Ecologically Correct Hōsōgyo

Question

The Hōsōgyo is a sea creature that roughly appears to be a humanoid shrimp

Their torso is shrimp-like, with a hard carapace on the pereon and a long, narrow abdomen like a tail. Their body is 2.2m long, with a 1.2m pereon an 1m long abdomen. They also have a 60cm wide tail fan. They only have 4 limbs, which are attached to the pereon. These limbs are flippers, with the front pair ending in a hand capable of grasping and the rear pair being stronger for land movement. The rear pair is a little longer than the front pair, being 1.2m and 1m respectively. The head is ape-like, but with proportions and dentition more similar to a horse. It is about 30cm long from the occiput to the lips. They have vertebrate eyes, ape-like ears, a humanoid tongue, a simian nose, and antennae like a shrimp

They are found only on the coasts of Japan. This is because they spend daytime in the sea and nighttime on land. A portion of their gills can process air with similar efficiency to human lungs. They are quite curious and inventive (though not so much as humans), and they also care for their young like apes do

The Earth they inhabit is roughly the same as in reality, at least for what would be relevant in this question

Given the above information, are there any niches that this Hōsōgyo could realistically fill?

Answer 1

One thing you hear Farrelly often from paleobiologists is that you can learn a lot from how an animal lives just by studying its body. For example: when it comes to vertebrates, a creature with horse-like dentition is a creature that seems to need its teeth for extensive chewing and very minor slicing, something particularly important when your diet is mostly based on plant matter.

As for what kind of plant matter, my best bet would be that it's composed mostly of seagrass, algae and other types of aquatic vegetation. The reason for this is that their habit of staying in the sea during the day and going to land at night is very reminiscent of diurnal sea lion species, leaving me to believe that these creatures too would go to land mostly to sleep and be active during the day. In this regard, they'd function a lot like dugongs and sea cows, living mostly in shallower waters grazing on the bottom vegetation. At night however they'd retreat to land and sleep, potentially in small herds. Basically, their ecological function would be mostly that of primary consumers, likely relying on their size and armor to remain safe from predators, as well as potential approach of herding to ensure safety in numbers l.

Now: what would keep them in check population-wise? I'm not the most knowledgeable about Japanese fauna, but I have a feeling that they'd have much fewer predators as they grew up, with the juveniles probably being at the most risk, since their smaller size would leave them vulnerable to anything capable of breaking through their armor, ESPECIALLY so if these creatures also shed their exoskeleton like arthropods in order to grow, because then they'd be an easy snack for larger tanukis, dogs, foxes, wild cats and a number of other small to medium carnivores, as well as bears. As they grew however the biggest threat I'd see them having to face would be bears, particularly Japanese brown bears, despite their diet not being that reliant on meat, and of course humans trying to capture them for culinary purposes.

In the aquatic habitat however, while juveniles could be vulnerable to sharks, I have a feeling that they'd mainly need to worry about one creature: the orca, since these creatures have the entirety of japan within their oceanic range and can eat moose with little effort.