Hi Barbara,
I'm unsure about the theme, could be snowflake (雪花 - yukibana) or sea cucumber (海鼠 - namako).
The hammered surface (tsuchime-ji) could suggest a katchūshi or Myōchin school (or Shōami, of course...), early to mid Edo period, maybe.
Just my not-so-educated opinion (waiting some more knowledged people would chime in).