically induced coma, former hosts of SCP-5405 are unlikely to survive beyond one week.
Addendum 5405-1: Reproductive Cycle of SCP-5405
SCP-5405 is an obligate parasite, reliant on its host to complete its reproductive cycle. Uniquely for a parasite however, SCP-5405 form lifelong bonds with instances of the opposite sex. Because these pair bonds are reliant on their hosts maintaining a bond, SCP-5405 pairs coordinate their hosts to be
Once mature, SCP-5405 instances will begin to periodically transmit a specialized mating signal in their immediate vicinity, gradually expanding the range of the transmission. While not precisely known, cases of A and B instances bonding over distances in excess of 1,300 km have been observed.
SCP-5405s anomalous abilities manifest in its natural reproductive cycle. Once they reach adulthood, SCP-5405 A instances parasitize a suitable host and begin transmitting their anomalous mating call. The contents of this signal are poorly understood, but the increase in brain activity and body heat of hosts during the process indicate that it requires significant mental and physical energy. If this call is received and accepted by an SCP-5405 B instance, the two instances will form a permanent pair bond.
Though not precisely known, cases in which hosts traveled over 6,000 km to reach each other have been reported.
Generally, bonded SCP-5405 hosts are difficult to distinguish from the normal human population. Factors correlated to bonded pairs, such as long-term marriage and a higher-than-average number of children, are typically insufficient to identify their anomalous nature. Under severe pressures, such as physical danger or financial insecurity, bonded pairs will exhibit extreme behavior in the interest of protecting their bonded spouse and any offspring. These incidents are the primary means by which bonded SCP-5405 pairs have been identified so far.
Once bonded, SCP-XXXX pairs will typically only communicate with their bonded partner, although they may also send warning signals to instances who are not their mate who approach too close.
Addendum 5405-2: Transcript of Interview with D-5559 and 5560
Yo
